Molting in Pancrustacea Is Characterized by Both Deeply Conserved and Recently Evolved Gene Modules.

Arthropods such as insects and crustaceans, which together form the monophyletic group Pancrustacea, possess a rigid chitinous exoskeleton that must be periodically shed through molting to allow growth and morphological change. Although molting is a deeply conserved developmental process across Arthropoda, our understanding of its molecular mechanisms is still largely derived from insect model species. Lineage-specific innovations and losses of molting-related genes raise fundamental questions about the extent of its conservation outside noninsect arthropods. Here, we investigate the evolutionary conservation of molting gene expression across five representative pancrustacean species using publicly available transcriptomic datasets. Changes in gene expression during molting are characterized by both deeply conserved and lineage-specific gene modules. Temporal gene expression analyses reveal that these lineage-specific signatures are not uniformly distributed across the molting process: the middle transitional phase is more lineage-specific, thereby exhibiting an inverse hourglass pattern. This is likely due to life-history-specific processes, development of the cuticle, and specialized structures of the exoskeleton. Overall, this study provides evidence for both the evolutionary conservation and divergence of this key postembryonic developmental process and highlights the modular architecture of the molting program.