The Million Mutation Project (MMP; Thomson et al., Genome Res. 23: 1749-1762, 2013) has become an outstanding resource for mutations in many C. elegans genes. More recently Timbers et al. (PLoS Genet. 12:
e1006235, 2016) used a subset of MMP strains to identify genes that could be mutated to give a dye-filling (Dyf) phenotype. Because most of the mutations in the MMP strains are homozygous, we were intrigued about the possibility that the MMP strains could be used to identify mutations having subtle phenotypic effects. We also wanted to assess how complete a screen utilizing the MMP strains would be. To this end we have undertaken a screen of the MMP strains to identify those with extreme to slight defects in the gentle touch response. Classical forward genetic screens (Chalfie and Sulston, 1981; Chalfie and Au, 1989) identified 18 genes necessary for gentle touch sensitivity, although others have also been found (e.g., among genes whose null phenotype is lethality). We have currently screened about 1700 of the 2007 MMP strains. 111 of these strains have an abnormal gentle touch response. 73 of these strains have mutations affecting the protein sequence, UTRs, or splice sites of genes previously known to mutate to touch insensitivity. In particular, we identified strains with mutations in all but one gene (
mec-17) found in our initial forward mutageneses.
mec-17 is extremely difficult to mutagenize to touch insensitivity; all four mutations in MMP strains failed to make the animals touch insensitive. Thus, the MMP collection, at least for viable phenotypes, appears to be essentially complete. Using MMP defects in the known touch genes that did and did not produce touch sensitivity, we were able to look for important domains within the encoded proteins. We are now identifying causative mutations in the remaining 38 strains, starting with the strains that exhibit a more extreme phenotypic defect. Because the MMP strains are sequenced, identifying the causative mutation should be relatively quick using genetic mapping. We are conducting several other screens using this resource and strongly recommend it to our colleagues.