C. elegans sexual fate is ultimately determined by the ratio of X chromosomes to autosomes. The number of X chromosomes is measured by several X-linked genes (X signal elements), such as
fox-1 and
sex-1. The high dose of X signal elements in XX animals functions to repress the
xol-1 gene leading to hermaphrodite development, while a low dose in XO animals allows for high
xol-1 levels and male development. The mechanism for counting autosomes is unknown, but may similarly rely on autosomal signaling genes.To identify potential autosomal signaling genes, we exploited the fact that mutations in both
fox-1 and
sex-1 result in hermaphrodite specific lethality without affecting male development. Loss of function mutations in genes required to signal autosomal dose should restore the X:A ratio and thus, suppress the lethality of
fox-1 sex-1 double mutants.We screened for suppressors of
fox-1 sex-1 lethality by mobilizing the Mos1 transposable element. This approach offers several advantages over chemical mutagenesis, primarily, ease of cloning. We have screened approximately 7500 genomes and identified 4 candidate suppressors carrying a Mos1 insertion. In 3 out of 4 cases the site of insertion occurred in intronic regions, while in one case,
y407, the insertion occurred in an exon of K10G6.3. For each insertion mutant, the corresponding ORF was disrupted by RNA interference in a
fox-1 sex-1 double mutant. RNAi of K10G6.3 phenocopied the suppression of
fox-1 sex-1 lethality similar to the Mos1 insertion. RNAi of the remaining ORFs had no effect on
fox-1 sex-1 lethality, suggesting that the RNAi may have been ineffective or that the Mos1 insertion may be tightly linked to a second, spontaneous mutation responsible for the suppression. Finally,
y407 suppresses mutations in other X signal element genes, but does not suppress mutations downstream of
xol-1.K10G6.3 encodes PQN-50, a novel protein containing glutamine and asparagine-rich repeats, a proline rich region, and several zinc fingers. A
pqn-50 transcriptional reporter revealed an unexpected neuronal expression pattern. We are continuing to characterize
pqn-50(
y407) at the molecular and genetic level to determine whether it is a bona fide autosomal signaling element. Efforts to clone additional autosomal elements are also ongoing.