How to align arthropod legs

Abstract

AbstractHow to align leg segments between the four groups of arthropods (insects, crustaceans, myriapods, and chelicerates) has tantalized generations of researchers, as this would answer over a century of speculation about the origins and homologies of the fascinating diversity of arthropod appendages and outgrowths. Here we compare the expression and loss-of-function phenotypes of leg patterning genes in crustaceans, insects, and arachnids using our own and previously published data. We find that all arthropod leg segments correspond to each other in a one-to-one fashion. This alignment suggests that chelicerates with seven leg segments incorporated a proximal leg segment into the body wall. In addition, this alignment suggests that insect and myriapod tracheae are convergent and homologous structures: each evolved via the independent internalization of an ancestral gill (respiratory exite) on the proximal-most leg segment of their shared ancestor. A framework for understanding the homologies of arthropod appendages opens up a powerful system for studying the origins of novel structures, the plasticity of morphogenetic fields across vast phylogenetic distances, and the convergent evolution of shared ancestral developmental fields.