Induction of vulval fates by the anchor cell is mediated by a conserved signal transduction pathway involving
let-60 Ras and the kinases
lin-45 Raf and
mpk-1/sur-1 MAP kinase. However, how this cascade causes cells to adopt vulval fates is not yet clear. Besides the kinases, three genes have been shown to function downstream of
let-60 in the genetic pathway for vulval induction,
lin-1,
lin-25 and
lin-31.
lin-31 encodes a putative transcription factor of the forkhead/HNF3 family. We have shown that
lin-25 encodes a novel protein (S.T and I.G.) and that
lin-1 encodes a putative nuclear protein with an ETS DNA binding domain (G.J.B. and H.R.H.). To order
lin-1,
lin-25 and
lin-31 with respect to one another in the pathway we have conducted genetic epistasis tests using null alleles of each gene.
lin-1 encodes a negative regulator of vulval induction,
lin-25 is a positive regulator and
lin-31 is required for the correct selection of vulval and non-vulval fates. In
lin-1 null mutants, all six VPCs adopt vulval fates. Lineage analysis of double mutants indicates that
lin-1 is epistatic to both
lin-25 and
lin-31. Since LIN-1 contains a number of potential MAP kinase phosphorylation sites and ETS-domain-containing proteins are known targets of MAP kinases in other organisms, it seems likely that LIN-1 is a direct target of MPK-1. Therefore, a simple interpretation of the epistasis results, that
lin-25 and
lin-31 act between
mpk-1/sur-1 and
lin-1, is unlikely to be correct. An alternative explanation is that
lin-25 and
lin-31 act in parallel to
lin-1. Our analysis of the pattern of VPC fates in
lin-1 null mutants suggests that
lin-1 does not mediate all the gonad-to-VPC signalling. Although all six VPCs adopt vulval fates in
lin-1 animals, the fate adopted by P6.p is still affected by the presence or absence of the gonad and anchor cell. Therefore gonad-to-vulva signalling cannot be a simple linear pathway. We are currently examining double mutants between
lin-1 and Ras pathway mutants to determine whether this non-linearity arises from branches in the Ras pathway, or whether multiple signalling events between the gonad and the VPCs are involved. We have also examined the interactions between
lin-1 and mutations in
lin-12, the receptor for the lateral signalling pathway.
lin-1 is epistatic to
lin-12(d)/+ but not to
lin-12(d)/lin-12(d) suggesting that the different vulval fates are determined by the relative rather than absolute levels of the outputs from the inductive and lateral signalling pathways.