BACKGROUND
spermatogenesis is a model that facilitates studies of the effect of gene mutations on mitosis, meiosis and the remodeling of many cell structures.
during spermatogenesis, primordial germ cells undergo an oriented mitotic division to replace themselves and to produce spermatogonia .
each spermatogonium undergoes four rounds of mitotic division, generating  <dig> spermatogonia that subsequently differentiate into spermatocytes within a cyst.
since the cytokinesis of mitotic divisions is incomplete, the spermatogonia are connected by ring channels.
then, all  <dig> spermatocytes go through two rounds of meiotic divisions, resulting in a cyst of  <dig> haploid, round-shaped spermatids.
the meiotic cytokinesis is also incomplete so that the spermatids remain interconnected.
during the coalescence stage in early spermatids, the mitochondria aggregate to the side of the nucleus, where the centriole resides  <cit> .
by the onion stage of spermatid differentiation, a dramatic transformation of the mitochondrial mass occurs.
the individual mitochondria fuse into two giant mitochondria, which are arranged in a densely-packed sphere consisting of many layers of wrapped mitochondrial membranes  <cit> .
this onion-like structure is termed the nebenkern.
at the elongation stage, the flagellar axoneme elongates, resulting in a dramatic change in the shape of the spermatid  <cit> .
the two interlocked subunits of the nebenkern unfold and extend along with the growing axoneme.
despite the structural changes of the two mitochondrial derivatives, both mitochondrial subunits remain aligned and associated with axoneme.
as spermatids begin to elongate, their heads, containing nuclei, remain aligned toward the testis wall and their tails are turned aside toward the testicular apex.
simultaneously, the cyst slides down along the testis wall, changing its shape from a disc-like structure to a bundle of elongating spermatids with the nuclear regions of spermatids polarized toward the base of the testis  <cit> .
following the flagellar elongation, the spermatid nucleus transforms its shape from a spherical structure to a long, thin needle.
subsequently, the process of individualization is initiated by the formation of the individualization complex , containing the actin cones at the head region of the spermatid bundle  <cit> .
individualization occurs in a cystic bulge, progressing along the entire length of the spermatid bundle.
during individualization, membrane remodeling takes place, the channels connecting spermatids are destroyed, and syncytial organization of a cyst is lost  <cit> .
following coiling of the sperm bundle, mature sperm are released into the testis lumen and then pass into the seminal vesicle.
although the spermatogenesis process has been well defined, only a limited number of genes whose mutations affect this process has been described, and the role of their protein products are mostly unknown.
mer <dig>  a mutation  in the gene encoding drosophila merlin, whose ortholog in human is named the neurofibromatosis  <dig>  gene  <cit> , leads to male sterility  <cit> .
male flies with a mer <dig> allele are viable but sterile; however, the cause of sterility is unknown.
merlin shares a high degree of homology to the ezrin, radixin, and moesin  proteins, which belong to the protein  <dig>  superfamily, linking the actin cytoskeleton to the plasma membrane  <cit> .
interaction of the erm proteins with the actin cytoskeleton is thought to be important for the determination of the cell-shape, cell adhesion, cell motility, cytokinesis, and intracellular signaling  <cit> .
in addition to its interaction with the actin cytoskeleton, merlin can bind to microtubules and regulate the microtubule cytoskeleton  <cit> .
studies in mammalian cells show that merlin mediates contact inhibition of proliferation  <cit> .
merlin inactivation leads to tumor formation in several cell types in mammals  <cit> .
merlin is evolutionally conserved  <cit> .
the merlin homolog in drosophila melanogaster shows extensive sequence homology to the human protein  <cit> .
this similarity between the fly and human merlin proteins extends over the entire amino acid sequences with the greatest similarity in the amino terminus of the ferm domain   <cit> .
the homology between the fly and human merlin proteins also exists in the carboxyl terminus, a region in which merlin diverges from the erm-family members  <cit> .
the drosophila merlin gene  is located at the 18d-e region of the x chromosome.
d-mer has been shown to regulate cell proliferation and survival through the hippo signaling pathway  <cit> .
in addition, merlin promotes endocytosis of several membrane signaling receptors  <cit> .
furthermore, d-mer is non-autonomously required to maintain polarity of posterior follicle cells in the oocyte and to limit their proliferation  <cit> .
to better understand the cause of sterility in merlin mutant flies, we carried out an extensive analysis of cellular events in spermatogenesis.
we showed that the merlin protein was concentrated in the mitochondrial derivatives, and that merlin mutations affected meiosis, cyst polarization, nuclear shaping, and axoneme-nebenkern association.
RESULTS
the sterility phenotype in merlin mutants is rescued by the introduction of a wild-type merlin gene
the mer <dig> allele carries a missense mutation , and male flies hemizygous for mer <dig> are viable but sterile  <cit> .
upon examination of the testis, we noted that seminal vesicles from the mer <dig> males were smaller than those from the control fm <dig> flies .
in addition, very few sperm were found in the mer <dig> seminal vesicles, and they were immotile, in contrast to those seen in the control fm <dig> siblings obtained from the same cross.
acetic acid/orcein staining showed that the mer <dig> testis had fewer sperm heads in each bundle than the control testis .
also, the sperm in the bundle were arranged in a more disorganized fashion.
previously, lajeunesse et al.
