BUDD, Graham E; ERIKSSON, B. Joakim; WENNBERG, Sofia; Uppsala University; Uppsala University; Uppsala University: The evolution of terminal patterning in arthropods

Living arthropods represent one of the three classically “eusegmented” clades, the others being annelids and vertebrates, but the relationship between the patterning of these groups is unclear. Such problems may be resolved in two ways; i) by tracing back relevant characters into the stem groups of the extant taxa; ii) by examining the character states in extant sister groups. The arthropod stem group remains controversial, but its broad outline at least is now widely accepted. Its most important feature is the development of segmentation in the epidermal ectoderm, a feature that appears to be lacking in extant onychophorans. This feature thus appears to be convergent between at least arthropods and the annelids. Evolution of terminal addition is less certain, because of the lack of definite arthropod embryonic (as opposed to larval) stages from the fossil record. Nevertheless, terminal addition appears to be the most likely basal state for the Arthropoda. The complex development of segmentation in the arthropod head can also be traced, although several controversial issues remain to be solved, including the evolution of the structure labelled as the “labrum” in various clades. The question of segmentation in the sister group to the arthropods, the Cycloneuralia (nematodes) is an interesting one that is currently being addressed. As well as the well-known serially repeated structures in taxa such as kinorhynchs and nematodes, developmental studies on a much wider range of these taxa are required before the evolution (or lack) of homologous segmental structures can be established in these mostly poorly-studied taxa. Answering this question would have a profound impact on our view of how deep the evolution of segmental structures, is rooted in metazoan phylogeny, and how their ontogenesis has evolved.