Multicellular development requires proper gene expression to specify cell fate. One family of transcription factors, the bHLH (basic Helix-loop-Helix) proteins, influences cell fate by altering gene expression in target cells. The bHLH proteins bind DNA through their basic domain as either homodimers or heterodimers. Two C. elegans bHLH proteins CeE/Da and CeTwist that are homologs of Drosophila Daughterless and Twist, respectively, are believed to cooperate based on in vitro DNA binding evidence and in vivo reporter activation studies (Harfe, et al.). CeE/Da and CeTwist have overlapping protein expression in M mesoblast derivatives that become vulva and uterine muscles and in defecation muscles (Krause, et al. and Harfe, et al.) making them reasonable candidates to function together in muscle specification of these cells. We have identified a semidominant mutant allele (
n2170) that is both egg-laying defective (Egl) and constipated (Con). The mutation maps to LG X in the region of
hlh-8 (the CeTwist gene). Because the mutant phenotypes affect tissues in which CeTwist is found, we focused on
hlh-8 as the possible locus for the
n2170 mutation. We sequenced the
hlh-8 locus in the
n2170 mutant and found a missense mutation that changes an amino acid invariant among all Twist family members in the basic domain of CeTwist. Both the Egl and Con phenotypes of
n2170 are relieved in transgenic animals expressing wild type CeTwist. Phalloidin and antibody staining reveal that the anal depressor and the intestinal muscles are missing in the
n2170 mutant and are restored in the CeTwist rescued animals. We propose that the semidominance of
n2170 results from interference of functional CeTwist:CeE/Da heterodimers and are currently investigating the nature of the
n2170 CeTwist defect. Harfe, B., Vaz Gomes, A., Kenyon, C., Liu, K., Krause, M., and Fire, A. (1998) Genes & Development, in press Krause, M., Park, M., Zhang, J., Yuan, J., Harfe, B., Xu, S., Greenwald, I., Cole, M., Paterson, B., and Fire, A. (1997) Development 124:2179-89.