BACKGROUND
a coordinated tempo-spatial expression of transcription factors is required for cell type specification and the differentiation of the three germ layers into distinct organ systems during bilaterian ontogeny  <cit> .
studies on ecdysozoan and deuterostome model organisms have elucidated the multifold roles of transcription factors during development; however, a major gap in knowledge does exist for the lophotrochozoa, one of the three bilaterian superphyla  <cit> .
for inferences concerning the evolutionary conservation versus plasticity of transcription factor expression patterns and their putative roles, a broad comparative approach is imperative.
among the lophotrochozoans, mollusca is the most speciose phylum and exhibits extremely diverse body plans and adaptative capacities ranging from worm-like spicule-bearing aplacophorans and sessile bivalved clams to highly motile squids .
no less diverse are the ontogenies, with direct and indirect developers and larval forms ranging from trochophore and pericalymma larvae to veligers  <cit> .
for the detection of settlement cues, predator avoidance, location of prey, as well as navigation in a complex environment, the larvae are equipped with sensory organs.
the most prominent larval sensory organ is the apical organ that is present in representatives of all molluscan classes investigated so far, except for cephalopods .
it is equipped with flask-shaped  cells and in some mollusks with additional peripheral cells.
a so-called ampullary system has been described in polyplacophoran larvae and appears to constitute an apomorphy of the class  <cit> .
it comprises four pairs of sensory cells that are located dorsolaterally and ventrolaterally in the episphere of the trochophore.
cells are of the ampullary sensory cell type, exhibit serotonin-like and fmrfamide-like immunoreactivity and are innervated by the cerebral commissure  <cit> .
another larval sensory organs, the so-called post-anal organ, only appears to exist in protobranch bivalves  <cit> .
it is composed of sensory cells with two types of cilia and a cavity in proximity to the anus.
coleoid cephalopods do not possess larvae as direct developers but a plethora of sensory organs such as complex camera eyes.
these have been subject of a recent study suggesting that a vast number of genes expressed in the squid camera-type eye is not found in the transcriptomes of the nautiloid pinhole eye  <cit> .
sensory organs as well as other epidermal epithelia or the nervous system are ectodermal derivatives  <cit> .
in mollusks, sensory organs have been described by histological and immunochemical techniques, however, the molecular underpinnings of their development and evolution remain largely obscure .fig.
<dig> putative interrelationships of recent mollusks.
two evolutionary scenarios based on recent phylogenomic studies ; arguing for the aculifera- conchifera concept
members of the paired box  gene family encode transcription factors that are involved in a variety of processes such as the development of excretory organs, the differentiation of the musculature, neuroectoderm specification, and the development of auditory/geosensory systems  <cit> .
in bilaterians, nine orthologous groups of pax proteins are known that can be distinguished on the basis of the presence or absence of highly conserved structural domains  <cit> .
members of the pax2/5/ <dig> group exhibit a conserved n-terminal paired domain, an octapeptide, a partial homeodomain, and a c-terminal transactivation domain  <cit> .
the n-terminal domain and the homeodomain serve as dna binding sites, the octapeptide is the site for activation or repression of the gene, and the c-terminal domain is the protein/protein interaction domain  <cit> .
in vertebrates and ecdysozoans, pax2/5/ <dig> is involved among others in the genesis of the auditory/geosensory system as well as in the formation of the midbrain/hindbrain boundary  <cit> .
for lophotrochozoans, so far only one study has been published on the expression domains and the putative function of pax2/5/ <dig> during development.
in this study, pax2/5/ <dig> expression was found in the statocysts, i.e.
the geosensory organ, of the gastropod veliger larva  <cit> .
in order to elucidate putative roles of pax2/5/ <dig> during molluscan development, we investigated the expression of its orthologs in representatives of both major molluscan lineages, the aculifera and the conchifera  <cit>  .
as a polyplacophoran, acanthochitona crinita has been shown to exhibit putative ancestral features of the aculifera  <cit> , while the conchiferan nucula tumidula is a protobranch that holds a basal position among the bivalves and possesses a unique pericalymma-type larva  <cit> .
a. crinita and n. tumidula do not possess statocysts in their larvae and exhibit a less centralized cns.
in contrast, the conchiferan cephalopod idiosepius notoides has a highly centralized cns and statocysts.
this study will test whether pax2/5/ <dig> is expressed in ectodermal, endodermal, or mesodermal derivatives during the ontogeny of all three molluscan species and it will shed light on the shared pax2/5/ <dig> expression among bilaterian representatives.
RESULTS
pax2/5/ <dig> gene orthologs and phylogenetic analysis
the multiple sequence alignment of pax2/5/ <dig> orthologs shows the highly conserved n-terminal paired domain, the octapeptide domain, and the lysine-arginine rich region .
in particular the octapeptide is less conserved among the bilaterian pax2/5/ <dig> orthologs.
the c-terminal partial homeodomain was not included in the alignment since it is even less well conserved.
the phylogenetic analysis included proteins of all pax families and demonstrates that all three molluscan pax2/5/ <dig> amino acid sequences cluster with their bilaterian orthologs .fig.
<dig> alignments of the molluscan pax2/5/ <dig> domains and their bilaterian orthologs.
a paired domain alignment of the predicted amino acids of various metazoan pax genes including acr-pax2/5/ <dig>  ntu-pax2/5/ <dig>  and ino-pax2/5/ <dig> .
this alignment was used for the maximum-likelihood consensus tree shown in fig.
<dig>  b alignment of pax proteins of various metazoans including acr-pax2/5/ <dig>  ntu-pax2/5/ <dig>  and ino-pax2/5/ <dig> .
the conserved n-terminal paired domain , the octapeptide , and the lysine-arginine-rich region  are shown, while the partial homeodomain and the c-terminal transactivation domain are omitted.
genbank accession numbers of all encoding genes used are listed in table 1
fig.
<dig> phylogenetic analysis of pax proteins.
maximum-likelihood consensus tree with  <dig> bootstrap replicates constructed from the alignment shown in fig.
2a.
note that acr-pax2/5/ <dig>  ntu-pax2/5/ <dig>  and ino-pax2/5/ <dig>  cluster with the predicted proteins of their bilaterian pax2/5/ <dig> orthologs .
pax protein families are labeled in different colors
pax2/5/ <dig> expression in the polyplacophoran acanthochitona crinita
the gastrula stage is reached between 3– <dig> h post fertilization  at 20 °c.
during this stage acr-pax2/5/ <dig> is expressed in ectodermal cells of the apical region .
early trochophore larvae hatch at approximately  <dig> hpf.
they are lecithotrophic and are slightly ovoid in shape, with an apical organ and apical tuft in the episphere .
the apical organ comprises three serotonin-like immunoreactive flask-shaped cells and faint fmrfamide-like immunoreactivity .
two other groups comprising four fmrfamide-like immunoreactive cells each are located in the episphere close to the prototroch .
the episphere is separated from the hyposphere by two rows of trochoblasts that give rise to the prototroch.
acr-pax2/5/ <dig> is expressed in different domains, here referred to as “groups”, located mainly in the larval episphere .
several acr-pax2/5/8-expressing cells are located in the ectoderm of the dorsal episphere adjacent to both rows of trochoblasts .
in addition, two other groups are located bilaterally and anterior to group  <dig> .
each group  <dig> comprises approximately four ectodermal cells that are located subepidermally and project dendrites through the epidermis .
two single acr-pax2/5/8-expressing cells are located bilaterally, immediately adjacent to the two rows of trochoblasts and more ventrally to group  <dig> in the episphere .
in the center of the ventral episphere, a group of 5– <dig> ectodermal acr-pax2/5/8-expressing cells is present .
the only acr-pax2/5/ <dig> expression domain that is located in the hyposphere of the early trochophore is situated in the region of the nascent shell fields in the dorsal hyposphere .
the apical organ does not express acr-pax2/5/ <dig> .
in further developed trochophore larvae , acr-pax2/5/8-expression is less pronounced in the episphere, while acr-pax2/5/ <dig> expression in the hyposphere gains in intensity .
this and other acr-pax2/5/8-expressing cell groups are located in similar expression domains as described for earlier developmental stages .
acr-pax2/5/8-expressing cells are present in the expression domains of groups  <dig>   <dig>  and  <dig> of previous stages .
furthermore, serotonin-like immunoreactive cell somata are located in corresponding regions, such as the lateroventral ampullary cells corresponding to acr-pax2/5/8-expressing group  <dig>  or the additional sensory cells that are located in the dorsal episphere that correspond to group  <dig> .
in the expression domains of acr-pax2/5/8-expressing groups  <dig> and  <dig>  fmrfamide-like immunoreactive cell somata are located .
in further developed trochophore larvae , the apical organ does not exhibit acr-pax2/5/ <dig> expression, however, very close to it, acr-pax2/5/8-expressing cells are located .
in metamorphic competent trochophore larvae , the acr-pax2/5/ <dig> expression pattern is similar to the one of earlier trochophores  .
strong acr-pax2/5/ <dig> expression is present in cells of the nascent shell fields .
these acr-pax2/5/8-expressing cells are located in the epidermis and project dendrites to the periphery .
in addition, acr-pax2/5/8-expressing cells are numerous in groups 1– <dig> during earlier development .
interestingly, all these acr-pax2/5/ <dig> expression domains house fmrfamide-like-immunoreactive  cells .fig.