<cit>  demonstrated that ectopic expression of a mer+ or mer <dig> transgene using a ubiquitously-expressed gal <dig> driver  in a mer <dig>  background rescued mer <dig> lethality.
in addition, insertion of a cosmid construct , carrying the entire d-mer gene, was capable of rescuing various mer mutations.
we conducted a similar experiment to test whether a mer+ transgene could rescue the sterility of the mer <dig> allele.
table  <dig> shows that males carrying both mer <dig> and p{cosmer+} were viable and fertile.
to ensure germline expression of the mer+ or mer3transgene, we cloned the mer+ and mer <dig> sequences into the puasp vector, and used them to transform embryos.
we showed that both puasp-mer+ and puasp-mer <dig> could rescue the lethality of mer <dig> mutation when ectopically activated by the act5c-gal <dig> driver .
however, only mer+ over-expression restored the fertility of flies with the mer <dig> mutation.
the merlin protein is dynamically redistributed during meiosis of spermatocytes and is concentrated in the nebenkern of spermatids
to understand the cause of sterility in the merlin mutant flies, we studied the subcellular localization of the merlin protein in the control fm <dig> and mer <dig> testes at various stages of spermatogenesis.
in the cysts from the control testis, merlin expression was detected in the cellular cortex of spermatocytes , as seen in somatic tissues and earlier germ cells  <cit> .
this cortical localization became more pronounced in spermatocytes during prometaphase and metaphase of meiosis .
merlin was found to redistribute, and more intense staining was observed in the area associated with the microtubules of the central spindle in telophase .
during cytokinesis, intense merlin staining remained associated with the microtubules but the intensity was less in the region of the contractile ring .
a similar merlin distribution pattern was seen during the second meiotic division .
at the onion stage, merlin was concentrated in the nebenkern, a specialized structure formed by the fusion of mitochondria during spermatid differentiation .
this intense merlin staining in the nebenkern remained throughout the comet stage of spermatid elongation, during which the nebenkern split into two parts .
note that concentrated merlin immunoreactivity was clearly seen in the two subunits of the nebenkern in the spermatid .
in the control cyst containing mature sperm, merlin staining continued to be present in the elongated nebenkern .
in addition, merlin was seen as a bright punctate dot in the acrosomal region, a golgi apparatus-derived structure developed over the anterior part of the sperm's head.
we also performed a similar immunostaining on the mer <dig> testis.
although we could detect merlin staining in the mer <dig> cyst at the comet stage, the mer <dig> spermatid nuclei were scattered throughout the cyst, and the arrangement of spermatids appeared irregular .
the ability of the antibody to detect merlin staining in the mer <dig> cyst suggests that the missense mutation in mer <dig> did not affect antibody recognition.
using the same antibody, lajeunesse  <cit>  previously detected a similar cortical localization of merlin in both the wild-type and mer <dig> imaginal discs.
however, the mer <dig> mutation clearly affects merlin function as the spermatids in the mer <dig> cyst were poorly arranged.
merlin mutations affect meiotic cytokinesis of spermatocytes, cyst polarization and nuclear shaping during spermatid elongation, and spermatid individualization
next, we examined if there were any abnormalities during early steps of spermatogenesis in the mer <dig> testis.
we dissected testes from both the control and mer <dig> mutant flies.
following staining with acetic acid/orcein, the testes were squashed and examined according to ashburner  <cit> .
although we did not find any abnormalities during mitosis of spermatogonia or spermatocyte growth, we observed three types of abnormalities during meiosis of spermatocytes from the mer <dig> testis, as compared with the control testis.
the first type of abnormality is shown in figure 3a, demonstrating a spermatid containing two nuclei of equal size and two nebenkerns.
this type of abnormality was likely caused by cytokinesis failure during the second meiotic division.
the second type of abnormality is tripolar spindle in a spermatocyte going through the second meiotic division .
this result suggests incomplete cytokinesis in the previous meiotic division.
the third type of abnormality is four-polar spindle in a spermatocyte undergoing the second meiotic division .
note that a secondary mer <dig> spermatocyte contained two pairs of telophase nuclei .
each pair of nuclei was situated with its own spindle, and two spindles shared a common mid zone .
this is in contrast with wild-type spermatocytes in telophase showing two daughter nuclei separate from each other along the central spindle .
the four-polar spindle represents another case of abnormal cytokinesis in the first meiotic division.
it should be mentioned that we detected the first type of abnormality in about 5% of the mutant cysts, while the second and third types of abnormalities appeared less frequent.
following meiosis, spermatid elongation ensues  <cit> .
prior to spermatid elongation, spermatid nuclei group at a defined area of the cyst wall in a process referred to as cyst polarization  <cit> .
we noted that, at this stage, spermatid nuclei were detected as a group in the control cyst .
intriguingly, spermatid nuclei in the mer <dig> cyst were grouped in two locations , and in some other cysts, nuclei appeared more scattered .
we also found a similar abnormality in nuclear grouping in the mer <dig> cyst.
although mer <dig> is larva-lethal  <cit> , we isolated some rare hemizygous mer <dig> male pupae from the y mer4/binsn stock, and a few of them were able to grow to pharates adults.
when examining testis preparations from these mer <dig> males, we detected spermatid nuclei arranged in two groups  or in a scattered manner in all of the mutant cysts .
also, the cyst polarization defects were more frequently seen in mer <dig>  than mer <dig>  cysts.
these results indicate that merlin mutations affect cyst polarization.
the final stage of spermatogenesis is the process of individualization, followed by sperm coiling  <cit> .
the individualization process is initiated at the head of the spermatid cyst, and involves the formation and movement of the actin cones from the head region of the spermatid bundle to the caudal end  <cit> .
analogous to the previous finding, we detected the actin cones, which moved caudally as a bundle in the control cyst .
note that the sperm heads were grouped in one end of the control cyst, consistent to that seen at the cyst polarization stage .
also, the sperm heads became needle-shaped.
however, we observed that the sperm nuclei in the mer <dig> cyst appeared scattered  and had variable morphology; some were round, while others were needle shape .
unlike the control cyst, the mer <dig> cyst had the actin cones located at multiple sites .
dispersed actin cones together with scattered nuclei were also found in the mer <dig> cyst .
merlin mutant cysts display abnormalities in nebenkern-axoneme association
since we detected intense merlin staining in the nebenkern at the onion stage, we employed electron microscopy to further examine nebenkern transformation from the structure containing two mitochondrial derivatives at the late stage of spermatid elongation into a configuration filled by electron dense material, called the paracrystalline body, at the end of the individualization stage  <cit> .
thin sections of testes from the control fm <dig>  mer <dig>  and mer <dig> males were analyzed under a transmission electron microscope.
