PhD Student
Advisor: Georg Brenneis

Unit for Integrative Zoology, Department of Evolutionary Biology
University of Vienna

Abstract

Hox genes are a family of highly conserved homeodomain transcription factors that play crucial roles in the regionalization of the developing anterior-posterior axis of most animals. This includes arthropods where they act as key regulators of segment identity specification. Due to this important developmental role, changes in their spatio-temporally staggered expression domains, their function and their regulation likely contributed to the incredible disparity of arthropod body plans. Accordingly, Hox genes have been in the focus of many comparative studies, yielding invaluable insights into arthropod body patterning and the homologization of segments and appendages across groups.

Sea spiders (Pycnogonida) are a marine chelicerate lineage that shows an unmistakable, considerably derived adult body plan. Uniquely among chelicerates, the pycnogonid prosoma bears a specialized ovigeral limb (used for egg-carrying) between the pedipalpal appendage and the first leg, thus encompassing one more prosomal segment than characteristic for other chelicerates. At the same time, pycnogonids lack a segmented opisthosoma; their last leg-bearing segment features instead only a small posterior anal tubercle. Despite these body plan peculiarities, sea spiders have hitherto been largely exempt from Hox gene expression surveys, not least due to challenges with obtaining developmental material.

To address this knowledge gap I investigated the spatio-temporal expression patterns of the nine identified Hox genes during postembryonic development of the sea spider Pycnogonum litorale. Special focus was placed on the ovigeral segment and the posterior body pole to compare the extent and specific combinations of Hox gene expression in these regions with other chelicerate taxa.