<dig> expression of acr-pax2/5/ <dig> in early larvae of the polyplacophoran acanthochitona crinita.
dorsal –ventral , apical -abapical , and left -right  axes indicate the orientation.
blastopores/mouth labeled by asterisks and the location of the apical organ is encircled.
a-b consecutive optical sections through a gastrula showing acr-pax2/5/8-expressing cells ).
c confocal reflection scan highlighting same acr-pax2/5/8-expressing cells  in apical ectoderm as shown in a-b
d three serotonin-like immunoreactive neurons of the apical organ in a  <dig> hpf early trochophore.
e-h optical sections from dorsal to ventral through an early trochophore  showing acr-pax2/5/8-expressing domains 1– <dig>  in the episphere and single acr-pax2/5/8-expressing cells in the hyposphere ).
e upper inset: several acr-pax2/5/8-expressing cells  are located in the ectoderm of the dorsal episphere adjacent to the prototroch .
lower inset: this very early trochophore  exhibits faint fmrfamide-like immunoreactive signal  in the region of the apical organ and in two groups of four fmrfamide-like immunoreactive ectodermal cells each in the episphere close to the prototroch.
f two other bilateral groups of acr-pax2/5/8-expressing cells  are located anterior to group  <dig> and additional acr-pax2/5/8-expressing cells lie in the region of the nascent shell fields in the hyposphere .
g two acr-pax2/5/8-expressing ectodermal cells  are located bilaterally in the episphere adjacent to the trochoblasts.
h a group of 5– <dig> ectodermal acr-pax2/5/8-expressing cells  lies in the central ventral episphere.
i-k same specimen as shown in e-h from the left side  to the mid-sagittal plane .
l apical view of same specimen as shown in e-k with acr-pax2/5/ <dig> expression domains  <dig>   <dig>  and  <dig>  abbreviations: bc, blastocoel; f, foot; pt, prototroch.
scale bars: 20 μm
fig.
<dig> sketch drawings of pax2/5/ <dig> expression in acanthochitona crinita, nucula tumidula, and idiosepius notoides.
dorsal –ventral , apical -abapical , and anterior -posterior  axes indicate the orientation.
mouth is marked by asterisk in all panels.
pax2/5/8-expressing cell groups are indicated by red numbers in early trochophore larvae of acanthochitona crinita .
additional pax2/5/8-expressing cells are located in the hyposphere of further developed larvae .
a pericalymma larva  and a settled individual  of nucula tumidula express pax2/5/ <dig> in the mantle.
note that pax2/5/ <dig> is not expressed in the test cells  that constitute the outermost cell layer.
a stage  <dig> individual  and a stage 24– <dig> individual  of idiosepius notoides express pax2/5/ <dig> in various domains.
red dashed line indicates trajectory of esophagus and internal yolk.
abbreviations: an, anus; at, apical tuft, ey, eye; f, foot; fn, funnel; g, gill; ib, interbasal lobe; m, mantle; ms, middle subesophageal mass; s, shell; sc, statocyst; sg, shell gland; se, supraesophageal mass; stm, stomach; ps, posterior subesophageal mass; pt, prototroch; y, yolk.
scale bars: 50 μm
fig.
<dig> expression of acr-pax2/5/ <dig> during late larval development and in settled individuals of acanthochitona crinita.
dorsal –ventral , apical -abapical , anterior -posterior , and left -right  axes indicate the orientation, the red dashed circles the apical organ, and the asterisks label the mouth.
cell nuclei are labeled in blue, cilia in green in b & right inset of j).
a-d
acr-pax2/5/ <dig> is expressed in ectodermal cells of the episphere , the shell fields ), the dorso-lateral  and ventral episphere , and close to the apical organ  in  <dig> hpf old trochophores  to ventral ).
b a further developed,  <dig> hpf old, trochophore  and a slightly younger specimen  with serotonin-like immunoreactive latero-ventral ampullary cells , additional sensory cells , apical organ cells , and cells in the ventral hyposphere .
e-h in competent trochophores   to ventral ), acr-pax2/5/ <dig> is expressed in cells of the shell fields , in cells of group  <dig>   <dig>  and  <dig> , and in two groups of cells  along the developing foot , which are connected via a slender bridge of cells.
inset in g
acr-pax2/5/8-expressing cells close to the apical organ.
i-k lateral view of trochophore  shown in .
j dashed box marks location of the small inset that depicts a confocal reflection scan of acr-pax2/5/8-expressing subepidermal flask-shaped cells  in the shell fields.
right large inset: fmrfamide-like immunoreactive cell somata  in the shell fields  and close to the trochoblasts.
k
acr-pax2/5/ <dig> expression in the posterior mantle fold .
l no acr-pax2/5/ <dig> expression was observed in postmetamorphic  specimens .
the staining of the perinotum is unspecific .
abbreviations: at, apical tuft; pt, prototroch.
scale bars: 20 μm  10 μm)
an additional acr-pax2/5/ <dig> expression domain is present close to the lateral borders of the foot .
both groups  <dig> are interconnected via a thin band of acr-pax2/5/8-expressing cells that runs along the abapical side of the mouth .
few acr-pax2/5/8-expressing cell are present in the posterior mantle fold , while no acr-pax2/5/8-expressing cells are located in the region of the apical organ .
settled animals  commence to reduce their episphere and the foot becomes more pronounced.
metamorphosis is largely completed with the shedding of the prototroch, although the eighth shell plate is not formed until probably several weeks after settlement.
early settled animals solely possess few faintly stained acr-pax2/5/8-expressing cells in the region of the shell fields  and only non-specific staining was observed in the perinotum that surrounds the plates .
pax2/5/ <dig> expression in the bivalve nucula tumidula
embryos of n. tumidula reach the 2– <dig> cell stage by  <dig> hpf at  <dig>  °c and the zygotes gastrulate by approximately  <dig> hpf.
during gastrulation, ntu-pax2/5/ <dig> is expressed in approximately five ectodermal cells close to the blastopore .
early pericalymma larvae swim by approximately  <dig> hpf, are roundish, and possess 5– <dig> ntu-pax2/5/8–expressing cells on the future dorsal side below the test-cells .
slightly further developed pericalymmae are characterized by an apical organ with a ciliary tuft, while mouth and anus are located on the abapical side .
they are covered with large, multiciliary calymma  cells.
subsequently, three rows of test cells bear three bands of longer cilia, while shorter cilia still cover the area around the apical tuft as well as the abapical side around mouth and anus.
these larvae express ntu-pax2/5/ <dig> in the area of the dorsal mantle  and the expression domain also includes the shell gland that develops as an invagination after gastrulation on the future dorsal side .
during subsequent development the apical-abapical axis elongates and the foot develops on the ventral side.
meanwhile, the shell field on the dorsal side has secreted two shell valves.
further developed larvae express ntu-pax2/5/ <dig> in more ventral and apical mantle regions .
late pericalymma larvae express ntu-pax2/5/ <dig> along the dorsal mantle region as well as in ventral domains of the mantle .
larvae that are metamorphic competent are more ovoid in shape than earlier larvae and express ntu-pax2/5/ <dig> mostly in dorsal domains but also in ventral ones .
lecithotrophic larvae swim until 15– <dig> dpf in the water column and finally settle.
during the settlement process that may last for several hours, larvae appear to probe the substrate and metamorphosis commences with shedding and ingestion of the test cells and the apical organ .
larvae express ntu-pax2/5/ <dig> mainly bilaterally in two postero-ventral domains of the central mantle .
faint expression is still visible on the dorsal side close to the hinge of the developing valves .
in individuals that have lost their test cells, ntu-pax2/5/ <dig> is expressed in the posteroventral region of the mantle and along the mantle margin .
early juveniles express ntu-pax2/5/ <dig> in lateral portions of the ventral mantle .fig.
<dig> expression of ntu-pax2/5/ <dig> during larval development of the bivalve nucula tumidula.
dorsal –ventral , apical -abapical , and left -right  axes indicate the orientation.
blastopore and mouth is marked with an asterisk and the position of the apical organ is encircled.
a
ntu-pax2/5/ <dig> expression in approximately five cells  close to the blastopore of a 24 h post fertilization  old gastrula.
inset: sketch drawing of ntu-pax2/5/ <dig> expression  in a gastrula .
b-c 5– <dig> cells  express ntu-pax2/5/ <dig> below the test cells and close to the mouth of the early pericalymma larva .
d confocal reflection  and autofluorescence  scan of a single optical section of an early larva ).
e a further developed specimen  with ntu-pax2/5/ <dig> expression in the mantle .
f
ntu-pax2/5/ <dig> is expressed in the mantle including the shell gland  .
g confocal reflection  and autofluorescence  scan of a single optical section of the same specimen as shown in .
h this further developed larva  expresses ntu-pax2/5/ <dig> bilaterally along the dorsal and apical mantle .
i late larvae  express ntu-pax2/5/ <dig> along the dorsal and ventral mantle .
j  <dig> dpf old larva of the same stage as the one shown in i with stronger ntu-pax2/5/ <dig> expression in the dorsal and ventral  mantle.
k same specimen as shown in j with ntu-pax2/5/ <dig> expression in the dorsal and ventral mantle  but not the foot .
l sketch drawing summarizing the ntu-pax2/5/ <dig> expression domains  in a metamorphic competent pericalymma larva before settlement.
abbreviations: an, anus; at, apical tuft; e, esophagus; f, foot; pc, perivisceral cavity; tc, test cell; s, shell; stm, stomach.
scale bars: 30 μm
fig.