as shown in figure 6a, we observed that at the late elongation stage, each spermatid in the control cyst contained one major and one minor mitochondrial derivative associated with one axoneme.
a paracrystalline body could be seen within the major mitochondrial derivative.
however, we found that some of the spermatids in the mer <dig> cyst contained two paracrystalline bodies within the major mitochondrial derivative .
also, some spermatids had two axonemes.
similarly, two paracrystalline bodies within the major mitochondrial derivative were frequently seen in the elongating spermatids of the mer <dig> cyst .
it should be mentioned that, unlike the spermatids in the control cyst, which displayed cell-cell contact, the spermatids in the mer <dig> cyst were loosely arranged , and those in the mer <dig> cyst were grossly disorganized .
in addition, some cytoplasmic shedding was present in the mer <dig> cysts , and excessive amount of cytoplasmic fragments was seen in the mer <dig> cyst .
at the individualization stage, each of the  <dig> spermatids in the control cyst contained one axoneme associated with the major and minor mitochondrial derivatives .
furthermore, the paracrystalline body almost filled the entire major mitochondrial derivative.
while some spermatids in the mer <dig> cyst at a similar developmental stage had a paracrystalline body-filled major mitochondrial derivative with axoneme, others containing two paracrystalline body-filled mitochondrial derivatives with abnormal shape or three paracrystalline body-filled mitochondrial derivatives together with one axoneme were also seen .
in addition, spermatids having one or two paracrystalline-filled mitochondrial derivatives but lacking axoneme were observed.
also, the spermatids remained loosely contacted with each other in the mer <dig> cyst.
the paired arrangement of axoneme with the mitochondrial derivatives was often lost in the spermatids of the mer <dig> cyst at the individualization stage .
spermatids with multiple paracrystalline body-filled mitochondrial derivatives but without axoneme, or with two axonemes, were found.
also, cytoplasmic fragmentation together with condensed cytoplasmic remnants and gigantic cytoplasmic bodies were also seen in the mer <dig> cyst.
consistent with previous observations  <cit> , we noted that mature spermatids in the control cyst had a substantial reduction in the amount of cytoplasm and a significant reduction in the size of the minor mitochondrial derivative at the end of the individualization stage.
the major mitochondrial derivative is filled by a dark-staining paracrystalline material .
each of the  <dig> individualized spermatids in the control cyst displayed a highly ordered axoneme-nebenkern pair.
in contrast, a gross disorganization in the arrangement of the individualized spermatids in the mer <dig> cyst was detected .
although some spermatids exhibited the axoneme-nebenkern pair, others appeared to be fused together or connected by a thin cytoplasmic extension.
in addition, the boundary between the cysts was not evident, and each cystic area contained less than  <dig> spermatids.
the most dramatic alteration was observed in the mer <dig> cyst .
although axoneme and nebenkern could be found, very little cytoplasmic material was seen, and the cell-cell boundary could not be easily identified.
insert in figure 6i illustrates that the structure of axoneme appeared to be intact.
despite this dramatic alteration, the 9+ <dig> microtubule-containing structure of axoneme appeared to be preserved in the merlin mutant cysts, indicating that merlin is not required for axoneme formation and elongation.
taken together, our results show that merlin mutations affect cytokinesis, cyst polarization, nuclear shaping, and spermatid individualization.
the observation that merlin is highly concentrated in the nebenkern suggests that merlin may pay a role in mitochondria formation and function during various stages of spermatogenesis.
discussion
spermatogenesis is a complicated developmental process, including mitosis, meiosis, cell shape changes, and remodeling of subcellular organelles from the nucleus to mitochondria  <cit> .
all of these events appear to involve cytoskeleton reorganization.
merlin has been shown to interact with the actin cytoskeleton and participate in the regulation of cell proliferation, cell adhesion, cell motility, and intracellular signaling  <cit> .
merlin can also interact with microtubules and regulate microtubule cytoskeleton  <cit> .
in addition, an interaction between merlin and the myosin heavy chain has been reported  <cit> .
consistent with findings that merlin interacts with key components of the cytoskeleton, our results showed that merlin mutations affected meiotic cytokinesis, cyst polarization, nuclear shaping, and spermatid individualization during spermatogenesis.
we also showed that the sterility phenotype of hemizygous male merlin mutants could be rescued by the introduction of a wild-type merlin gene.
the first abnormality that we found in the merlin mutants during spermatogenesis is cytokinesis failure during meiosis of spermatocytes.
cytokinesis is the process of dividing the cytoplasm and separating two daughter cells.
it involves the formation of a contractile ring and the central spindle, two interdependent structures that cooperatively interact throughout the process  <cit> .
the drosophila contractile ring is comprised of actin, non-muscle myosin ii, the regulatory light chain of myosin ii, and anillin.
intriguingly, we observed that the merlin expression pattern was closely associated with the microtubules of the spindle during cytokinesis.
given the fact that merlin interacts with microtubule, actin, and myosin heavy chain, it is possible that merlin may be involved in the assembly of the spindle and membrane addition.
further colocalization analysis will be needed to better understand the role of merlin in this meiotic process.
studies in mammalian cells indicate that merlin functions both as a growth and tumor suppressor  <cit> .
in drosophila, tumor suppressors often regulate cell proliferation in a tissue-specific manner  <cit> .
two genes previously reported to have tumor suppressor property in the male gonad are bam and bcgn.
mutations in these genes result in a large number of cells resembling those in early germline stages  <cit> .
mosaic analysis in the eye tissue has revealed that merlin mutant clones over-proliferate relative to normal sister clones; however, no tumors were found in the homozygous merlin mutant tissue  <cit> .
this result indicates that merlin mutations belong to the class of overgrowth mutations.
upon careful examination of the mer <dig> and mer <dig> testis tissues, we also did not find any tumors.
cyst polarization involves nuclear migration to a defined area of the cyst wall  <cit> .
although specific details about cyst polarization are not understood, it is envisaged that nuclear migration requires the participation of the actin cytoskeleton.
since merlin interacts and modulates the actin cytoskeleton and other cytoskeletal apparatus, merlin mutations could affect nuclear migration and lead to spermatid nuclei in two disorganized groups or more scattered in the mutant cysts as we have observed.
alternatively, the presence of merlin in the nebenkern suggests that merlin may play a role in cyst polarization through nebenkern.
nevertheless, merlin represents the first gene whose mutation affects cyst polarization in spermatogenesis.
previously, in a genetic screen for genes functioning in embryonic axis specification, macdougall et al.
