ced-10 Rac and mig-2 function redundantly and act with unc-73 trio to control the orientation of vulval cell divisions and migrations in Caenorhahditis elegans

Vulval development in the nematode Caenorhabiditis elegans can be divided into a fate specification phase controlled in part by let-60 Ras, and a fate execution phase involving stereotypical patterns of cell division and migration controlled in part by lin-17 Frizzled. Since the small GTPase Rac has been implicated as a downstream target of both Ras and Frizzled and influences cytoskeletal dynamics, we investigated the role of Rac signaling during each phase of vulval development. We show that the Rac gene ced-10 and the Rac-related gene mig-2 are redundantly required for the proper orientation of certain vulval cell divisions, suggesting a role in spindle positioning. red-10 Rae and mig-2 are also redundantly required for vulval cell migrations and play a minor role in vulval fate specification. Constitutively active and dominant-negative mutant forms of mig-2 cause vulval defects that are very similar to those seen in red-10;mig-2 double loss-of-function mutants, indicating that they interfere with the functions of both red-10 Rae and mig-2. Mutations in unc-73 (a Trio-like guanine nucleotide exchange factor) cause similar vulval defects, suggesting that UNC-73 is an exchange factor for both CED-10 and MIG-2. We discuss the similarities and differences between the cellular defects seen in Rae mutants and let-60 Ras or lin-17 Frizzled mutants. (C) 2001 Elsevier Science.