Evolutionary dynamics of the arthropod molting machinery.

Exoskeletons define arthropods, providing support for segmented bodies and appendages while protecting against environmental stress and predation. Although ubiquitous, this evolutionarily variable feature has enabled arthropods to occupy diverse lifestyles and ecological niches, contributing to their unrivaled diversity. Because the chitinous cuticle is rigid, exoskeletons must be periodically shed and replaced as animals grow. Arthropods therefore develop through discrete molts, with conserved phases that progress from pre-molt preparation to ecdysis and post-molt maturation. These are tightly regulated developmental transitions controlled by a molecular toolkit comprising neuropeptides, hormone-synthesizing enzymes, receptors, and the early, fate, and late gene sets that activate and execute the molting process. Although genetic studies in model species have identified many components, major knowledge gaps remain, especially in noninsect arthropods. Advances in genome sequencing now enable comparative genomic analyses across diverse, previously understudied arthropod lineages to begin to address these gaps. We present a comprehensive comparative genomic survey of arthropods, sampling all four subphyla: Chelicerata, Myriapoda, Crustacea, and Hexapoda. Orthology inference and gene copy-number analyses contrast stable and dynamic components of the molting machinery across the phylum. Ancestral state reconstructions and phylogenetic reconciliations reveal gene duplication and loss dynamics and the evolutionary histories of key molting gene families. The inclusion of newly generated myriapod genomes addresses a major taxonomic gap and enables inferences of gene repertoire changes in the Mandibulata ancestor. The broad taxonomic representation enables a phylum-wide assessment to systematically evaluate, refine, and revise current understanding of the evolutionary dynamics of the molting genetic toolkit.