<cit>  found that gurken  mrna localization is altered in mer mutant embryos.
normally, grk signal instructs about  <dig> follicle cells to adopt a posterior fate.
in turn, the posterior follicle cells send a polarizing signal back to the oocyte.
consequently, it induces the reorganization of oocyte microtubules, determining the localization of different mrna and oocyte nuclear migration in the oocyte.
despite a broad expression pattern of merlin in the egg chamber  <cit> , merlin appears to be specifically required non-autonomously only in a small group of follicular cells to maintain the polarity of posterior follicle cells and to limit their proliferation  <cit> .
this finding is consistent with our data showing merlin also has a role in cyst polarization during the spermatid pre-elongation period.
it should be mentioned that merlin mutations also affect nuclear shaping during spermatid elongation as we observed a few spermatids with a round but not needle-shaped head in the mutant cysts.
recent studies  <cit>  have suggested a possible contribution of acrosome to nuclear shaping because defective acrosome development leads to round-headed sperm in mice.
the biogenesis of the acrosome, a derivative of the golgi complex important for sperm-egg penetration, requires the formation of the transient microtubule-containing manchette caudally to the acrosome  <cit> .
in addition, the assembly of an f-actin-containing cytoskeletal plate, called acroplaxome, serves as an anchor for the developing acrosome to the nuclear envelope  <cit> .
given the well-established relationship between merlin and the actin filament or microtubule, merlin mutations may affect any of the cytoskeleton-mediated structures required for acrosome formation.
it is interesting to note that we have detected intense merlin staining in the acrosomal region, suggesting a possible role of merlin in this organelle.
actin is a major cytoskeletal component of the ic, and individualization is accomplished by the assembly of the cytoskeletal-membrane complex at the nuclear end of the cyst  <cit> .
our results showing scattered nuclei and dispersed actin cones found in the merlin mutant cysts are consistent with the idea that spermatid nuclei provide a physical scaffolding for the assembly of the ic.
intriguingly, several drosophila mutants with scattered nuclei, including chc <dig> , scat <dig> , cbx <dig> , ecr <dig> , also display the dispersed ic phenotype  <cit> .
the gene clathrin heavy chain has been shown to participate in a number of biological processes, including receptor-mediated endocytosis, neurotransmitter secretion, and sperm individualization.
the gene scattered is involved in golgi-to-vacuole transport, retrograde transport from endosome to golgi, and spermatid individualization.
the gene crossbronx, encoding a ubiquitin-protein ligase, is important for the ubiquitin cycle and spermatid individualization.
the gene ecdysone receptor, whose protein product responds to hormone stimuli, is essential for embryonic development and organogenesis, including spermatid development.
curiously, merlin has been shown to promote endocytosis of several signaling receptors  <cit> .
however, how this set of genes is connected to the individualization process remains to be determined.
spermatid individualization involves membrane remodeling and the outcome of this process is endowing each spermatid with its own plasma membrane and simultaneously removing most of the syncytial cytoplasm from between sperm tails as it proceeds  <cit> .
at present, we do not know how merlin mutations lead to excessive cytoplasmic remnants and poorly organized spermatids at the end of individualization.
although merlin may have a role in membrane remodeling as previously suggested, it is possible that spermatogenesis is such an orchestrated process that perturbation in each stage results in specific abnormalities, which could subsequently affect the following events.
merlin has been shown to localize underneath the plasma membrane at cell-cell junctions and other actin-rich sites  <cit> .
the detection of a high concentration of merlin protein in the nebenkern at the onion stage and its maintenance throughout mature sperm formation imply a unique role of merlin in mitochondria formation and function.
the presence of two or multiple paracrystalline bodies in the major mitochondrial derivative of merlin mutant spermatids could result from merlin dysfunction, leading to such an abnormal nebenkern structure.
a similar abnormality has also been observed in the mutants defective in the gene fuzzy onion , encoding a gtpase  <cit>  or rotund , coding for a rac gtpase activating protein  <cit> ).
however, it is not known whether merlin function links to these signaling molecules in regulating nebenkern formation.
one fundamental function that mitochondria provide is the production of atp, which serves as an energy source.
the nebenkern structure is pivotal to sperm motility.
although rare sperm could still be found in the mer <dig> testis, it is likely that without normal merlin function, sperm motility is impaired.
it should be mentioned that merlin is not the only growth suppressor whose loss results in male sterility.
like merlin, the tumor suppressor for lung cancer  <dig>  protein, an immunoglobulin superfamily molecule predominantly expressed in the brain, lung and testis, plays important roles in cell adhesion and tumor invasion in mammals  <cit> .
interestingly, tslc1-deficient mice also produce round spermatids and are sterile.
thus, it will be important to see whether merlin deficiency in the testis has any effect on the fertility of mice.
CONCLUSIONS
drosophila merlin mutant flies are viable but sterile, and the sterility phenotype is rescued by the introduction of a wild-type merlin gene.
merlin mutations affect meiotic cytokinesis of spermatocytes, cyst polarization and nuclear shaping during spermatid elongation, and spermatid individualization.
the merlin protein is enriched in the nebenkern and this mitochondrial localization is maintained throughout sperm formation.
these results suggest a role of merlin in mitochondria formation and function during various stages of spermatogenesis.
further investigation of the action of merlin in mitochondria is warranted.

