Genes that implement the hermaphrodite mode of dosage compensation in Caenorhabditis elegans.

We report a genetic characterization of several essential components of the dosage compensation process in Caenorhabditis elegans. Mutations in the genes dpy-26, dpy-27, dpy-28, and the newly identified gene dpy-29 disrupt dosage compensation, resulting in elevated X-linked gene expression in XX animals and an incompletely penetrant maternal-effect XX-specific lethality. These dpy mutations appear to cause XX animals to express each set of X-linked genes at a level appropriate for XO animals. XO dpy animals are essentially wild type. Both the viability and the level of X-linked gene expression in XX animals carrying mutations in two or more dpy genes are the same as in animals carrying only a single mutation, consistent with the view that these genes act together in a single process (dosage compensation). To define a potential time of action for the gene dpd-28 we performed reciprocal temperature-shift experiments with a heat sensitive allele. The temperature-sensitive period for lethality begins 5 hr after fertilization at the 300-cell stage and extends to about 9 hr, a point well beyond the end of cell proliferation. This temperature-sensitive period suggests that dosage compensation is functioning in XX animals by mid-embryogenesis, when many zygotically transcribed genes are active. While mutations in the dpy genes have no effect on the sexual phenotype of otherwise wild-type XX or XO animals, they do have a slight feminizing effect on animals whose sex-determination process is already genetically perturbed. The opposite directions of the feminizing effects on sex determination and the masculinizing effects on dosage compensation caused by the dpy mutations are inconsistent with the wild-type dpy genes acting to coordinately control both processes. Instead, the feminizing effects are most likely an indirect consequence of disruptions in dosage compensation caused by the dpy mutations. Based on the cumulative evidence, the likely mechanism of dosage compensation in C. elegans involves reducing X-linked gene expression in XX animals to equal that in XO animals via the action of the dpy genes.