<dig> expression of ntu-pax2/5/ <dig> during postlarval development of the bivalve nucula tumidula.
dorsal –ventral , anterior -posterior , and left -right  axes indicate the orientation.
a
ntu-pax2/5/ <dig> expression in the mantle of a  <dig> dpf old larva during onset of metamorphosis.
ntu-pax2/5/ <dig> expression is restricted to the mantle  margin and a bilateral expression domain located in the postero-ventral central region but not the foot .
faint expression  is still visible on the dorsal side of the animal close to the hinge .
note the test cells  being ingested by this specimen.
b confocal reflection  and autofluorescence  scan of a single optical section of the specimen shown in  .
all test cells and the apical organ  are shed and engulfed during metamorphosis.
c
ntu-pax2/5/ <dig> expression in a settled  <dig> dpf old individual that has engulfed all test cells.
note the ntu-pax2/5/ <dig> expression along the mantle margin .
inset: ventral view of a similar staged settled animal.
d confocal reflection  and autofluorescence  scan of a single optical section of the same  <dig> dpf old specimen as shown in  .
a different confocal plane of the boxed area is shown in the inset highlighting test cells in the stomach  of the settled larva.
e
ntu-pax2/5/ <dig> expression along the ventral mantle margin in a further developed specimen  with no test cells visible in the stomach .
note the lack of ntu-pax2/5/ <dig> expression along the dorsal mantle area close to the hinge.
f posterior view of the same specimen as shown in e. scale bars: 50 μm
pax2/5/ <dig> expression in the cephalopod idiosepius notoides
as a typical cephalopod, i. notoides is a direct developer with discoidal cleavage.
hatching of the juvenile occurs within 9– <dig> days after oviposition at 25 °c.
in the present study, we adhere to the common designation of embryological orientation in cephalopods, i.e., the mantle apex is considered dorsal and the cephalic region including the brachial crown that surrounds the mouth is considered ventral.
the funnel is located posteriorly, while the opposite side including the supraesophageal mass is considered anterior.
anlagen of the cns, the arms, the funnel, and the mantle are discernable from stage  <dig> onward .
early stage  <dig> individuals of the pygmy squid i. notoides express ino-pax2/5/ <dig> in the mantle, the funnel, and all ten arms .
faint ino-pax2/5/8-expression is present in the developing gills of stage  <dig> individuals .
during subsequent development, ino-pax2/5/ <dig> expression is located in the same domains as in stage  <dig> individuals .
ino-pax2/5/ <dig> is additionally expressed in the tissue that interconnects the arms  and in nervous tissue of the optic ganglia .
stage  <dig> individuals strongly express ino-pax2/5/ <dig> in all arms, the funnel, eyes, and the epidermis of the cephalopedal region .
additionally, ino-pax2/5/8-expressing cells are present in the region of the interbasal lobes that both belong to the posterior basal lobes of the brain .
during stages 24– <dig>  ino-pax2/5/ <dig> is also expressed in the eyes .
stage  <dig> individuals still express ino-pax2/5/ <dig> in the gills and in large areas of the epidermis of the cephalopedal region .
ino-pax2/5/ <dig> expression is only located in the proximal portions of all arms and the proximal portion of the funnel .
ino-pax2/5/ <dig> expression domains in the cns include the anterior basal and the interbasal lobes that both belong to the supraesophageal mass .
the vertical lobe complex as highest integration center does not express ino-pax2/5/ <dig> .
ino-pax2/5/ <dig> is also faintly expressed in the posterior subesophageal mass .
the posterior perikaryal layer of the middle subesophageal mass also expresses ino-pax2/5/ <dig> .
in late prehatching developmental stages such as stage  <dig>  ino-pax2/5/ <dig> is expressed in the gills , in the posterior perikaryal layer of the middle subesophageal mass, and around the buccal mass .
ino-pax2/5/ <dig> expression in the interbasal lobes persists in stage  <dig> individuals .
faint ino-pax2/5/ <dig> expression is still present at the base of the funnel .fig.
<dig> expression of ino-pax2/5/ <dig> during early prehatching development of the cephalopod idiosepius notoides.
dorsal –ventral , anterior -posterior , and left -right  axes indicate the orientation.
a
ino-pax2/5/ <dig> is expressed in the mantle , the funnel , and all five arm pairs  of stage  <dig> individuals.
note the faint ino-pax2/5/ <dig> expression in the gills .
same specimen as shown in  with ino-pax2/5/ <dig> expression in the 2-5th arm pair .
c lateral view of same specimen as shown in  and  exhibiting expression of ino-pax2/5/ <dig> in all five arm pairs .
d stage  <dig> individuals show ino-pax2/5/ <dig> expression in the mantle, the fifth arm pair , and the funnel.
note the faint ino-pax2/5/ <dig> expression in the gills .
e
ino-pax2/5/ <dig> expression in portions of the optic ganglia  close to the eyes  in the same specimen as shown in  f same specimen as shown in  with ino-pax2/5/ <dig> expression in the cerebral ganglia .
g lateral view of the same specimen as shown in  inset shows detail of ino-pax2/5/ <dig> expression in the arms .
abbreviations: y, yolk sac.
scale bars:  150 μm
fig.
<dig> expression of ino-pax2/5/ <dig> during late prehatching development of the cephalopod idiosepius notoides.
dorsal –ventral , anterior -posterior , and left -right  axes indicate the orientation.
yolk sac  removed in  arms are labeled with numbers.
a stage  <dig> individuals express ino-pax2/5/ <dig> in all arms, the eyes , the epidermis of the cephalopedal region, the interbasal lobes , the posterior basal lobes , and the anterior basal lobes .
note the unspecific staining around the shell gland .
b same stage  <dig> individual as shown in  note ino-pax2/5/ <dig> expression in the proximal optic lobes , the eyes, and the funnel .
c
ino-pax2/5/ <dig> is expressed in smaller portions of the body of stage  <dig> individuals compared to earlier stage individuals.
ino-pax2/5/ <dig> is expressed in the gills , the eyes, and in the proximal but not the distal arm portions.
note ino-pax2/5/ <dig> expression in the interbasal lobes .
d the interbasal lobes  and anterior basal lobes of the supraesophageal mass  express ino-pax2/5/ <dig>  the posterior perikaryal layer of the middle subesophageal mass  as well as the posterior subesophageal mass  express ino-pax2/5/ <dig>  ino-pax2/5/ <dig> expression could not be observed in the distal funnel of this stage  <dig> individual.
please note the unspecific staining around the statocyst .
e-h in stage  <dig> individuals, ino-pax2/5/ <dig> is restricted to few expression domains such as the gills, the posterior perikaryal layer of the middle subesophageal mass, the interbasal lobes , tissue around the buccal mass , and the base of the funnel.
trajectory of esophagus is indicated by red dashed line in  abbreviations: bk, beak; is, ink sac; iy, internal yolk; se, supraesophageal mass.
scale bars:  150 μm; e-h: 200 μm
discussion
evolution of pax2/5/ <dig> proteins
our phylogenetic analysis confirms the identities of all three molluscan pax2/5/ <dig> proteins presented herein.
the sequences of acanthochitona crinita, idiosepius notoides, and nucula tumidula exhibit a paired domain, an octapeptide, the lysine-arginine rich region, as well as the partial homeodomain  diagnostic for pax2/5/ <dig>
pax2/5/ <dig> is expressed in the development of multimodal sensory systems of mollusks
among bilaterians, pax2/5/ <dig> expression is particularly well-investigated in the deuterostomia and ecdysozoa  <cit> .
studies on various deuterostomes revealed that orthologs of pax2/5/ <dig> play a role during development of auditory/ geosensory organs .
interestingly, the phylogenetically distantly related gastropod mollusk haliotis asinina also expresses pax2/5/ <dig> in its geosensory organ, the statocysts  <cit> .
this present study demonstrated that homeobox genes may be recruited into the development of organs with similar function but of different evolutionary origin .
it also poses the question as to how pax2/5/ <dig> is recruited during the ontogeny of organisms without auditory and equilibrial sense.
in our study, we included the chiton acanthochitona crinita and the bivalve nucula tumidula, both lophotrochozoan representatives without larval statocysts, in order to assess the role of pax2/5/ <dig> during their development.
pax2/5/ <dig> is expressed in several groups of ectodermal cells in the episphere and the hyposphere of the polyplacophoran trochophore.
although a. crinita and n. tumidula possess larval apical  organs, no pax2/5/ <dig> expression was detected in components of this organ.
in a. crinita cells of groups 2– <dig> are located in a similar location as cells of the larval ampullary sensory system that is an autapomorphy of polyplacophora .
cells of the ampullary system are flask-shaped and probably chemosensory rather than mechanosensory  <cit> .
pax2/5/ <dig> expression and fmrfamide-like immunoreactivity may be co-localized in ectodermal cells in the dorsal episphere adjacent to the prototroch .
further pax2/5/8-expressing ectodermal cells  are located in the ventral episphere and might correspond to the four serotonin-like immunoreactive flask-shaped cells .
the immunochemical and molecular phenotype, their flask-shaped somata, and their subepithelial location argue for a sensory role of the above-mentioned cells  <cit> .
this might also be true for the pax2/5/8-expressing groups  <dig> that are located on the ventral side along the borders of the foot of advanced trochophore larvae .
both groups are connected to each other via a small band along the abapical side of the mouth/ apical side of the foot .
this expression domain also includes a region where the “schwabe organ”, a putative chemosensory organ of a polyplacophoran ingroup, is located  <cit> .
although the chitonid a. crinita has not been studied with respect to the presence of this organ, it is likely that it is not present in a. crinita since other congeneric species of the chitonida lack this sensory structure.
