Meeting Abstract

26.1  Wednesday, Jan. 5  Chelicerate tagmosis: inferred homeotic development in extinct taxa WOLFE, Joanna M.*; LEGG, David A.; Yale University; Imperial College London jo.wolfe@yale.edu

Arthropod tagmosis is the differentiation of segments along the anterior/posterior axis, producing appendages specialized for sensation, feeding, and locomotion. Characters relating to segment and limb number and morphology are important in constructing phylogenies that include fossil taxa. Chelicerates are a diverse group, including the extant spiders, mites, harvestmen, scorpions, xiphosurans, and the extinct trigonotarbids, eurypterids, chasmataspids, and synziphosurans. The extant pygnogonids (sea spiders) may also belong to this group. The chelicerate body is divided into two major tagmata: prosoma and opisthosoma. There is variation, however, within this bauplan (e.g. number of segments in a limb, fusion of dorsal tergites). Developmentally, patterns of tagmosis are associated with changes in the expression of Hox genes, segment polarity genes, and limb gap genes. The distribution of extant taxa for which Hox expression patterns are known is sparse. Therefore, we studied morphological traits for which the developmental basis is understood in extant chelicerates. We constructed a new phylogeny for chelicerates from over 400 morphological characters. Representative extant taxa with sequence or developmental gene expression data were included, as well as fossils with unique patterns of tagmosis or other morphological traits. Using this morphological topology, ancestral character states were inferred. Results under different models of character evolution (parsimony, likelihood, Bayesian) are compared.