A Polychaete hunchbackOrtholog

We report the first characterization of a segmentation gene homologue in the basal polychaete Capitella capitatausing a pan-annelid cross-species antibody to the hunchback-like gene product. In flies, the gap segmentation gene hunchback(hb) encodes a C2H2zinc-finger transcription factor that plays a pivotal role in patterning the anterior region of the fly body plan. The hborthologue in Capitella(Cc-hb) is expressed maternally and in all micromere and macromere cells throughout cleavage. At gastrulation, nuclear Cc-hb protein is expressed in the micromere-derived surface epithelium that undergoes epiboly and in the large vegetal blastomeres that gradually become internalized. During organogensis, Cc-hb is expressed in the developing gut epithelium, the prostomial and pygidial epithelium, and in a subset of differentiated neurons in the adult central nervous system. Cc-hb is not expressed in the segmental precursor cells in the trunk. The Cc-hb expression domains in Capitellaare similar to those reported for the leech hborthologue (LZF2), and many of the observed differences between the annelid classes correlate with changes in life history. The lack of detectable annelid hb protein in the trunk at the time of AP pattern formation in leech and in polychaete suggests that the anterior organizing function of hbin flies originated in the arthropod or insect lineage.