pax2/5/ <dig> expressing cells are also located in the posterior mantle fold that exhibits the chemosensory osphradia of adult chitons .
the majority of these cells is located in central regions of each shell field and exhibits processes that penetrate the epidermis and the nascent calcareous layers of the shell plates .
these processes exhibit fmrfamide-like immunoreactivity in earlier developmental stages than the associated cell somata .
judging by their location and morphology, these pax2/5/8-expressing and fmrfamide-like immunoreactive cells are probably esthetes.
esthetes are primarily photoreceptive organs that constitute a polyplacophoran autoapomorphy and penetrate the surface of the shell plates of adult specimens .
during ontogeny, esthetes may first be identified by the narrow holes of their canals in shell plates of metamorphosed specimens  <cit> .
in the pygmy squid, pax2/5/ <dig> is expressed in various organs that are known to be equipped with sensory cells.
during a short developmental time frame , ino-pax2/5/ <dig> is expressed in the eyes , resembling the situation of the adult gastropod haliotis asinina  <cit> .
the brachial crown, i.e.
all five arm pairs, expresses pax2/5/ <dig> in epidermal cells until stage  <dig> .
in stage  <dig> individuals, pax2/5/ <dig> expression is restricted to the proximal part of each arm/tentacle.
expression fades in subsequent developmental stages.
cephalopod arms and tentacles house a plethora of sensory cells.
although cell proliferation experiments during cephalopod arm growth are still lacking, it is tempting to speculate that pax2/5/ <dig> is expressed in ectodermal sensory cells in regions of high proliferation activity, such as the proximal portion of the brachial crown in stage  <dig> specimens.
the funnel shows a similar pattern as the brachial crown.
during early development the entire funnel expresses pax2/5/ <dig>  while only the proximal portion expresses pax2/5/ <dig> in subsequent stages until it fades entirely in late prehatching stages.
here, pax2/5/ <dig> appears to be expressed in the epidermis as it has also been reported for vertebrates  <cit> .
along the lateral sides of the foot the future gills develop and pax2/5/ <dig> expression predates their development.
expression of pax2/5/ <dig> in the molluscan mantle
pax2/5/ <dig> is expressed in different regions of the developing mantle of the aculiferan polyplacophoran acanthochitona crinita, the basal bivalve nucula tumidula, and the cephalopod idiosepius notoides.
this corroborates data on the veliger larva of haliotis asinina, where pax2/5/ <dig> is expressed in the anterior region of the mantle that is known to be richly equipped with sensory cells  <cit> .
in contrast to the esthetes as distinct expression domain of a. crinita, pax2/5/ <dig> is rather broadly expressed in the mantle of n. tumidula and i. notoides.
during earlier development of n. tumidula, pax2/5/ <dig> expression is, however, first restricted to the dorsal mantle epithelium, including the shell gland.
in contrast, the shell gland of h. asinina does not exhibit pax2/5/ <dig> expression .
shortly before metamorphosis, pax2/5/ <dig> expression shifts to the ventral side of the mantle and disappears in the dorsal region after metamorphosis.
this may be explained by the onset of sensory cell development on the ventral side of the mantle that is exposed to water currents in the postmetamorphic, settled animal.
other ectodermal expression domains comprise the cephalopedal epidermis of stage  <dig> individuals of i. notoides .
pax2/5/ <dig> plays a role in cephalopod brain regionalization
studies on mouse and drosophila embryos revealed that several gene orthologs play a role in brain regionalization.
the anterior-most brain regions of both organisms, i.e.
the murine forebrain/midbrain as well as the fruit fly’s protocerebrum/deuterocerebrum, express otd/otx .
the adjacent more posterior brain regions, that are patterned in a spatially collinear fashion by anterior hox gene orthologs, are separated from the anterior-most brain regions by a pax2/5/ <dig> expression domain .
interestingly, a similar molecular fingerprint is manifested in the developing brain of the cephalopod idiosepius notoides  and other cephalopods investigated.
here, pax2/5/ <dig> expression is restricted to the cerebral ganglia during early development.
in subsequent developmental stages transcripts are restricted to the anterior basal and the interbasal lobes that are located slightly anterior to the esophagus, thus dividing the cephalopod brain into a supraesophageal and a subesophageal mass .
anterior portions of the posterior subesophageal mass that are located close to the esophagus also exhibit faint pax2/5/ <dig> expression.
notably, brain lobes that are located more anteriorly, such as the median basal lobe or the vertical lobe complex, do not express pax2/5/ <dig> in i. notoides.
in late-stage prehatching individuals, pax2/5/ <dig> is only expressed in the interbasal lobes as well as in the posterior perikaryal layer of the middle subesophageal mass.fig.
<dig>
pax2/5/ <dig> has been independently recruited in brain regionalization in various bilaterian subgroups.
expression of otx, pax2/5/ <dig>  and anterior hox genes during brain formation in a lophotrochozoan , an ecdysozoan , and a deuterostome .
anterior-most brain regions face up and posterior-most brain regions face down.
the cephalopod vertical lobe does not express otx, pax2/5/ <dig>  or hox genes.
the superior frontal, subpedunculate, and median basal lobes express otx in sepia officinalis  <cit>  and the anterior basal and interbasal lobes express pax2/5/ <dig> in idiosepius notoides .
the posterior subesophageal mass and the posterior perikaryal layer of the middle subesophageal mass express pax2/5/ <dig> in i. notoides .
anterior hox genes are expressed in the palliovisceral and pedal ganglia/ subesophageal mass of other cephalopods  <cit> .
sketch modified after  <cit>
otx is expressed in the supraesophageal mass of the cuttlefish sepia officinalis and the pygmy squid i. notoides .
compared to pax2/5/ <dig> it is, however, expressed in more anterior regions of the supraesophageal mass; i.e., in components of the vertical lobe complex  and the median basal lobe of the supraesophageal mass.
in contrast to pax2/5/ <dig> expression, no otx expression was found in the subesophageal mass .
in cephalopods, hox genes are expressed in the cns with exception of the cerebral ganglia/supraesophageal mass  <cit> .
although no collinear spatial expression is reported from cephalopods, the otx- and pax2/5/ <dig> expression patterns adhere to the overall pattern reported for drosophila and mouse  <cit> .
data on other bilaterians suggest a six <dig> expression domain in the anterior-most region of the cephalopod brain, i.e.
the vertical lobe complex .
surprisingly, pax2/5/ <dig> is not expressed in the developing nervous system of the polyplacophorans, bivalves, or gastropods investigated so far .
in contrast to cephalopods, the fruit fly, or mouse none of the latter mollusks possess a highly centralized brain.
pax2/5/ <dig> expression is restricted to the ectoderm in mollusks
among lophotrochozoans, pax2/5/ <dig> expression during development has only been studied in mollusks .
in all mollusks investigated so far, pax2/5/ <dig> is exclusively expressed in ectodermal domains such as the epidermis, the shell gland, the mantle, sensory cells, as well as the nervous system .
considering that pax2/5/ <dig> is consistently expressed in ectodermal epithelia in all metazoans investigated so far, with solely the onychophora and the chordata expressing pax2/5/ <dig> in mesodermal domains such as the nephridia, the most parsimonious explanation is an independent recruitment of pax2/5/ <dig> in mesodermal epithelia development in these clades .
in nematodes and cnidarians the pax2/5/ <dig> orthologs are involved in the formation of endodermal structures such as the hindgut or the endoderm at the aboral end of the pharynx .
since the anterior-most and posterior-most portions of the digestive tract are located at the endodermal-ectodermal boundary, the pax2/5/ <dig> expression domains in the nematode hindgut and the aboral end of the pharynx of the cnidarian nematostella vectensis might be an evolutionary reminiscent of ancestral ectodermal expression domains in the ancestors of these taxa .fig.
<dig>
pax2/5/ <dig> expression domains during ontogeny of various metazoan representatives.
a comparison among metazoan animals suggests that the ancestral role of pax2/5/ <dig> was probably restricted to ectodermal domains while chordates and onychophorans recruited orthologs into the development of mesodermal organ systems.
the few data on lophotrochozoa suggest that pax2/5/ <dig> expression adheres to the ancestral state, i.e.
patterning of certain ectodermal domains.
in addition, they show that among mollusks pax2/5/ <dig> apparently lost its role in the formation of the nervous system  with exception of the cephalopods, while the mantle including shell gland strongly express pax2/5/ <dig>  data on pax2/5/ <dig>  pax <dig>  pax <dig>  pax <dig>  paxb-expression:  <cit>  ;  <cit>  ;  <cit>  ;  <cit>  ; present study ;  <cit> , present study ;  <cit>  ;  <cit>  ;  <cit>  ;  <cit>  .
cladogram simplified after  <cit>
CONCLUSIONS
the present study shows that pax2/5/ <dig> is expressed in multimodal sensory systems in mollusks such as the esthetes and the ampullary system of polyplacophorans, the eyes of squids, but not in the larval apical organ of a. crinita and n. tumidula.
pax2/5/ <dig> expression probably predates sensory cell development during ontogenesis, since pax2/5/8-expressing cells are present in regions where the future sensory cells are situated.
compared to other bilaterians, pax2/5/ <dig> is not expressed in the development of the less centralized nervous system of bivalves, gastropods, and polyplacophorans, and hence it most likely lost its role in brain development in these mollusks.
interestingly, pax2/5/ <dig> is expressed along the trajectory of the esophagus that divides the cephalopod brain into a supraesophageal and a subesophageal mass.
together with otx and hox genes pax2/5/ <dig> might have been recruited into brain regionalization, thus representing an extreme case of convergent evolution of gene function to the situation found in vertebrates  and insects .
since pax2/5/ <dig> is largely expressed in ectodermal domains throughout the bilateria, its ancestral role was most likely in the differentiation of this outer most germ layer of bilaterians.

