SAVAGE, R.M.; Williams College: The Divergent Roles of the Segmentation Gene hunchback in Annelids and Arthropods

My lab investigates the mechanisms responsible for the specification of the anteroposterior (AP) axis in annelids. With this objective in mind, we are characterizing orthologs to the Drosophila gap gene hunchback (hb) in each annelid class. The hb gene encodes a C2H2 zinc-finger transcription factor that plays a pivotal role in insect AP pattern formation. We have used a cross-species antibody to compare the expression patterns of hunchback-like protein in leeches (Helobdella), oligochaetes (Tubifex-with T. Shimizu at Hokkaido University, Japan), and polychaetes (Capitella). Hb-like protein is expressed in the micromeres (and its epithelial derivatives), the gut and the nervous system in all three classes, and the class-specific differences in expression correlate with changes in life history. Yet annelid hb orthologs are not expressed in the segmental precursor cells in the trunk at the time of AP pattern formation. The combination of the comparative expression data across annelid classes, a phylogenetic analysis and the evolution of structural domains within the hb protein support the idea that the anterior organizing function of hb in flies originated in the arthropod and/or insect lineage.