I. Mutations in genes involved in the process of dosage compensation, sex determination, and/or the assessment of the X chromosome to autosome ratio often adversely affect only XO(XY) or XX animals. In Caenorhabditis s in five genes (
dpy-21,
dpy-27, ferential expression in XX animals, whereas there is no visible phenotype in XO animals. This has been interpreted as a disruption of dosage compensation, leading to a general increase of X-linked gene expression in XX animals. Biochemical evidence suggests that this interpretation is valid, at least for the genes
dpy-21, py-28 and
egl-16 (WBG v. 9 n. 1: Meyer and Casson, Villeneuve and Meyer). We have identified a new mutation resulting in XO-specific lethality. This maternal-effect mutation,
y2, is located on chromosome II between the markers
unc-85 and
dpy-10 (but is not under the deficiency mnDf30). It was isolated in a general screen for sex specific lethals (WBG v. 8 n. 3 ) . Briefly, hermaphrodites homozygous for
him5(e1490) were mutagenized using EMS and descendant F2s were cloned onto individual plates. Their broods were scored for evidence of sex specific lethality; a
y2 brood devoid of males was identified as a candidate for further study. We observed that
y2/y2 sons of
y2/+ animals are both viable and functionally normal, but
y2/y2 animals produced only daughters. Mutations with this phenotype could result from 1) inviability of XO animals, 2) suppression of
him-5(
e1490), or 3) sexual transformation of XO animals into hermaphrodites. The possibility that
y2 fails to produce nullo-X ova was eliminated by mating males bearing the X- linked marker
lin-14 with
y2 him-5(
e1490) hermaphrodites: equal numbers of
lin-14 and wild-type males were generated. Further evidence in support of
y2 causing XO inviability rather than the failure to form XO eggs comes from progeny counts.
y2;
him-5(
e1490) strains produce about 40% inviable offspring, primarily as dead eggs. These inviable zygotes can be accounted for by the normal distribution of 30% XO males and 10% dead eggs associated with
him-5(
e1490). ( There is also some inviability and infertility among hermaphrodite progeny of
y2 (non-him) animals.) The evidence that
y2 fails to transform XO animals into hermaphrodites is based on the failure to produce Unc non-Rol [XO] hermaphrodites in crosses of
y2/+ males with
y2 rol-6(
e187); 1) hermaphrodites. In addition, only half the expected number of male progeny were produced, consistent
y2/y2 being a XO specific lethal. Finally we showed, by constructing double mutants with the sex transformer genes
tra-1(
e1099 and
e1488),
tra-2(
e1095), and
her-1(
n695sd), that this lethality is karyotype rather than sex specific;
y2/y2 XX pseudomale offspring of
y2/y2 mothers are viable. We also noted that
y2 had no morphological effects on the degree of transformation of these mutants, suggesting that the effects of
y2 on the sex determination pathway are minimal. We were interested in exploring whether the bias toward XO lethality is associated with a reduction in the expression of X-linked genes in
y2 animals. Using genetic assays developed by L. De long (
lin14) and P. Meneely (
lin-15) we have shown that this mutation results in the hypo-expression of at least these two X-linked loci. By these assays the reduced X expression in
y2 is a simple zygotic phenotype of both XO and XX animals. We have also put
y2 in combination with mutations in
dpy-21, ulting strains are inviable, contrary to our naive expectation that simple rescue of
y2 by these mutations (or vice versa) might occur. Additionally, 2 copies of the X-chromosome duplications mnDp25 and mnDp10 fail to restore XO viability. We are presently determining the relative levels of X-linked gene expression in these strains. Additional alleles of the the gene defined by
y2 as well as suppressors of this mutation are being sought to aid our analysis of this gene. II. The mutation
her-1(
n695sd) causes an incomplete masculinization of XX animals. Mutations in the chauvinistic dpy genes (
dpy-21,
dpy-27, n the partial suppression of the transformed tail phenotype of
her-1(
n695sd). We felt that a clonal reversion screen of
n695 would give us a method of identifying new mutations in genes involved in the process of dosage compensation. We isolated 12 candidates from 6000 F2 clones. Two of these suppress not only the tail transformation of
n695 but also restore the HSNs (and hence egg-laying ability) to these animals. Both are extragenic, one mapping to chromosome I and one to III; neither appear to have a visible phenotype independent of
n695. These mutations appear specific for
n695, as they do not suppress
egl-41, another gene which displays a weak Tra phenotype.
egl-16(
n485) does not block the suppression of
n695 by these mutations, which can be contrasted to the fact that
egl-16 does block the suppression of
n695 by
dpy-21(
e428) (Villeneuve, personal communication). We also isolated several enhancers of
n695 transformation, which are in the process of being characterized.