 a coordinated tempo-spatial expression of transcription factors is required for cell type specification and the differentiation of the three germ layers into distinct organ systems during bilaterian ontogeny. studies on ecdysozoan and deuterostome model organisms have elucidated the multifold roles of transcription factors during development; however, a major gap in knowledge does exist for the lophotrochozoa, one of the three bilaterian superphyla. for inferences concerning the evolutionary conservation versus plasticity of transcription factor expression patterns and their putative roles, a broad comparative approach is imperative. among the lophotrochozoans, mollusca is the most speciose phylum and exhibits extremely diverse body plans and adaptative capacities ranging from worm-like spicule-bearing aplacophorans and sessile bivalved clams to highly motile squids. no less diverse are the ontogenies, with direct and indirect developers and larval forms ranging from trochophore and pericalymma larvae to veligers. for the detection of settlement cues, predator avoidance, location of prey, as well as navigation in a complex environment, the larvae are equipped with sensory organs. the most prominent larval sensory organ is the apical organ that is present in representatives of all molluscan classes investigated so far, except for cephalopods. it is equipped with flask-shaped cells and in some mollusks with additional peripheral cells. a so-called ampullary system has been described in polyplacophoran larvae and appears to constitute an apomorphy of the class. it comprises four pairs of sensory cells that are located dorsolaterally and ventrolaterally in the episphere of the trochophore. cells are of the ampullary sensory cell type, exhibit serotonin-like and fmrfamide-like immunoreactivity and are innervated by the cerebral commissure. another larval sensory organs, the so-called post-anal organ, only appears to exist in protobranch bivalves. it is composed of sensory cells with two types of cilia and a cavity in proximity to the anus. coleoid cephalopods do not possess larvae as direct developers but a plethora of sensory organs such as complex camera eyes. these have been subject of a recent study suggesting that a vast number of genes expressed in the squid camera-type eye is not found in the transcriptomes of the nautiloid pinhole eye. sensory organs as well as other epidermal epithelia or the nervous system are ectodermal derivatives. in mollusks, sensory organs have been described by histological and immunochemical techniques, however, the molecular underpinnings of their development and evolution remain largely obscure.fig. putative interrelationships of recent mollusks. two evolutionary scenarios based on recent phylogenomic studies; arguing for the aculifera- conchifera concept members of the paired box gene family encode transcription factors that are involved in a variety of processes such as the development of excretory organs, the differentiation of the musculature, neuroectoderm specification, and the development of auditory/geosensory systems. in bilaterians, nine orthologous groups of pax proteins are known that can be distinguished on the basis of the presence or absence of highly conserved structural domains. members of the pax2/5/ group exhibit a conserved n-terminal paired domain, an octapeptide, a partial homeodomain, and a c-terminal transactivation domain. the n-terminal domain and the homeodomain serve as dna binding sites, the octapeptide is the site for activation or repression of the gene, and the c-terminal domain is the protein/protein interaction domain. in vertebrates and ecdysozoans, pax2/5/ is involved among others in the genesis of the auditory/geosensory system as well as in the formation of the midbrain/hindbrain boundary. for lophotrochozoans, so far only one study has been published on the expression domains and the putative function of pax2/5/ during development. in this study, pax2/5/ expression was found in the statocysts, i.e. the geosensory organ, of the gastropod veliger larva. in order to elucidate putative roles of pax2/5/ during molluscan development, we investigated the expression of its orthologs in representatives of both major molluscan lineages, the aculifera and the conchifera. as a polyplacophoran, acanthochitona crinita has been shown to exhibit putative ancestral features of the aculifera, while the conchiferan nucula tumidula is a protobranch that holds a basal position among the bivalves and possesses a unique pericalymma-type larva. a. crinita and n. tumidula do not possess statocysts in their larvae and exhibit a less centralized cns. in contrast, the conchiferan cephalopod idiosepius notoides has a highly centralized cns and statocysts. this study will test whether pax2/5/ is expressed in ectodermal, endodermal, or mesodermal derivatives during the ontogeny of all three molluscan species and it will shed light on the shared pax2/5/ expression among bilaterian representatives.  pax2/5/ gene orthologs and phylogenetic analysis the multiple sequence alignment of pax2/5/ orthologs shows the highly conserved n-terminal paired domain, the octapeptide domain, and the lysine-arginine rich region. in particular the octapeptide is less conserved among the bilaterian pax2/5/ orthologs. the c-terminal partial homeodomain was not included in the alignment since it is even less well conserved. the phylogenetic analysis included proteins of all pax families and demonstrates that all three molluscan pax2/5/ amino acid sequences cluster with their bilaterian orthologs.fig. alignments of the molluscan pax2/5/ domains and their bilaterian orthologs. a paired domain alignment of the predicted amino acids of various metazoan pax genes including acr-pax2/5/ ntu-pax2/5/ and ino-pax2/5/. this alignment was used for the maximum-likelihood consensus tree shown in fig.  b alignment of pax proteins of various metazoans including acr-pax2/5/ ntu-pax2/5/ and ino-pax2/5/. the conserved n-terminal paired domain, the octapeptide, and the lysine-arginine-rich region are shown, while the partial homeodomain and the c-terminal transactivation domain are omitted. genbank accession numbers of all encoding genes used are listed in table 1 fig. phylogenetic analysis of pax proteins. maximum-likelihood consensus tree with bootstrap replicates constructed from the alignment shown in fig. 2a. note that acr-pax2/5/ ntu-pax2/5/ and ino-pax2/5/ cluster with the predicted proteins of their bilaterian pax2/5/ orthologs. pax protein families are labeled in different colors pax2/5/ expression in the polyplacophoran acanthochitona crinita the gastrula stage is reached between 3– h post fertilization at 20 °c. during this stage acr-pax2/5/ is expressed in ectodermal cells of the apical region. early trochophore larvae hatch at approximately hpf. they are lecithotrophic and are slightly ovoid in shape, with an apical organ and apical tuft in the episphere. the apical organ comprises three serotonin-like immunoreactive flask-shaped cells and faint fmrfamide-like immunoreactivity. two other groups comprising four fmrfamide-like immunoreactive cells each are located in the episphere close to the prototroch. the episphere is separated from the hyposphere by two rows of trochoblasts that give rise to the prototroch. acr-pax2/5/ is expressed in different domains, here referred to as “groups”, located mainly in the larval episphere. several acr-pax2/5/8-expressing cells are located in the ectoderm of the dorsal episphere adjacent to both rows of trochoblasts. in addition, two other groups are located bilaterally and anterior to group. each group comprises approximately four ectodermal cells that are located subepidermally and project dendrites through the epidermis. two single acr-pax2/5/8-expressing cells are located bilaterally, immediately adjacent to the two rows of trochoblasts and more ventrally to group in the episphere. in the center of the ventral episphere, a group of 5– ectodermal acr-pax2/5/8-expressing cells is present. the only acr-pax2/5/ expression domain that is located in the hyposphere of the early trochophore is situated in the region of the nascent shell fields in the dorsal hyposphere. the apical organ does not express acr-pax2/5/. in further developed trochophore larvae, acr-pax2/5/8-expression is less pronounced in the episphere, while acr-pax2/5/ expression in the hyposphere gains in intensity. this and other acr-pax2/5/8-expressing cell groups are located in similar expression domains as described for earlier developmental stages. acr-pax2/5/8-expressing cells are present in the expression domains of groups and of previous stages. furthermore, serotonin-like immunoreactive cell somata are located in corresponding regions, such as the lateroventral ampullary cells corresponding to acr-pax2/5/8-expressing group or the additional sensory cells that are located in the dorsal episphere that correspond to group. in the expression domains of acr-pax2/5/8-expressing groups and fmrfamide-like immunoreactive cell somata are located. in further developed trochophore larvae, the apical organ does not exhibit acr-pax2/5/ expression, however, very close to it, acr-pax2/5/8-expressing cells are located. in metamorphic competent trochophore larvae, the acr-pax2/5/ expression pattern is similar to the one of earlier trochophores. strong acr-pax2/5/ expression is present in cells of the nascent shell fields. these acr-pax2/5/8-expressing cells are located in the epidermis and project dendrites to the periphery. in addition, acr-pax2/5/8-expressing cells are numerous in groups 1– during earlier development. interestingly, all these acr-pax2/5/ expression domains house fmrfamide-like-immunoreactive cells.fig. expression of acr-pax2/5/ in early larvae of the polyplacophoran acanthochitona crinita. dorsal –ventral, apical -abapical, and left -right axes indicate the orientation. blastopores/mouth labeled by asterisks and the location of the apical organ is encircled. a-b consecutive optical sections through a gastrula showing acr-pax2/5/8-expressing cells ). c confocal reflection scan highlighting same acr-pax2/5/8-expressing cells in apical ectoderm as shown in a-b d three serotonin-like immunoreactive neurons of the apical organ in a hpf early trochophore. e-h optical sections from dorsal to ventral through an early trochophore showing acr-pax2/5/8-expressing domains 1– in the episphere and single acr-pax2/5/8-expressing cells in the hyposphere ). e upper inset: several acr-pax2/5/8-expressing cells are located in the ectoderm of the dorsal episphere adjacent to the prototroch. lower inset: this very early trochophore