To define further the genetic pathway of vulval development, we have isolated extragenic suppressors and enhancers of
lin-15 mutations. Iin- 15 mutations perturb the determination of vulval cell fates and cause a Multivulva (Muv) phenotype. Mosaic analysis (Herman & Hedgecock, Nature 348: 169-171, 1990) suggests that
lin-15 acts in the hypodermis to inhibit the expression of vulval-specific cell lineages. In a screen of 76,000 haploid genomes, we isolated 156 mutations that suppress and 13 mutations that enhance lin-lS
(n765). Eighty of the suppressor mutations cause a vulvaless (Vul) phenotype. These mutations include alleles of
let-23,
let-60,
let-341,
lin-2,
lin-7,
lin-10,
lin-24,
lin-33,
lin-39, sem-S,
unc-83, and
unc-84. At least five genes (
let-23,
let-60, let
(n2018),
let-341, and sem-S) appear to be involved in signalling between the anchor cell and the cells of the vulval equivalence group. The anchor cell produces an inductive signal that is required for the formation of the vulva. Mutations in these genes cause a Vul phenotype that is identical to the phenotype observed when the anchor cell is killed. That mutations in these genes also suppress the Muv phenotype of
lin-15 mutants ( which are Muv both in the presence and absence of the anchor cell) suggests that they are involved in the reception of the anchor cell signal, possibly acting in the vulval cells themselves. Mutations in these genes share a variety of phenotypes (e.g. strong alleles cause larval lethality), suggesting that these genes might define components used in cell signalling in other aspects of development. Interestingly, sem-S is also involved in the cell signalling that controls the migration of the sex myoblasts (see abstract by Stern, Clark and Horvitz). Two of these five genes have been shown to be similar to genes involved in signal transduction pathways in mammals:
let-23 is similar to the EGF receptor (Aroian et al., Nature 348: 693-699, 1990) and let- 60 is a ras protein (Han & Sternberg, Cell 63: 921-931,1990). We have cloned sem-S (Stem, Clark and Horvitz) and
lin-15 to investigate how these genes function at the molecular level and how they interact with
let-23 and
let-60. Another gene identified in our
lin-15 suppressor screen,
lin-39, might function in defining the spatial identity of cells located within the central body region. Iin-39 mutations cause P(3-8) to express fates characteristic of other P cells and perturb the anterior migration of QR. We have rescued
lin-39 mutants by germline transformation.
ceh-15, a homeobox-containing gene (Kamb et al., PNAS 86: 4372-4376, 1989), is located within the genomic fragment that rescues
lin-39. Kenyon et al. have also rescued
lin-39 mutants with
ceh-15 clones. We are currently determining whether
ceh-15 is
lin-39.