Comparative transcriptomics suggests a potential realizator gene for carapace expansion in longtail tadpole shrimp, Triops longicaudatus (Branchiopoda: Notostraca).

The origin of morphological innovation has been extensively studied within evolutionary developmental biology (evo-devo). Recent studies have demonstrated that the developmental module for double-layered epithelial outgrowths is conserved between the insect wings and branchiopod crustacean carapace, thereby introducing homology among these diverse structures. However, evo-devo studies on the branchiopod crustacean carapace have been primarily limited to a single species, the water flea Daphnia magna, leaving the gene regulatory network governing carapace development not comprehensively understood. Furthermore, realizator genes downstream of the character identity mechanism (ChIM) for bilayered epithelial development remain inadequately described. In this study, we analyzed tissue-specific transcriptional profiles in the developing longtail tadpole shrimp, Triops longicaudatus. We observed significant upregulation of papilin in the carapace-bearing head, along with its expression in both the carapace and the trunk limb lobes. Based on these results, we hypothesize that differential expression of papilin is involved in the disproportional growth of Triops carapace. Our findings will contribute to elucidating the diversification of double-layered epithelial outgrowths across distant arthropod lineages.