exhibits faint fmrfamide-like immunoreactive signal in the region of the apical organ and in two groups of four fmrfamide-like immunoreactive ectodermal cells each in the episphere close to the prototroch. f two other bilateral groups of acr-pax2/5/8-expressing cells are located anterior to group and additional acr-pax2/5/8-expressing cells lie in the region of the nascent shell fields in the hyposphere. g two acr-pax2/5/8-expressing ectodermal cells are located bilaterally in the episphere adjacent to the trochoblasts. h a group of 5– ectodermal acr-pax2/5/8-expressing cells lies in the central ventral episphere. i-k same specimen as shown in e-h from the left side to the mid-sagittal plane. l apical view of same specimen as shown in e-k with acr-pax2/5/ expression domains and abbreviations: bc, blastocoel; f, foot; pt, prototroch. scale bars: 20 μm fig. sketch drawings of pax2/5/ expression in acanthochitona crinita, nucula tumidula, and idiosepius notoides. dorsal –ventral, apical -abapical, and anterior -posterior axes indicate the orientation. mouth is marked by asterisk in all panels. pax2/5/8-expressing cell groups are indicated by red numbers in early trochophore larvae of acanthochitona crinita. additional pax2/5/8-expressing cells are located in the hyposphere of further developed larvae. a pericalymma larva and a settled individual of nucula tumidula express pax2/5/ in the mantle. note that pax2/5/ is not expressed in the test cells that constitute the outermost cell layer. a stage individual and a stage 24– individual of idiosepius notoides express pax2/5/ in various domains. red dashed line indicates trajectory of esophagus and internal yolk. abbreviations: an, anus; at, apical tuft, ey, eye; f, foot; fn, funnel; g, gill; ib, interbasal lobe; m, mantle; ms, middle subesophageal mass; s, shell; sc, statocyst; sg, shell gland; se, supraesophageal mass; stm, stomach; ps, posterior subesophageal mass; pt, prototroch; y, yolk. scale bars: 50 μm fig. expression of acr-pax2/5/ during late larval development and in settled individuals of acanthochitona crinita. dorsal –ventral, apical -abapical, anterior -posterior, and left -right axes indicate the orientation, the red dashed circles the apical organ, and the asterisks label the mouth. cell nuclei are labeled in blue, cilia in green in b & right inset of j). a-d acr-pax2/5/ is expressed in ectodermal cells of the episphere, the shell fields ), the dorso-lateral and ventral episphere, and close to the apical organ in hpf old trochophores to ventral ). b a further developed, hpf old, trochophore and a slightly younger specimen with serotonin-like immunoreactive latero-ventral ampullary cells, additional sensory cells, apical organ cells, and cells in the ventral hyposphere. e-h in competent trochophores to ventral ), acr-pax2/5/ is expressed in cells of the shell fields, in cells of group and, and in two groups of cells along the developing foot, which are connected via a slender bridge of cells. inset in g acr-pax2/5/8-expressing cells close to the apical organ. i-k lateral view of trochophore shown in. j dashed box marks location of the small inset that depicts a confocal reflection scan of acr-pax2/5/8-expressing subepidermal flask-shaped cells in the shell fields. right large inset: fmrfamide-like immunoreactive cell somata in the shell fields and close to the trochoblasts. k acr-pax2/5/ expression in the posterior mantle fold. l no acr-pax2/5/ expression was observed in postmetamorphic specimens. the staining of the perinotum is unspecific. abbreviations: at, apical tuft; pt, prototroch. scale bars: 20 μm 10 μm) an additional acr-pax2/5/ expression domain is present close to the lateral borders of the foot. both groups are interconnected via a thin band of acr-pax2/5/8-expressing cells that runs along the abapical side of the mouth. few acr-pax2/5/8-expressing cell are present in the posterior mantle fold, while no acr-pax2/5/8-expressing cells are located in the region of the apical organ. settled animals commence to reduce their episphere and the foot becomes more pronounced. metamorphosis is largely completed with the shedding of the prototroch, although the eighth shell plate is not formed until probably several weeks after settlement. early settled animals solely possess few faintly stained acr-pax2/5/8-expressing cells in the region of the shell fields and only non-specific staining was observed in the perinotum that surrounds the plates. pax2/5/ expression in the bivalve nucula tumidula embryos of n. tumidula reach the 2– cell stage by hpf at  °c and the zygotes gastrulate by approximately hpf. during gastrulation, ntu-pax2/5/ is expressed in approximately five ectodermal cells close to the blastopore. early pericalymma larvae swim by approximately hpf, are roundish, and possess 5– ntu-pax2/5/8–expressing cells on the future dorsal side below the test-cells. slightly further developed pericalymmae are characterized by an apical organ with a ciliary tuft, while mouth and anus are located on the abapical side. they are covered with large, multiciliary calymma cells. subsequently, three rows of test cells bear three bands of longer cilia, while shorter cilia still cover the area around the apical tuft as well as the abapical side around mouth and anus. these larvae express ntu-pax2/5/ in the area of the dorsal mantle and the expression domain also includes the shell gland that develops as an invagination after gastrulation on the future dorsal side. during subsequent development the apical-abapical axis elongates and the foot develops on the ventral side. meanwhile, the shell field on the dorsal side has secreted two shell valves. further developed larvae express ntu-pax2/5/ in more ventral and apical mantle regions. late pericalymma larvae express ntu-pax2/5/ along the dorsal mantle region as well as in ventral domains of the mantle. larvae that are metamorphic competent are more ovoid in shape than earlier larvae and express ntu-pax2/5/ mostly in dorsal domains but also in ventral ones. lecithotrophic larvae swim until 15– dpf in the water column and finally settle. during the settlement process that may last for several hours, larvae appear to probe the substrate and metamorphosis commences with shedding and ingestion of the test cells and the apical organ. larvae express ntu-pax2/5/ mainly bilaterally in two postero-ventral domains of the central mantle. faint expression is still visible on the dorsal side close to the hinge of the developing valves. in individuals that have lost their test cells, ntu-pax2/5/ is expressed in the posteroventral region of the mantle and along the mantle margin. early juveniles express ntu-pax2/5/ in lateral portions of the ventral mantle.fig. expression of ntu-pax2/5/ during larval development of the bivalve nucula tumidula. dorsal –ventral, apical -abapical, and left -right axes indicate the orientation. blastopore and mouth is marked with an asterisk and the position of the apical organ is encircled. a ntu-pax2/5/ expression in approximately five cells close to the blastopore of a 24 h post fertilization old gastrula. inset: sketch drawing of ntu-pax2/5/ expression in a gastrula. b-c 5– cells express ntu-pax2/5/ below the test cells and close to the mouth of the early pericalymma larva. d confocal reflection and autofluorescence scan of a single optical section of an early larva ). e a further developed specimen with ntu-pax2/5/ expression in the mantle. f ntu-pax2/5/ is expressed in the mantle including the shell gland. g confocal reflection and autofluorescence scan of a single optical section of the same specimen as shown in. h this further developed larva expresses ntu-pax2/5/ bilaterally along the dorsal and apical mantle. i late larvae express ntu-pax2/5/ along the dorsal and ventral mantle. j dpf old larva of the same stage as the one shown in i with stronger ntu-pax2/5/ expression in the dorsal and ventral mantle. k same specimen as shown in j with ntu-pax2/5/ expression in the dorsal and ventral mantle but not the foot. l sketch drawing summarizing the ntu-pax2/5/ expression domains in a metamorphic competent pericalymma larva before settlement. abbreviations: an, anus; at, apical tuft; e, esophagus; f, foot; pc, perivisceral cavity; tc, test cell; s, shell; stm, stomach. scale bars: 30 μm fig. expression of ntu-pax2/5/ during postlarval development of the bivalve nucula tumidula. dorsal –ventral, anterior -posterior, and left -right axes indicate the orientation. a ntu-pax2/5/ expression in the mantle of a dpf old larva during onset of metamorphosis. ntu-pax2/5/ expression is restricted to the mantle margin and a bilateral expression domain located in the postero-ventral central region but not the foot. faint expression is still visible on the dorsal side of the animal close to the hinge. note the test cells being ingested by this specimen. b confocal reflection and autofluorescence scan of a single optical section of the specimen shown in. all test cells and the apical organ are shed and engulfed during metamorphosis. c ntu-pax2/5/ expression in a settled dpf old individual that has engulfed all test cells. note the ntu-pax2/5/ expression along the mantle margin. inset: ventral view of a similar staged settled animal. d confocal reflection and autofluorescence scan of a single optical section of the same dpf old specimen as shown in. a different confocal plane of the boxed area is shown in the inset highlighting test cells in the stomach of the settled larva. e ntu-pax2/5/ expression along the ventral mantle margin in a further developed specimen with no test cells visible in the stomach. note the lack of ntu-pax2/5/ expression along the dorsal mantle area close to the hinge. f posterior view of the same specimen as shown in e. scale bars: 50 μm pax2/5/ expression in the cephalopod idiosepius notoides as a typical cephalopod, i. notoides is a direct developer with discoidal cleavage. hatching of the juvenile occurs within 9– days after oviposition at 25 °c. in the present study, we adhere to the common designation of embryological orientation in cephalopods, i.e., the mantle apex is considered dorsal and the cephalic region including the brachial crown that surrounds the mouth is considered ventral. the funnel is located posteriorly, while the opposite side including the supraesophageal mass is considered anterior. anlagen of the cns, the arms, the funnel, and the mantle are discernable from stage onward. early stage individuals of the pygmy squid i. notoides express ino-pax2/5/ in the mantle, the funnel, and all ten arms. faint ino-pax2/5/8-expression is present in the developing gills of stage individuals. during