 spermatogenesis is a model that facilitates studies of the effect of gene mutations on mitosis, meiosis and the remodeling of many cell structures. during spermatogenesis, primordial germ cells undergo an oriented mitotic division to replace themselves and to produce spermatogonia. each spermatogonium undergoes four rounds of mitotic division, generating spermatogonia that subsequently differentiate into spermatocytes within a cyst. since the cytokinesis of mitotic divisions is incomplete, the spermatogonia are connected by ring channels. then, all spermatocytes go through two rounds of meiotic divisions, resulting in a cyst of haploid, round-shaped spermatids. the meiotic cytokinesis is also incomplete so that the spermatids remain interconnected. during the coalescence stage in early spermatids, the mitochondria aggregate to the side of the nucleus, where the centriole resides. by the onion stage of spermatid differentiation, a dramatic transformation of the mitochondrial mass occurs. the individual mitochondria fuse into two giant mitochondria, which are arranged in a densely-packed sphere consisting of many layers of wrapped mitochondrial membranes. this onion-like structure is termed the nebenkern. at the elongation stage, the flagellar axoneme elongates, resulting in a dramatic change in the shape of the spermatid. the two interlocked subunits of the nebenkern unfold and extend along with the growing axoneme. despite the structural changes of the two mitochondrial derivatives, both mitochondrial subunits remain aligned and associated with axoneme. as spermatids begin to elongate, their heads, containing nuclei, remain aligned toward the testis wall and their tails are turned aside toward the testicular apex. simultaneously, the cyst slides down along the testis wall, changing its shape from a disc-like structure to a bundle of elongating spermatids with the nuclear regions of spermatids polarized toward the base of the testis. following the flagellar elongation, the spermatid nucleus transforms its shape from a spherical structure to a long, thin needle. subsequently, the process of individualization is initiated by the formation of the individualization complex, containing the actin cones at the head region of the spermatid bundle. individualization occurs in a cystic bulge, progressing along the entire length of the spermatid bundle. during individualization, membrane remodeling takes place, the channels connecting spermatids are destroyed, and syncytial organization of a cyst is lost. following coiling of the sperm bundle, mature sperm are released into the testis lumen and then pass into the seminal vesicle. although the spermatogenesis process has been well defined, only a limited number of genes whose mutations affect this process has been described, and the role of their protein products are mostly unknown. mer a mutation in the gene encoding drosophila merlin, whose ortholog in human is named the neurofibromatosis gene, leads to male sterility. male flies with a mer allele are viable but sterile; however, the cause of sterility is unknown. merlin shares a high degree of homology to the ezrin, radixin, and moesin proteins, which belong to the protein superfamily, linking the actin cytoskeleton to the plasma membrane. interaction of the erm proteins with the actin cytoskeleton is thought to be important for the determination of the cell-shape, cell adhesion, cell motility, cytokinesis, and intracellular signaling. in addition to its interaction with the actin cytoskeleton, merlin can bind to microtubules and regulate the microtubule cytoskeleton. studies in mammalian cells show that merlin mediates contact inhibition of proliferation. merlin inactivation leads to tumor formation in several cell types in mammals. merlin is evolutionally conserved. the merlin homolog in drosophila melanogaster shows extensive sequence homology to the human protein. this similarity between the fly and human merlin proteins extends over the entire amino acid sequences with the greatest similarity in the amino terminus of the ferm domain. the homology between the fly and human merlin proteins also exists in the carboxyl terminus, a region in which merlin diverges from the erm-family members. the drosophila merlin gene is located at the 18d-e region of the x chromosome. d-mer has been shown to regulate cell proliferation and survival through the hippo signaling pathway. in addition, merlin promotes endocytosis of several membrane signaling receptors. furthermore, d-mer is non-autonomously required to maintain polarity of posterior follicle cells in the oocyte and to limit their proliferation. to better understand the cause of sterility in merlin mutant flies, we carried out an extensive analysis of cellular events in spermatogenesis. we showed that the merlin protein was concentrated in the mitochondrial derivatives, and that merlin mutations affected meiosis, cyst polarization, nuclear shaping, and axoneme-nebenkern association.  the sterility phenotype in merlin mutants is rescued by the introduction of a wild-type merlin gene the mer allele carries a missense mutation, and male flies hemizygous for mer are viable but sterile. upon examination of the testis, we noted that seminal vesicles from the mer males were smaller than those from the control fm flies. in addition, very few sperm were found in the mer seminal vesicles, and they were immotile, in contrast to those seen in the control fm siblings obtained from the same cross. acetic acid/orcein staining showed that the mer testis had fewer sperm heads in each bundle than the control testis. also, the sperm in the bundle were arranged in a more disorganized fashion. previously, lajeunesse et al. demonstrated that ectopic expression of a mer+ or mer transgene using a ubiquitously-expressed gal driver in a mer background rescued mer lethality. in addition, insertion of a cosmid construct, carrying the entire d-mer gene, was capable of rescuing various mer mutations. we conducted a similar experiment to test whether a mer+ transgene could rescue the sterility of the mer allele. table shows that males carrying both mer and p{cosmer+} were viable and fertile. to ensure germline expression of the mer+ or mer3transgene, we cloned the mer+ and mer sequences into the puasp vector, and used them to transform embryos. we showed that both puasp-mer+ and puasp-mer could rescue the lethality of mer mutation when ectopically activated by the act5c-gal driver. however, only mer+ over-expression restored the fertility of flies with the mer mutation. the merlin protein is dynamically redistributed during meiosis of spermatocytes and is concentrated in the nebenkern of spermatids to understand the cause of sterility in the merlin mutant flies, we studied the subcellular localization of the merlin protein in the control fm and mer testes at various stages of spermatogenesis. in the cysts from the control testis, merlin expression was detected in the cellular cortex of spermatocytes, as seen in somatic tissues and earlier germ cells. this cortical localization became more pronounced in spermatocytes during prometaphase and metaphase of meiosis. merlin was found to redistribute, and more intense staining was observed in the area associated with the microtubules of the central spindle in telophase. during cytokinesis, intense merlin staining remained associated with the microtubules but the intensity was less in the region of the contractile ring. a similar merlin