17Proc. Arthropod. Embryol. Soc. Jpn. 54, 17–18 (2022)Ⓒ 2022 Arthropodan Embryological Society of Japan
ISSN 1341–1527

Molecular Evolutionary Biology of Carapace Formationin Crustaceans＊
Yasuhiro SHIGA
School of Life Sciences, Tokyo University of Pharmacy and Life Sciences, 1432–1, Horinouti, Hachioji, Tokyo 19–0392,
Japan
E-mail: yshiga@toyaku.ac.jp

More than 60,000 species belonging to thesubphylum Crustacea are known to date and, in many
species, a characteristic flat structure consisting oftwo epithelial cell layers is formed on the dorsal side.
These flat structures, called carapaces, show high levelsof morphological variation and are known to havevarious functions, depending on subgroups, including
provision of hydrodynamic advantages, protection of
the body, formation of a feeding chamber, a respirationchamber, and a brood (egg) chamber (Olesen, 2013).
In 1909, a Scottish zoologist William ThomasCalman first suggested that the carapace is a primitive
structure in the crustacean lineage (Calman, 1909).
Since then, the evolutionary origin of the carapace has
been the subject of extensive study and debate, however,extensive morphological diversity in the dorsal part ofcrustacean head (cephalothorax) prevents conclusive
answer from morphological comparisons among
lineages. Several questions on the origin of the carapace,including whether the carapace is a primitive structurewith a single origin or whether it had evolved multiple
times, from which segment(s) it originated, andwhether it is related to structures in the body of otherarthropods, remains unresolved for over a century(Fryer, 1996; Olesen, 2013). The EvoDevo studies
may provide good opportunities to address thesequestions, by exploring the molecular mechanisms that
underlie the development of crustacean carapaces andexamining how these mechanisms evolved.
Fossil records suggest that the bivalved carapacesof crustaceans (and crustaceanomorphs) have evolvedin the early Cambrian, approximately 525 million
years ago (Xian-guang et al., 2004), long before theemergence of the insect wings. The water flea Daphnia
magna Straus, 1820 (Branchiopoda: Anomopoda) has abivalved carapace which consists of a double epidermal
cellular layer decorated with bristles on its margin.
＊These morphological features are also found in thewings of insects, leading us to hypothesize that some of
the regulatory genes that play key roles in patterning theinsect wings may have similar roles in the margin of theDaphnia carapace.
To test this hypothesis, full length cDNAs for D.
magna orthologs of a TEA-domain transcriptional
factor scalloped (sd) and a transcriptional cofactors
vestigial (vg), both often considered to be the mastergenes of insect wings, and an Wnt-family secretory
signaling factor wingless (wg) were cloned, and theirexpressions during embryogenesis were analyzed.
Double immunostaining and in situ hybridization
revealed that the expression domains of Wg, Vg and Sd
fully overlap in developing Daphnia carapace marginand co-expression of these was not observed in otherparts of the embryo. RNAi knockdown experiments
demonstrated that Sd and Wg play essential roles incarapace outgrowth. The sole Hox protein expressed indeveloping Daphnia carapace was Sex combs reduced(Scr) and CRISPR/Cas9-mediated Scr gene knockout
resulted in severely reduced carapace, suggestingstrongly that the Daphnia carapace is derived from thefirst and second maxillary segments (Mx1 and 2).
Taken together, our data suggest that Daphnia
carapace is formed in Scr-expressing domains (Mx1and 2 segments) and is folded at the marginal Wg/Vg/
Sd co-expressing cells before extending posteriorly.
If Calmanʼs hypothesis on evolution of crustacean
carapaces is the case, then one can expect the existenceof similar genetic mechanism for carapace developmentin other modern crustaceans. To address this, my grouphas extended our research to study on other crustacean
species, including the American tadpole shrimp
Triops longicaudatus (LeConte, 1846) (Branchiopoda:Notostraca), the American brine shrimp Artemia
franciscana Kellogg, 1906 (Branchiopoda: Anostraca),Contribution to the symposium “Acquisition of Functional Diversity of Insects” in the 57th Annual Meeting of the Arthropodan EmbryologicalSociety of Japan, July 9–10, 2021, Ushiku-numa, Ibaraki, Japan.

18

and the red swamp crawfish Procambarus clarkii(Girard, 1852) (Malacostraca: Decapoda).
References
Calman, W.T. (1909) Crustacea. In E.R. Lankester (ed.), A
Treatise on Zoology. Adam and Charles Black, London.
Fryer, G. (1996) The carapace of the branchiopod Crustacea.
Philosophical Transactions of the Royal Society of London
B, Biological Sciences, 351, 1703–1712.

Olesen, J. (2013) The crustacean carapace: Morphology,function, development, and phylogenetic history. In L.
Watling and M. Thiel (eds.), Functional Morphology andDiversity, pp. 103–139. Oxford University Press, Oxford.
Xian-guang, H., R.J. Aldridge, J. Bergström, D.J. Siveter and F.
Xiang-Hong (2004) The Cambrian Fossils of Chengjiang,
China: The Flowering of Early Animal Life. Blackwell
Publishing, Oxford.