subsequent development, ino-pax2/5/ expression is located in the same domains as in stage individuals. ino-pax2/5/ is additionally expressed in the tissue that interconnects the arms and in nervous tissue of the optic ganglia. stage individuals strongly express ino-pax2/5/ in all arms, the funnel, eyes, and the epidermis of the cephalopedal region. additionally, ino-pax2/5/8-expressing cells are present in the region of the interbasal lobes that both belong to the posterior basal lobes of the brain. during stages 24– ino-pax2/5/ is also expressed in the eyes. stage individuals still express ino-pax2/5/ in the gills and in large areas of the epidermis of the cephalopedal region. ino-pax2/5/ expression is only located in the proximal portions of all arms and the proximal portion of the funnel. ino-pax2/5/ expression domains in the cns include the anterior basal and the interbasal lobes that both belong to the supraesophageal mass. the vertical lobe complex as highest integration center does not express ino-pax2/5/. ino-pax2/5/ is also faintly expressed in the posterior subesophageal mass. the posterior perikaryal layer of the middle subesophageal mass also expresses ino-pax2/5/. in late prehatching developmental stages such as stage ino-pax2/5/ is expressed in the gills, in the posterior perikaryal layer of the middle subesophageal mass, and around the buccal mass. ino-pax2/5/ expression in the interbasal lobes persists in stage individuals. faint ino-pax2/5/ expression is still present at the base of the funnel.fig. expression of ino-pax2/5/ during early prehatching development of the cephalopod idiosepius notoides. dorsal –ventral, anterior -posterior, and left -right axes indicate the orientation. a ino-pax2/5/ is expressed in the mantle, the funnel, and all five arm pairs of stage individuals. note the faint ino-pax2/5/ expression in the gills. same specimen as shown in with ino-pax2/5/ expression in the 2-5th arm pair. c lateral view of same specimen as shown in and exhibiting expression of ino-pax2/5/ in all five arm pairs. d stage individuals show ino-pax2/5/ expression in the mantle, the fifth arm pair, and the funnel. note the faint ino-pax2/5/ expression in the gills. e ino-pax2/5/ expression in portions of the optic ganglia close to the eyes in the same specimen as shown in f same specimen as shown in with ino-pax2/5/ expression in the cerebral ganglia. g lateral view of the same specimen as shown in inset shows detail of ino-pax2/5/ expression in the arms. abbreviations: y, yolk sac. scale bars: 150 μm fig. expression of ino-pax2/5/ during late prehatching development of the cephalopod idiosepius notoides. dorsal –ventral, anterior -posterior, and left -right axes indicate the orientation. yolk sac removed in arms are labeled with numbers. a stage individuals express ino-pax2/5/ in all arms, the eyes, the epidermis of the cephalopedal region, the interbasal lobes, the posterior basal lobes, and the anterior basal lobes. note the unspecific staining around the shell gland. b same stage individual as shown in note ino-pax2/5/ expression in the proximal optic lobes, the eyes, and the funnel. c ino-pax2/5/ is expressed in smaller portions of the body of stage individuals compared to earlier stage individuals. ino-pax2/5/ is expressed in the gills, the eyes, and in the proximal but not the distal arm portions. note ino-pax2/5/ expression in the interbasal lobes. d the interbasal lobes and anterior basal lobes of the supraesophageal mass express ino-pax2/5/ the posterior perikaryal layer of the middle subesophageal mass as well as the posterior subesophageal mass express ino-pax2/5/ ino-pax2/5/ expression could not be observed in the distal funnel of this stage individual. please note the unspecific staining around the statocyst. e-h in stage individuals, ino-pax2/5/ is restricted to few expression domains such as the gills, the posterior perikaryal layer of the middle subesophageal mass, the interbasal lobes, tissue around the buccal mass, and the base of the funnel. trajectory of esophagus is indicated by red dashed line in abbreviations: bk, beak; is, ink sac; iy, internal yolk; se, supraesophageal mass. scale bars: 150 μm; e-h: 200 μm discussion evolution of pax2/5/ proteins our phylogenetic analysis confirms the identities of all three molluscan pax2/5/ proteins presented herein. the sequences of acanthochitona crinita, idiosepius notoides, and nucula tumidula exhibit a paired domain, an octapeptide, the lysine-arginine rich region, as well as the partial homeodomain diagnostic for pax2/5/ pax2/5/ is expressed in the development of multimodal sensory systems of mollusks among bilaterians, pax2/5/ expression is particularly well-investigated in the deuterostomia and ecdysozoa. studies on various deuterostomes revealed that orthologs of pax2/5/ play a role during development of auditory/ geosensory organs. interestingly, the phylogenetically distantly related gastropod mollusk haliotis asinina also expresses pax2/5/ in its geosensory organ, the statocysts. this present study demonstrated that homeobox genes may be recruited into the development of organs with similar function but of different evolutionary origin. it also poses the question as to how pax2/5/ is recruited during the ontogeny of organisms without auditory and equilibrial sense. in our study, we included the chiton acanthochitona crinita and the bivalve nucula tumidula, both lophotrochozoan representatives without larval statocysts, in order to assess the role of pax2/5/ during their development. pax2/5/ is expressed in several groups of ectodermal cells in the episphere and the hyposphere of the polyplacophoran trochophore. although a. crinita and n. tumidula possess larval apical organs, no pax2/5/ expression was detected in components of this organ. in a. crinita cells of groups 2– are located in a similar location as cells of the larval ampullary sensory system that is an autapomorphy of polyplacophora. cells of the ampullary system are flask-shaped and probably chemosensory rather than mechanosensory. pax2/5/ expression and fmrfamide-like immunoreactivity may be co-localized in ectodermal cells in the dorsal episphere adjacent to the prototroch. further pax2/5/8-expressing ectodermal cells are located in the ventral episphere and might correspond to the four serotonin-like immunoreactive flask-shaped cells. the immunochemical and molecular phenotype, their flask-shaped somata, and their subepithelial location argue for a sensory role of the above-mentioned cells. this might also be true for the pax2/5/8-expressing groups that are located on the ventral side along the borders of the foot of advanced trochophore larvae. both groups are connected to each other via a small band along the abapical side of the mouth/ apical side of the foot. this expression domain also includes a region where the “schwabe organ”, a putative chemosensory organ of a polyplacophoran ingroup, is located. although the chitonid a. crinita has not been studied with respect to the presence of this organ, it is likely that it is not present in a. crinita since other congeneric species of the chitonida lack this sensory structure. pax2/5/ expressing cells are also located in the posterior mantle fold that exhibits the chemosensory osphradia of adult chitons. the majority of these cells is located in central regions of each shell field and exhibits processes that penetrate the epidermis and the nascent calcareous layers of the shell plates. these processes exhibit fmrfamide-like immunoreactivity in earlier developmental stages than the associated cell somata. judging by their location and morphology, these pax2/5/8-expressing and fmrfamide-like immunoreactive cells are probably esthetes. esthetes are primarily photoreceptive organs that constitute a polyplacophoran autoapomorphy and penetrate the surface of the shell plates of adult specimens. during ontogeny, esthetes may first be identified by the narrow holes of their canals in shell plates of metamorphosed specimens. in the pygmy squid, pax2/5/ is expressed in various organs that are known to be equipped with sensory cells. during a short developmental time frame, ino-pax2/5/ is expressed in the eyes, resembling the situation of the adult gastropod haliotis asinina. the brachial crown, i.e. all five arm pairs, expresses pax2/5/ in epidermal cells until stage. in stage individuals, pax2/5/ expression is restricted to the proximal part of each arm/tentacle. expression fades in subsequent developmental stages. cephalopod arms and tentacles house a plethora of sensory cells. although cell proliferation experiments during cephalopod arm growth are still lacking, it is tempting to speculate that pax2/5/ is expressed in ectodermal sensory cells in regions of high proliferation activity, such as the proximal portion of the brachial crown in stage specimens. the funnel shows a similar pattern as the brachial crown. during early development the entire funnel expresses pax2/5/ while only the proximal portion expresses pax2/5/ in subsequent stages until it fades entirely in late prehatching stages. here, pax2/5/ appears to be expressed in the epidermis as it has also been reported for vertebrates. along the lateral sides of the foot the future gills develop and pax2/5/ expression predates their development. expression of pax2/5/ in the molluscan mantle pax2/5/ is expressed in different regions of the developing mantle of the aculiferan polyplacophoran acanthochitona crinita, the basal bivalve nucula tumidula, and the cephalopod idiosepius notoides. this corroborates data on the veliger larva of haliotis asinina, where pax2/5/ is expressed in the anterior region of the mantle that is known to be richly equipped with sensory cells. in contrast to the esthetes as distinct expression domain of a. crinita, pax2/5/ is rather broadly expressed in the mantle of n. tumidula and i. notoides. during earlier development of n. tumidula, pax2/5/ expression is, however, first restricted to the dorsal mantle epithelium, including the shell gland. in contrast, the shell gland of h. asinina does not exhibit pax2/5/ expression. shortly before metamorphosis, pax2/5/ expression shifts to the ventral side of the mantle and disappears in the dorsal region after metamorphosis. this may be explained by the onset of sensory cell development on the ventral side of the mantle that is exposed to water currents in the postmetamorphic, settled animal. other ectodermal expression domains comprise the cephalopedal epidermis of stage individuals of i. notoides. pax2/5/ plays a role in cephalopod brain regionalization studies on mouse and drosophila embryos revealed that