Dominant mutations affecting muscle structure in Caenorhabditis elegans that map near the actin gene cluster

By examining F1 progeny of mutagenized Caenorhabditis elegans larvae, we recovered several dominant mutations which affect muscle structure. Five of these new mutations resulted in phenotypes unlike the previously recognized unc-54 and unc-15 dominant alleles. Mapping studies placed all five mutations in the same small region of linkage group V. Polarized light, fluorescence and electron microscopic studies showed that a prominent feature of the disorganized myofilament lattice is the abnormal placement of thin filaments within the body wall muscle cells. Pharyngeal musculature is also affected by three of the mutations when homozygous. Of the five mutations only three are homozygous viable. All three of these have unusually high intragenic reversion rates either spontaneously (approximately 10(-6)) or after ethyl methanesulfonate mutagenesis (2 X 10(-5)), suggesting that reversion occurs through loss of function mutations. No unlinked suppressor mutations were found. The dominance of the mutations, the effect on thin filaments and the reversion properties suggested that these new dominant mutations lie in a gene or genes specifying a structural component of the thin filament. The positioning of a set of three actin sequences in the same region (Files et al., 1983) led us to speculate that these mutations lie in actin genes.