During C. elegans development, the decision to develop a female (XX) or male (XO) soma is determined by the ratio of the number of X chromosomes to the number of sets of autosomes (X:A ratio). The resulting signal is transmitted through a cascade of interacting genes and determines the activity of the most downstream factor of this cascade, the transcription factor TRA-1A (TRA, transformer). Strong loss-of-function (lf) mutations in the
tra-1 gene lead to a complete transformation of the XX soma into a male soma while weak
tra-1(lf) mutations result in a partial masculinization of the XX soma. A partially masculinized XX soma overall resembles a wild-type XX soma, however, frequently the hermaphrodite-specific HSN neurons are absent and the male-specific CEM neurons are present. The HSNs and CEMs are the only neurons in C. elegans that are sex-specific as a result of programmed cell death. The specification of the cell death fate of these cells therefore seems to be particularly sensitive to changes in
tra-1 dosage. In a screen for mutants, in which the CEMs inappropriately survive in XX animals, we identified mutations in a previously not described gene, referred to as
tra-4, which is required for complete feminization of the XX soma.
bc45, a lf mutation in the
tra-4 gene, causes a partial masculinization of the XX soma: 73% of the HSNs undergo programmed cell death and 80% of the CEMs escape programmed cell death. In addition, in
tra-4(
bc45) L1 hermaphrodites the B cells and the ventral left coelomocytes have a male appearance or position, respectively, indicating weak masculinization. Double mutant analyses suggest that
tra-4 acts upstream of
fem-1,
fem-2, and
fem-3 (fem, feminization of XX and XO animals), and downstream of
her-1 (her, hermaphrodization of XO animals) and possibly
tra-2.
tra-4 might therefore be a negative regulator of the fem genes. The
tra-4 locus encodes a protein with predicted nuclear localization signals and C2H2 type Zinc finger domains, which indicates a role in transcriptional regulation.
bc45 is a missense mutation leading to the exchange of a conserved amino acid in one of the C2H2 DNA-binding motives, suggesting that
bc45 might not be a null allele. The analysis of the expression pattern of a functional Ptra-4GFP::TRA-4 construct indicates that
tra-4 is expressed in most if not all somatic nuclei at all developmental stages in animals of both sexes. Using various approaches, we are currently searching for targets of the TRA-4 protein.