several gene orthologs play a role in brain regionalization. the anterior-most brain regions of both organisms, i.e. the murine forebrain/midbrain as well as the fruit fly’s protocerebrum/deuterocerebrum, express otd/otx. the adjacent more posterior brain regions, that are patterned in a spatially collinear fashion by anterior hox gene orthologs, are separated from the anterior-most brain regions by a pax2/5/ expression domain. interestingly, a similar molecular fingerprint is manifested in the developing brain of the cephalopod idiosepius notoides and other cephalopods investigated. here, pax2/5/ expression is restricted to the cerebral ganglia during early development. in subsequent developmental stages transcripts are restricted to the anterior basal and the interbasal lobes that are located slightly anterior to the esophagus, thus dividing the cephalopod brain into a supraesophageal and a subesophageal mass. anterior portions of the posterior subesophageal mass that are located close to the esophagus also exhibit faint pax2/5/ expression. notably, brain lobes that are located more anteriorly, such as the median basal lobe or the vertical lobe complex, do not express pax2/5/ in i. notoides. in late-stage prehatching individuals, pax2/5/ is only expressed in the interbasal lobes as well as in the posterior perikaryal layer of the middle subesophageal mass.fig. pax2/5/ has been independently recruited in brain regionalization in various bilaterian subgroups. expression of otx, pax2/5/ and anterior hox genes during brain formation in a lophotrochozoan, an ecdysozoan, and a deuterostome. anterior-most brain regions face up and posterior-most brain regions face down. the cephalopod vertical lobe does not express otx, pax2/5/ or hox genes. the superior frontal, subpedunculate, and median basal lobes express otx in sepia officinalis and the anterior basal and interbasal lobes express pax2/5/ in idiosepius notoides. the posterior subesophageal mass and the posterior perikaryal layer of the middle subesophageal mass express pax2/5/ in i. notoides. anterior hox genes are expressed in the palliovisceral and pedal ganglia/ subesophageal mass of other cephalopods. sketch modified after otx is expressed in the supraesophageal mass of the cuttlefish sepia officinalis and the pygmy squid i. notoides. compared to pax2/5/ it is, however, expressed in more anterior regions of the supraesophageal mass; i.e., in components of the vertical lobe complex and the median basal lobe of the supraesophageal mass. in contrast to pax2/5/ expression, no otx expression was found in the subesophageal mass. in cephalopods, hox genes are expressed in the cns with exception of the cerebral ganglia/supraesophageal mass. although no collinear spatial expression is reported from cephalopods, the otx- and pax2/5/ expression patterns adhere to the overall pattern reported for drosophila and mouse. data on other bilaterians suggest a six expression domain in the anterior-most region of the cephalopod brain, i.e. the vertical lobe complex. surprisingly, pax2/5/ is not expressed in the developing nervous system of the polyplacophorans, bivalves, or gastropods investigated so far. in contrast to cephalopods, the fruit fly, or mouse none of the latter mollusks possess a highly centralized brain. pax2/5/ expression is restricted to the ectoderm in mollusks among lophotrochozoans, pax2/5/ expression during development has only been studied in mollusks. in all mollusks investigated so far, pax2/5/ is exclusively expressed in ectodermal domains such as the epidermis, the shell gland, the mantle, sensory cells, as well as the nervous system. considering that pax2/5/ is consistently expressed in ectodermal epithelia in all metazoans investigated so far, with solely the onychophora and the chordata expressing pax2/5/ in mesodermal domains such as the nephridia, the most parsimonious explanation is an independent recruitment of pax2/5/ in mesodermal epithelia development in these clades. in nematodes and cnidarians the pax2/5/ orthologs are involved in the formation of endodermal structures such as the hindgut or the endoderm at the aboral end of the pharynx. since the anterior-most and posterior-most portions of the digestive tract are located at the endodermal-ectodermal boundary, the pax2/5/ expression domains in the nematode hindgut and the aboral end of the pharynx of the cnidarian nematostella vectensis might be an evolutionary reminiscent of ancestral ectodermal expression domains in the ancestors of these taxa.fig. pax2/5/ expression domains during ontogeny of various metazoan representatives. a comparison among metazoan animals suggests that the ancestral role of pax2/5/ was probably restricted to ectodermal domains while chordates and onychophorans recruited orthologs into the development of mesodermal organ systems. the few data on lophotrochozoa suggest that pax2/5/ expression adheres to the ancestral state, i.e. patterning of certain ectodermal domains. in addition, they show that among mollusks pax2/5/ apparently lost its role in the formation of the nervous system with exception of the cephalopods, while the mantle including shell gland strongly express pax2/5/ data on pax2/5/ pax pax pax paxb-expression:;;;; present study;, present study;;;;. cladogram simplified after  the present study shows that pax2/5/ is expressed in multimodal sensory systems in mollusks such as the esthetes and the ampullary system of polyplacophorans, the eyes of squids, but not in the larval apical organ of a. crinita and n. tumidula. pax2/5/ expression probably predates sensory cell development during ontogenesis, since pax2/5/8-expressing cells are present in regions where the future sensory cells are situated. compared to other bilaterians, pax2/5/ is not expressed in the development of the less centralized nervous system of bivalves, gastropods, and polyplacophorans, and hence it most likely lost its role in brain development in these mollusks. interestingly, pax2/5/ is expressed along the trajectory of the esophagus that divides the cephalopod brain into a supraesophageal and a subesophageal mass. together with otx and hox genes pax2/5/ might have been recruited into brain regionalization, thus representing an extreme case of convergent evolution of gene function to the situation found in vertebrates and insects. since pax2/5/ is largely expressed in ectodermal domains throughout the bilateria, its ancestral role was most likely in the differentiation of this outer most germ layer of bilaterians. 
mollusks represent the largest lophotrochozoan phylum and exhibit highly diverse body plans. in mollusks, pax2/5/ is only expressed in derivatives of the ectoderm and hence an ancestral role in molluscan ectoderm differentiation is inferred. pax2/5/8-expressing cells are present in regions where the future sensory cells such as the polyplacophoran esthetes are situated and hence pax2/5/ expression probably predates sensory cell development during ontogeny. expression domains include esthetes and the ampullary system in polyplacophorans as well as the eyes of cephalopods. in addition, pax2/5/ is expressed in multimodal sensory systems in mollusks such as the esthetes and the ampullary system of polyplacophorans as well as the eyes of cephalopods. our results, along with those on otx- and hox-gene expression, demonstrate that the cephalopod condition is similar to that in mouse and fruit fly, with otx being expressed in the anterior-most brain region and a pax2/5/ expression domain separating the otx-domain from a hox-gene expressing posterior brain region. thus, pax2/5/ appears to have been recruited independently into regionalization of non-homologous complex brains of organisms as different as squid, fruit fly, and mouse. in contrast, pax2/5/ is expressed among others in brain lobes along the trajectory of the esophagus that divides the cephalopod brain. previous studies have demonstrated that transcription factors such as pax genes play important roles during their development. pax2/5/ is expressed in regions where sensory cells develop subsequently during ontogenesis.
 mollusks represent the largest lophotrochozoan phylum and exhibit highly diverse body plans. previous studies have demonstrated that transcription factors such as pax genes play important roles during their development. accordingly, in ecdysozoan and vertebrate model organisms, orthologs of pax2/5/ are among others involved in the formation of the midbrain/hindbrain boundary, the auditory/geosensory organ systems, and the excretory system. methods pax2/5/ expression was investigated by in situ hybridization during the development of representatives of the two major molluscan subclades, aculifera and conchifera.  compared to the investigated polyplacophoran and bivalve species that lack larval statocysts as geosensory organs and elaborate central nervous systems, cephalopods possess highly centralized brains and statocysts. pax2/5/ is expressed in regions where sensory cells develop subsequently during ontogenesis. expression domains include esthetes and the ampullary system in polyplacophorans as well as the eyes of cephalopods. no pax2/5/ expression was observed in the less centralized cns of bivalve, polyplacophoran, and gastropod embryos, thus arguing for a loss of pax2/5/ involvement in cns development in these lineages. in contrast, pax2/5/ is expressed among others in brain lobes along the trajectory of the esophagus that divides the cephalopod brain.  our results, along with those on otx- and hox-gene expression, demonstrate that the cephalopod condition is similar to that in mouse and fruit fly, with otx being expressed in the anterior-most brain region and a pax2/5/ expression domain separating the otx-domain from a hox-gene expressing posterior brain region. thus, pax2/5/ appears to have been recruited independently into regionalization of non-homologous complex brains of organisms as different as squid, fruit fly, and mouse. in addition, pax2/5/ is expressed in multimodal sensory systems in mollusks such as the esthetes and the ampullary system of polyplacophorans as well as the eyes of cephalopods. pax2/5/8-expressing cells are present in regions where the future sensory cells such as the polyplacophoran esthetes are situated and hence pax2/5/ expression probably predates sensory cell development during ontogeny. in mollusks, pax2/5/ is only expressed in derivatives of the ectoderm and hence an ancestral role in molluscan ectoderm differentiation is inferred. keywords bivalviacephalopodcomplexityevolutionhomeoboxhoxlophotrochozoaneurogenesissensory cellpolyplacophoraissue-copyright-statement© the author 2015 