Hermaphrodites homozygous for
lin-25 mutations are vulvaless ( Ferguson and Horvitz, 1985. Genetics 110, 17-72). We are particularly interested in this gene because it may provide a link between two binary decisions during vulval precursor cell (VPC) fate specification: the vulvaVnon-vulval decision, controlled by
let-60 activity (Han and Sternberg, 1990. Genetics 126, 899-913; Beitel et al. , 1990. Nature 348, 503-509) and the 2 /non-2 decision, controlled by
lin-12 activity (Greenwald et al., 1983. Cell 34, 435-444). Two observations support this idea. Firstly, in
lin-25(1f) hermaphrodites no VPCs adopt the 2 fate: P.6p undergoes the 1 fate and the other VPCs adopt the 3 fate (Ferguson et al., 1987. Nature 326, 259267). Secondly, mutations in
lin-25 are epistatic to
let-60(nlO46gf), but
lin12(nl37gf) is epistatic to
lin-25(113. We have obtained evidence that
lin-25(
el446),
lin-25(nlO63) and a new allele,
lin-25(
ar90) ( which we isolated in a non-complementation screen) are null alleles. We generated a deficiency in the region, arDfl, which fails to complement markers from
uab-8 to
lin-25. We used the deficiency in genetic dosage studies with the above alleles and two temperature sensitive hypomorphic alleles;
lin25(n545ts) and
lin-25(nl 722). We have examined the interactions of
lin-26 mutations with some of the other genes involved in VPC fate determination. We found that the putative null alleles of
lin-25 are epistatic to Muv mutations in the genes; lin-l,
lin-15, and
lin-8;1in-9. In our efforts to clone
lin-25 we have mapped Tcl induced RFLPs in congenic strains kindly supplied by ATelfer.These strains contain BO chromosomal DNA in the
rol-4 to par-l interval (in which
lin-25 maps) in an N2 backround. The RFLP closest to
lin-25, which we have named arPl, maps 0.05mu to the right. We have cloned the DNA flanking arPl by inverse PCR and used it to isolate two lambda clones. These clones were fingerprinted by Alan Coulson and John Sulston to the cosmid C06Cl. We have established a metric for the region of 44 cosmid lengths/map unit by comparing the physical and genetic distances between arPI and a second cloned polymorphism present in the congenic strains, TCPARl. Using the metric we estimate that
lin-25 lies on one of the cosmids immediately to the left of C06C 1.