The sex-determination signal in C. elegans is the X:A ratio -- diploid XO animals are male and XX animals are hermaphrodite. The X:A ratio also sets the mode of dosage compensation -- XX animals turn down gene expression from their two X chromosomes to equal that from the single X in males. Most animals that select the inappropriate mode of dosage compensation die due to either overexpression or underexpression of X-linked genes. The goal of our research is to understand how the embryo calculates its X:A ratio. While the nature of the autosomal component of the signal is unknown, recent evidence suggests that animals count their X chromosomes by measuring the dose of a few discrete signal elements on X. We showed that the left end of X contains such sex-determination signal elements. XO animals with a large duplication of this region are dead, but they are rescued by downstream mutations that prevent the XX mode of dosage compensation. XX animals heterozygous for a deficiency spanning this region are Dpy, Egl and masculinized, a phenotype that is suppressed by a downstream mutation that prevents the XO mode of dosage compensation. Our analysis of smaller duplications and deficiencies, and of combinations thereof, suggested that the dose of each of three regions contributes to the extent of these phenotypes. We concluded that there are several signal elements near the left end of X. Our data also suggested that additional signal element(s) exist elsewhere on X. One such element may be
rox-1 (see Carmi et al. abstract, this meeting). To identify mutations in signal elements, we originally screened for suppressors of the almost complete XO-specific lethality caused by the combination of mnDp66 and yDp14. The two duplications together include all three signal-element regions at the left end of X. The screen yielded two strong suppressor mutations,
y303 and
y304, that were found to be mutations in
fox-1, a gene included in yDp14.
y303, a likely
fox-1 null allele, only weakly suppresses the complete XO-specific lethality caused by two copies of yDp14, suggesting that the total dose of signal elements is still too high in such males. We reasoned that we could identify mutations in additional signal elements by screening for surviving yDp14 homozygous males in the presence of the
fox-1(
y303) mutation. Using this new screen, we have so far screened 7000 genomes, and isolated 9 suppressor mutations. At least three of the mutations map to X and are not alleles of any known genes in the sex-determination/dosage compensation pathway. For one of these mutations, the XX animals are slightly Dpy and Egl, a phenotype characteristic of a dosage-compensation defect. The other two mutants have a wild-type XX phenotype in the presence of yDp14. All three are good candidates for being signal element mutations. We are continuing to characterize these X-linked mutants and to screen for more mutants. If there are autosomal signal elements, we expect this screen to identify only duplications or gain-of-function mutations of such elements. In this regard, at least one of the autosomal mutants appears to be dominant.