Hunchback-like protein is expressed in cleavage blastomeres, gastrula epithelium, and ciliary structures in...

We report the expression of Hunchback (Hb)-like protein during embryonic and larval development in two caenogastropods, Crepidula fornicata and Ilyanassa obsoleta. During the cleavage stages of these species, Hb-like protein is uniformly expressed in micromere and macromere nuclei. At gastrulation, gastropod Hb-like protein is expressed in the surface epithelium that undergoes epiboly. During organogenesis, gastropod Hb-like protein is expressed in the developing ciliated structures associated with feeding and locomotion. We find no detectable gradient or regionalization of Hb-like protein in gastropod embryos or larvae that resembles the graded Hb pattern of expression observed in dipteran insect embryos. Rather we found that the spatiotemporal expression profile of gastropod Hb-like protein is nearly identical to the Hb-like patterns obtained from the polychaete Capitella sp. I and is highly similar to those reported for clitellate annelids. Based upon the comparative data collected from both ecdysozoans and lophotrochozoan lineages, our results support the hypothesis that the role of Hb in anteroposterior patterning is a derived trait specific to arthropods, and that the ancestral function of lophotrochozoan Hb-like protein played a role in the formation of the cleavage-stage blastomeres and the gastrula epithelium and in structures associated with larval feeding and locomotion.