The
lin-12 and glp-l genes encode homologous transmembrane proteins ( l) that may act as receptors for cell interactions during development ( 2,3). The glp-l gene is required zygotically for germ line proliferation and maternally for embryogenesis (2, 4);
lin-12, in contrast, is involved in various postembryonic cell interactions in the soma, including those between vulval precursor cells (VPCs) that specify vulval fates (5). The novel allele glp-l (
q35) has a semidominant Multivulva phenotype (Muv) in addition to the typical recessive Glp phenotypes. We find that the Muv phenotype of glp-l (
q35) is similar to that of
lin-12(d) in three ways. First, VPCs that would normally fuse with the hypodermal syncytium (3 fate) follow a 2 -1ike vulval lineage instead. Second, the Muv phenotype is not dependent on the anchor cell. Third, the Muv phenotype results from increased or novel, rather than decreased, glp-l activity. Because glp-l (
q35) remains Muv in animals lacking
lin-12, this effect does not depend on
lin-12 activity. We conclude that the glp-l (
q35) product is functionally very similar to that of
lin-12. The glp-l (
q35) gene bears a nonsense mutation predicted to truncate the glp-l protein 122 amino acids from its carboxy terminus. The region which should be missing from the glp-l (
q35) protein includes a PEST sequence thought to destabilize proteins. A simple explanation of these results is that the carboxy terminus bears a negative regulatory domain that normally keeps glp-l activity off in the VPCs. In glp-l (
q35) animals, inappropriate glp-l activity may drive these cells to proliferate, and thereby channel them toward a 2 -like fate.