Inductive regulation of cell fusion in leech

Cell-cell fusion is a component of many different developmental processes, but little is known about how cell-cell fusion is regulated. Here we invest igate the regulation of a stereotyped cell-cell fusion event that occurs am ong the endodermal precursor cells of the glossiphoniid leech Helobdella ro busta. We find that this fusion event is regulated inductively by a cell th at does not itself fuse. We also show that biochemical arrest (by microinje ction with ricin A chain or ribonuclease A) of the inducer or either of the fusion partners prevents fusion, but only if the arrest is initiated durin g a critical period long before the time at which fusion normally occurs. I f the arrest occurs after this critical period, fusion occurs on schedule. These results suggest that both fusion partners play active roles in the pr ocess and that neither the induction nor the fusion itself requires concomi tant protein synthesis.