Cell lineage analysis of the expression of an engrailed homolog in leech embryos.

ht-en is an engrailed-class gene that is expressed during early development and neurogenesis in embryos of the leech Helobdella triserialis. During the early development of this annelid (stages 7-9), ht-en is expressed in each of the ectodermal and mesodermal teloblast lineages that contributes progeny to the definitive segments. ht-en is expressed transiently by individually identified cells within the segmentally iterated primary blast cell clones. Its expression is correlated with the age of the primary blast cell clone. After consegmental primary blast cell clones from the different teloblast lineages have come into segmental register, cells that express ht-en during stages 7-9 are clearly confined to a transverse region corresponding to the posterior portion of the segmental anlage, but not all cells within this region express ht-en. Only a minority of the identified cells that express ht-en during terminal differentiation of the segmental ganglia and body wall (stages 10-11) are descendants of cells that express ht-en in early development (stages 7-9).