Specification of ectodermal teloblast lineages in embryos of the oligochaete annelid Tubifex: involvement of novel cell-cell interactions.

In embryos of clitellate annelids (i.e. oligochaetes and leeches), four ectodermal teloblasts (ectoteloblasts N, O, P and Q) are generated on either side through a stereotyped sequence of cell divisions of a proteloblast, NOPQ. The four ectoteloblasts assume distinct fates and produce bandlets of smaller progeny cells, which join together to form an ectodermal germ band. The pattern of the germ band, with respect to the ventrodorsal order of the bandlets, has been highly preserved in clitellate annelids. We show that specification of ectoteloblast lineages in the oligochaete annelid Tubifex involves cell interaction networks distinct from those in leeches. Cell ablation experiments have shown that fates of teloblasts N, P and Q in Tubifex embryos are determined rigidly as early as their birth. In contrast, the O teloblast and its progeny are initially pluripotent and their fate becomes restricted to the O fate through an inductive signal emanating from the P lineage. In the absence of this signal, the O lineage assumes the P fate. These results differ significantly from those obtained in embryos of the leech Helobdella, suggesting the diversity of patterning mechanisms that give rise to germ bands with similar morphological pattern.