Segmental expression of an engrailed-class gene during early development and neurogenesis in an annelid.

ht-en protein, an annelid homolog of the Drosophila engrailed protein, is expressed during both early development and neurogenesis in embryos of the leech, Helobdella triserialis. In Helobdella as in Drosophila, early expression is in segmentally iterated stripes of cells within the posterior portion of the segment and later expression is in cells of the segmental ganglia. These findings suggest that dual expression of an en-class gene was present in a common ancestor of annelids and arthropods.