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 distinct sets of 'O' and 'P' progeny to the nervous system and epidermis of the mature leech. It has been thought that the fate-determining interactions in this 'O-P equivalence group' take place between the equipotent cells themselves. We show here that such intra-group interactions are neither necessary nor sufficient. Instead, transient contact with cells in another ectodermal lineage is necessary and sufficient to induce o/p blast cells to assume the P fate. In the absence of this contact they assume the O fate.