distribution pattern was seen during the second meiotic division. at the onion stage, merlin was concentrated in the nebenkern, a specialized structure formed by the fusion of mitochondria during spermatid differentiation. this intense merlin staining in the nebenkern remained throughout the comet stage of spermatid elongation, during which the nebenkern split into two parts. note that concentrated merlin immunoreactivity was clearly seen in the two subunits of the nebenkern in the spermatid. in the control cyst containing mature sperm, merlin staining continued to be present in the elongated nebenkern. in addition, merlin was seen as a bright punctate dot in the acrosomal region, a golgi apparatus-derived structure developed over the anterior part of the sperm's head. we also performed a similar immunostaining on the mer testis. although we could detect merlin staining in the mer cyst at the comet stage, the mer spermatid nuclei were scattered throughout the cyst, and the arrangement of spermatids appeared irregular. the ability of the antibody to detect merlin staining in the mer cyst suggests that the missense mutation in mer did not affect antibody recognition. using the same antibody, lajeunesse previously detected a similar cortical localization of merlin in both the wild-type and mer imaginal discs. however, the mer mutation clearly affects merlin function as the spermatids in the mer cyst were poorly arranged. merlin mutations affect meiotic cytokinesis of spermatocytes, cyst polarization and nuclear shaping during spermatid elongation, and spermatid individualization next, we examined if there were any abnormalities during early steps of spermatogenesis in the mer testis. we dissected testes from both the control and mer mutant flies. following staining with acetic acid/orcein, the testes were squashed and examined according to ashburner. although we did not find any abnormalities during mitosis of spermatogonia or spermatocyte growth, we observed three types of abnormalities during meiosis of spermatocytes from the mer testis, as compared with the control testis. the first type of abnormality is shown in figure 3a, demonstrating a spermatid containing two nuclei of equal size and two nebenkerns. this type of abnormality was likely caused by cytokinesis failure during the second meiotic division. the second type of abnormality is tripolar spindle in a spermatocyte going through the second meiotic division. this result suggests incomplete cytokinesis in the previous meiotic division. the third type of abnormality is four-polar spindle in a spermatocyte undergoing the second meiotic division. note that a secondary mer spermatocyte contained two pairs of telophase nuclei. each pair of nuclei was situated with its own spindle, and two spindles shared a common mid zone. this is in contrast with wild-type spermatocytes in telophase showing two daughter nuclei separate from each other along the central spindle. the four-polar spindle represents another case of abnormal cytokinesis in the first meiotic division. it should be mentioned that we detected the first type of abnormality in about 5% of the mutant cysts, while the second and third types of abnormalities appeared less frequent. following meiosis, spermatid elongation ensues. prior to spermatid elongation, spermatid nuclei group at a defined area of the cyst wall in a process referred to as cyst polarization. we noted that, at this stage, spermatid nuclei were detected as a group in the control cyst. intriguingly, spermatid nuclei in the mer cyst were grouped in two locations, and in some other cysts, nuclei appeared more scattered. we also found a similar abnormality in nuclear grouping in the mer cyst. although mer is larva-lethal, we isolated some rare hemizygous mer male pupae from the y mer4/binsn stock, and a few of them were able to grow to pharates adults. when examining testis preparations from these mer males, we detected spermatid nuclei arranged in two groups or in a scattered manner in all of the mutant cysts. also, the cyst polarization defects were more frequently seen in mer than mer cysts. these results indicate that merlin mutations affect cyst polarization. the final stage of spermatogenesis is the process of individualization, followed by sperm coiling. the individualization process is initiated at the head of the spermatid cyst, and involves the formation and movement of the actin cones from the head region of the spermatid bundle to the caudal end. analogous to the previous finding, we detected the actin cones, which moved caudally as a bundle in the control cyst. note that the sperm heads were grouped in one end of the control cyst, consistent to that seen at the cyst polarization stage. also, the sperm heads became needle-shaped. however, we observed that the sperm nuclei in the mer cyst appeared scattered and had variable morphology; some were round, while others were needle shape. unlike the control cyst, the mer cyst had the actin cones located at multiple sites. dispersed actin cones together with scattered nuclei were also found in the mer cyst. merlin mutant cysts display abnormalities in nebenkern-axoneme association since we detected intense merlin staining in the nebenkern at the onion stage, we employed electron microscopy to further examine nebenkern transformation from the structure containing two mitochondrial derivatives at the late stage of spermatid elongation into a configuration filled by electron dense material, called the paracrystalline body, at the end of the individualization stage. thin sections of testes from the control fm mer and mer males were analyzed under a transmission electron microscope. as shown in figure 6a, we observed that at the late elongation stage, each spermatid in the control cyst contained one major and one minor mitochondrial derivative associated with one axoneme. a paracrystalline body could be seen within the major mitochondrial derivative. however, we found that some of the spermatids in the mer cyst contained two paracrystalline bodies within the major mitochondrial derivative. also, some spermatids had two axonemes. similarly, two paracrystalline bodies within the major mitochondrial derivative were frequently seen in the elongating spermatids of the mer cyst. it should be mentioned that, unlike the spermatids in the control cyst, which displayed cell-cell contact, the spermatids in the mer cyst were loosely arranged, and those in the mer cyst were grossly disorganized. in addition, some cytoplasmic shedding was present in the mer cysts, and excessive amount of cytoplasmic fragments was seen in the mer cyst. at the individualization stage, each of the spermatids in the control cyst contained one axoneme associated with the major and minor mitochondrial derivatives. furthermore, the paracrystalline body almost filled the entire major mitochondrial derivative. while some spermatids in the mer cyst at a similar developmental stage had a paracrystalline body-filled major mitochondrial derivative with axoneme, others containing two paracrystalline body-filled mitochondrial derivatives with abnormal shape or three paracrystalline body-filled mitochondrial derivatives together with one axoneme were also seen. in addition, spermatids having one or two paracrystalline-filled mitochondrial derivatives but lacking axoneme were observed. also, the spermatids remained loosely contacted with each other in the mer cyst. the paired arrangement of axoneme with the mitochondrial