CELL FATE DETERMINATION IN AN ANNELID EQUIVALENCE GROUP

In embryos of glossiphoniid leeches such as Helobdella triserialis and H. robusta, pairs of adjacent 'o/p' ectodermal blast cells are known to be developmentally equipotent and yet eventually contribute distinc t sets of 'O' and 'P' progeny to the nervous system and epidermis of t he mature leech. It has been thought that the fate-determining interac tions in this 'O-P equivalence group' take place between the equipoten t cells themselves. We show here that such intra-group interactions ar e neither necessary nor sufficient. Instead, transient contact with ce lls in another ectodermal lineage is necessary and sufficient to induc e o/p blast cells to assume the P fate. In the absence of this contact they assume the O fate.