Cell fates in leech embryos with duplicated lineages.

We have examined the fates of the progeny of supernumerary embryonic stem cells (O/P teloblasts) generated by microinjecting polyadenylic acid into newborn O/P teloblasts in embryos of the leech, Helobdella triserialis. In normal development, each O/P teloblast generates a rostrocaudal column of daughter cells (primary blast cells) that contribute distinct segmentally iterated O or P sets of epidermal and neural progeny to the mature leech. Previous results suggest that primary blast cells derived from ipsilateral pairs of O/P teloblasts are equipotent and equivalent at birth; that they and their progeny assume distinct O or P fates according to hierarchical and position-dependent interactions; and that the P fate is the primary, or default, fate and the O fate is the secondary fate. In the work presented here, one O/P teloblast was experimentally induced to undergo a supernumerary equal division, and the developmental fates of the progeny of the three (two "duplicate" and one "nonduplicate") ipsilateral O/P teloblasts were determined at stages 8 and 10. We find that some supernumerary O/P teloblasts produce supernumerary P progeny, whereas others generate supernumerary O progeny. When three O/P-derived bandlets are present, bandlets derived from the duplicate O/P teloblasts give rise to progeny of the same (O or P) fate. When the nonduplicate bandlet is absent, the duplicate bandlets assume distinct O and P fates. These results suggest that ipsilateral sister O/P teloblasts, while equipotent, might not be equivalent.