derivatives was often lost in the spermatids of the mer cyst at the individualization stage. spermatids with multiple paracrystalline body-filled mitochondrial derivatives but without axoneme, or with two axonemes, were found. also, cytoplasmic fragmentation together with condensed cytoplasmic remnants and gigantic cytoplasmic bodies were also seen in the mer cyst. consistent with previous observations, we noted that mature spermatids in the control cyst had a substantial reduction in the amount of cytoplasm and a significant reduction in the size of the minor mitochondrial derivative at the end of the individualization stage. the major mitochondrial derivative is filled by a dark-staining paracrystalline material. each of the individualized spermatids in the control cyst displayed a highly ordered axoneme-nebenkern pair. in contrast, a gross disorganization in the arrangement of the individualized spermatids in the mer cyst was detected. although some spermatids exhibited the axoneme-nebenkern pair, others appeared to be fused together or connected by a thin cytoplasmic extension. in addition, the boundary between the cysts was not evident, and each cystic area contained less than spermatids. the most dramatic alteration was observed in the mer cyst. although axoneme and nebenkern could be found, very little cytoplasmic material was seen, and the cell-cell boundary could not be easily identified. insert in figure 6i illustrates that the structure of axoneme appeared to be intact. despite this dramatic alteration, the 9+ microtubule-containing structure of axoneme appeared to be preserved in the merlin mutant cysts, indicating that merlin is not required for axoneme formation and elongation. taken together, our results show that merlin mutations affect cytokinesis, cyst polarization, nuclear shaping, and spermatid individualization. the observation that merlin is highly concentrated in the nebenkern suggests that merlin may pay a role in mitochondria formation and function during various stages of spermatogenesis. discussion spermatogenesis is a complicated developmental process, including mitosis, meiosis, cell shape changes, and remodeling of subcellular organelles from the nucleus to mitochondria. all of these events appear to involve cytoskeleton reorganization. merlin has been shown to interact with the actin cytoskeleton and participate in the regulation of cell proliferation, cell adhesion, cell motility, and intracellular signaling. merlin can also interact with microtubules and regulate microtubule cytoskeleton. in addition, an interaction between merlin and the myosin heavy chain has been reported. consistent with findings that merlin interacts with key components of the cytoskeleton, our results showed that merlin mutations affected meiotic cytokinesis, cyst polarization, nuclear shaping, and spermatid individualization during spermatogenesis. we also showed that the sterility phenotype of hemizygous male merlin mutants could be rescued by the introduction of a wild-type merlin gene. the first abnormality that we found in the merlin mutants during spermatogenesis is cytokinesis failure during meiosis of spermatocytes. cytokinesis is the process of dividing the cytoplasm and separating two daughter cells. it involves the formation of a contractile ring and the central spindle, two interdependent structures that cooperatively interact throughout the process. the drosophila contractile ring is comprised of actin, non-muscle myosin ii, the regulatory light chain of myosin ii, and anillin. intriguingly, we observed that the merlin expression pattern was closely associated with the microtubules of the spindle during cytokinesis. given the fact that merlin interacts with microtubule, actin, and myosin heavy chain, it is possible that merlin may be involved in the assembly of the spindle and membrane addition. further colocalization analysis will be needed to better understand the role of merlin in this meiotic process. studies in mammalian cells indicate that merlin functions both as a growth and tumor suppressor. in drosophila, tumor suppressors often regulate cell proliferation in a tissue-specific manner. two genes previously reported to have tumor suppressor property in the male gonad are bam and bcgn. mutations in these genes result in a large number of cells resembling those in early germline stages. mosaic analysis in the eye tissue has revealed that merlin mutant clones over-proliferate relative to normal sister clones; however, no tumors were found in the homozygous merlin mutant tissue. this result indicates that merlin mutations belong to the class of overgrowth mutations. upon careful examination of the mer and mer testis tissues, we also did not find any tumors. cyst polarization involves nuclear migration to a defined area of the cyst wall. although specific details about cyst polarization are not understood, it is envisaged that nuclear migration requires the participation of the actin cytoskeleton. since merlin interacts and modulates the actin cytoskeleton and other cytoskeletal apparatus, merlin mutations could affect nuclear migration and lead to spermatid nuclei in two disorganized groups or more scattered in the mutant cysts as we have observed. alternatively, the presence of merlin in the nebenkern suggests that merlin may play a role in cyst polarization through nebenkern. nevertheless, merlin represents the first gene whose mutation affects cyst polarization in spermatogenesis. previously, in a genetic screen for genes functioning in embryonic axis specification, macdougall et al. found that gurken mrna localization is altered in mer mutant embryos. normally, grk signal instructs about follicle cells to adopt a posterior fate. in turn, the posterior follicle cells send a polarizing signal back to the oocyte. consequently, it induces the reorganization of oocyte microtubules, determining the localization of different mrna and oocyte nuclear migration in the oocyte. despite a broad expression pattern of merlin in the egg chamber, merlin appears to be specifically required non-autonomously only in a small group of follicular cells to maintain the polarity of posterior follicle cells and to limit their proliferation. this finding is consistent with our data showing merlin also has a role in cyst polarization during the spermatid pre-elongation period. it should be mentioned that merlin mutations also affect nuclear shaping during spermatid elongation as we observed a few spermatids with a round but not needle-shaped head in the mutant cysts. recent studies have suggested a possible contribution of acrosome to nuclear shaping because defective acrosome development leads to round-headed sperm in mice. the biogenesis of the acrosome, a derivative of the golgi complex important for sperm-egg penetration, requires the formation of the transient microtubule-containing manchette caudally to the acrosome. in addition, the assembly of an f-actin-containing cytoskeletal plate, called acroplaxome, serves as an anchor for the developing acrosome to the nuclear envelope. given the well-established relationship between merlin and the actin filament or microtubule, merlin mutations may affect any of the cytoskeleton-mediated structures required for acrosome formation. it is interesting to note that we have detected intense merlin staining in the acrosomal region, suggesting a possible role of merlin in this organelle. actin is a major cytoskeletal component of the ic, and