xol-1 is required in XO animals for proper sex determination and dosage compensation. Mutations in the gene
xol-1 result in XO-specific lethality and map between
mec-7 and
dpy-6 on the X chromosome. The lethal phenotype is recessive since stDp2/f;
xol-1 XO mutant animals are viable males. Two independently isolated mutations,
y9 and
y70, both exhibit XO- specific lethality and map to the same interval on X. In addition, they are thus far inseparable by recombination (<.01 m.u. apart), indicating that they are probably alleles of the same gene. The time of death for
xol-1 XO mutant animals extends from mid- embryogenesis to early in the L1 larval stage. It is likely that these XO animals die because, in the absence of
xol-1, the XX mode of dosage compensation is being expressed in XO animals, presumably resulting in under expression of X-linked genes. The XO lethality of the
xol-1 mutations can be rescued by mutations in
dpy-21,
dpy-27, and
dpy-28, which themselves elevate X- linked gene expression in XX animals. Presumably, mutations in these dpy genes prevent proper expression of the hermaphrodite mode of dosage compensation that is being inappropriately expressed in
xol-1 mutant XO animals and thereby return levels of X-linked gene expression to normal. Consistent with this idea is the finding that there are wild-type levels of X-linked mRNA in
dpy-28(
y1); and
dpy-21(
e428); XO animals. The XO animals rescued by mutations in
dpy-21, 26, 27, and 28 range in phenotype from fertile males to fertile hermaphrodites. Therefore, although both
y9 and
y70 were isolated as apparent 'suppressors' of the
dpy-28(
y1) mutation, though non-Dpy (unsuppressed) animals in the
y1;
y9 and
y1;
y70 strains are, in fact, XO animals that have been transformed into hermaphrodites by the
xol-1 mutations and are not suppressed XX animals. In a typical experiment that demonstrates transformation of XO animals into hermaphrodites,
y1;
y70 males were mated with
dpy-18(
e364) ); ) 1) hermaphrodites. Among the cross progeny were many Unc nonDpy (XO) hermaphrodites. Similar results have been obtained with mutations in
dpy-21 and
dpy-27. Although mutations in
dpy-26 have not yet been tested as directly, results from other experiments indicate that many
dpy-26; t animals are also transformed into hermaphrodites. The transformation of XO animals into hermaphrodites in the above strains suggested a role for
xol-1 in sex determination and prompted us to look directly at the dying embryos and L1 larvae in
xol-1(
y9) and
xol-1(
y70) mutant strains. It is possible to determine the sex of dying L1 larvae in these strains by observing cells that are sexually dimorphic at hatching. Preliminary observations suggest that B, Y, and HSN cells can undergo hermaphrodite-specific cell fates in dying
xol-1 mutant L1 larvae, implying that
xol-1 is required in XO animals for proper sexual development. It appears that mutations in
dpy-21, 26, 27, and 28 can suppress the dosage compensation defects but not the sex determination defects found in XO
xol-1 mutant animals. By contrast, mutations in
sdc-1, a gene required for hermaphrodite development and proper X expression in XX animals (Villeneuve and Meyer, Cell 48: 25-37, 1987), are fully epistatic to
xol-1 mutations in that they are able to suppress both the sex determination and the dosage compensation defects of
xol-1 mutations in XO animals. Specifically, all
xol-1(
y9) 5) XO animals are viable males (A. Villeneuve, personal comm.). The same result was observed with a newly identified gene,
sdc-2 (C. Nusbaum and B. Meyer, this WBG). In a possible model explaining
xol-1's function in XO animals, the wild-type
xol-1 gene ensures that the activities of
sdc-1, r genes involved in hermaphrodite sex determination and dosage compensation remain inactive in XO animals (see figure below). Mutations in
xol-1 also affect sex determination in XX animals. Although
xol-1 mutant XX animals are phenotypically wild type,
xol-1 mutations further masculinize XX animals that are already partially masculinized.
tra-2(strong) and
tra-3(strong) mutations result in the incomplete transformation of XX animals into non-mating pseudomales. Surprisingly, the
xol-1(
y9) mutation, in combination with these strong alleles or
n1106 (a weak allele of
tra-2) causes further transformation of some XX animals into mating males.
y9 also dramatically masculinizes
her-1(
n695sd) XX animals. In a
her-1(
n695sd) strain, 30% of the XX animals are nonmating pseudomales while the remaining 70% are Egl hermaphrodites (C. Trent, personal comm.). In a
her-1(
n695sd); strain, all of the XX animals are at least partially transformed into pseudomales and some are capable of mating. Furthermore,
y9 enhances the Tra phenotype of
sdc-1 XX animals, although mating males are not produced.
xol-1(
y70) also displays an XX phenotype similar to
y9 in that it fails to complement
y9 in the further masculinization of some
tra-2( strong) XX animals into mating males.
y70, however, appears to be much weaker than
y9 with respect to this XX sexual transformation phenotype. A loss-of-function mutation in
her-1 does not block
xol-1(
y9)'s masculinization of a strong
tra-2 mutation, implying that wild-type
her-1 product is not required for further masculinization. Furthermore, mutations in
fem-1, and strong mutations in
tra-1 are all epistatic to the
xol-1(
y9) mutation. Therefore, with respect to maculinization of XX animals,
xol-1 either interacts with the major sex determination pathway somewhere downstream of
her-1 and upstream of
tra-1 or interacts in a parallel pathway. A small, X-linked deficiency that spans the
xol-1 locus, uDf1 ( provided by the Chalfie lab), fails to complement the XX sex determination phenotype of
y9. Specifically, some
tra-2(strong);
y9/uDf1 XX animals are mating males. Preliminary experiments with uDf1 have revealed that
y9 and
y70 are probably loss of function mutations in
xol-1 and that the
xol-1 locus is haplo-insufficient with respect to its interactions with
her-1(
n695). Some XX animals of both genotypes
her-1(
n695);
y9/f and
her-1(
n695); uDf1/+ (D. Hsu and B. Meyer, this WBG) are mating males. Moreover, some XX animals of the triply heterozygous genotype
tra-2(strong)/+; 5)/+;
y9/+ are mating males (D. Hsu, personal comm.). In addition, neither
y9 nor
y70 is suppressed by the tRNA amber suppressor
sup-5(
e1464) in either one or two copies. Reversion of
xol-1(
y9 or
y70) has been undertaken by many members of our lab and has yielded many new alleles of previously identified genes. Using the
xol-1 reversion screen, we have isolated 13 alleles of
dpy-21 (one of which was induced in a mutator strain provided by the Anderson lab), 1 allele of
dpy-26, 5 alleles of
dpy-27, and 2 alleles of
sdc-1. 7 additional alleles of
tra-1 and 17 alleles of tra- 2 were also isolated. In addition, the
xol-1 reversion screen has allowed the isolation of a new Tra mutation on V and another one on X. Furthermore, 21 alleles of
sdc-2 have been isolated as suppressors of
y9 (one in a mutator background).