individualization is accomplished by the assembly of the cytoskeletal-membrane complex at the nuclear end of the cyst. our results showing scattered nuclei and dispersed actin cones found in the merlin mutant cysts are consistent with the idea that spermatid nuclei provide a physical scaffolding for the assembly of the ic. intriguingly, several drosophila mutants with scattered nuclei, including chc, scat, cbx, ecr, also display the dispersed ic phenotype. the gene clathrin heavy chain has been shown to participate in a number of biological processes, including receptor-mediated endocytosis, neurotransmitter secretion, and sperm individualization. the gene scattered is involved in golgi-to-vacuole transport, retrograde transport from endosome to golgi, and spermatid individualization. the gene crossbronx, encoding a ubiquitin-protein ligase, is important for the ubiquitin cycle and spermatid individualization. the gene ecdysone receptor, whose protein product responds to hormone stimuli, is essential for embryonic development and organogenesis, including spermatid development. curiously, merlin has been shown to promote endocytosis of several signaling receptors. however, how this set of genes is connected to the individualization process remains to be determined. spermatid individualization involves membrane remodeling and the outcome of this process is endowing each spermatid with its own plasma membrane and simultaneously removing most of the syncytial cytoplasm from between sperm tails as it proceeds. at present, we do not know how merlin mutations lead to excessive cytoplasmic remnants and poorly organized spermatids at the end of individualization. although merlin may have a role in membrane remodeling as previously suggested, it is possible that spermatogenesis is such an orchestrated process that perturbation in each stage results in specific abnormalities, which could subsequently affect the following events. merlin has been shown to localize underneath the plasma membrane at cell-cell junctions and other actin-rich sites. the detection of a high concentration of merlin protein in the nebenkern at the onion stage and its maintenance throughout mature sperm formation imply a unique role of merlin in mitochondria formation and function. the presence of two or multiple paracrystalline bodies in the major mitochondrial derivative of merlin mutant spermatids could result from merlin dysfunction, leading to such an abnormal nebenkern structure. a similar abnormality has also been observed in the mutants defective in the gene fuzzy onion, encoding a gtpase or rotund, coding for a rac gtpase activating protein ). however, it is not known whether merlin function links to these signaling molecules in regulating nebenkern formation. one fundamental function that mitochondria provide is the production of atp, which serves as an energy source. the nebenkern structure is pivotal to sperm motility. although rare sperm could still be found in the mer testis, it is likely that without normal merlin function, sperm motility is impaired. it should be mentioned that merlin is not the only growth suppressor whose loss results in male sterility. like merlin, the tumor suppressor for lung cancer protein, an immunoglobulin superfamily molecule predominantly expressed in the brain, lung and testis, plays important roles in cell adhesion and tumor invasion in mammals. interestingly, tslc1-deficient mice also produce round spermatids and are sterile. thus, it will be important to see whether merlin deficiency in the testis has any effect on the fertility of mice.  drosophila merlin mutant flies are viable but sterile, and the sterility phenotype is rescued by the introduction of a wild-type merlin gene. merlin mutations affect meiotic cytokinesis of spermatocytes, cyst polarization and nuclear shaping during spermatid elongation, and spermatid individualization. the merlin protein is enriched in the nebenkern and this mitochondrial localization is maintained throughout sperm formation. these results suggest a role of merlin in mitochondria formation and function during various stages of spermatogenesis. further investigation of the action of merlin in mitochondria is warranted. 
by cytological and electron microscopy analyses of the mer mer, and control testes at various stages of spermatogenesis, we show that merlin mutations affect meiotic cytokinesis of spermatocytes, cyst polarization and nuclear shaping during spermatid elongation, and spermatid individualization. also, merlin exhibits punctate staining in the acrosomal region of mature sperm. our results demonstrated for the first time the mitochondrial localization of merlin and suggest that merlin may play a role in mitochondria formation and function during spermatogenesis. at the onion stage, merlin is concentrated in the nebenkern of spermatids, and this mitochondrial localization is maintained throughout sperm formation. drosophila merlin, the homolog of the human neurofibromatosis gene, is important for the regulation of cell proliferation and receptor endocytosis. male flies carrying a mer allele, a missense mutation in the merlin gene, are viable but sterile; however, the cause of sterility is unknown. immunostaining demonstrates that the merlin protein is redistributed to the area associated with the microtubules of the central spindle in telophase and its staining is less in the region of the contractile ring during meiotic cytokinesis. the merlin protein is dynamically redistributed during meiosis of spermatocytes and is concentrated in the nebenkern of spermatids. we also demonstrate that the lethality and sterility phenotype of the mer mutant is rescued by the introduction of a wild-type merlin gene. merlin mutations affect spermatogenesis at multiple stages.
 drosophila merlin, the homolog of the human neurofibromatosis gene, is important for the regulation of cell proliferation and receptor endocytosis. male flies carrying a mer allele, a missense mutation in the merlin gene, are viable but sterile; however, the cause of sterility is unknown.  testis examination reveals that hemizygous mer mutant males have small seminal vesicles that contain only a few immotile sperm. by cytological and electron microscopy analyses of the mer mer, and control testes at various stages of spermatogenesis, we show that merlin mutations affect meiotic cytokinesis of spermatocytes, cyst polarization and nuclear shaping during spermatid elongation, and spermatid individualization. we also demonstrate that the lethality and sterility phenotype of the mer mutant is rescued by the introduction of a wild-type merlin gene. immunostaining demonstrates that the merlin protein is redistributed to the area associated with the microtubules of the central spindle in telophase and its staining is less in the region of the contractile ring during meiotic cytokinesis. at the onion stage, merlin is concentrated in the nebenkern of spermatids, and this mitochondrial localization is maintained throughout sperm formation. also, merlin exhibits punctate staining in the acrosomal region of mature sperm.  merlin mutations affect spermatogenesis at multiple stages. the merlin protein is dynamically redistributed during meiosis of spermatocytes and is concentrated in the nebenkern of spermatids. our results demonstrated for the first time the mitochondrial localization of merlin and suggest that merlin may play a role in mitochondria formation and function during spermatogenesis. 