This paper describes a mutator system in the nematode Caenorhabditis elegans var. Bergerac for the gene
unc-22. Of nine C. elegans and two C. briggsae strains tested only the Bergerac BO strain yielded mutant animals at a high frequency and the
unc-22 IV gene is a preferred mutational target. The forward spontaneous mutation frequency at the
unc-22 locus in Bergerac BO is about 1 X 10(-4), and most of these spontaneous
unc-22 mutations revert at frequencies between 2 X 10(-3) and 2 X 10(-4). Both the forward mutation frequency and the reversion frequency are sensitive to genetic background. Spontaneous
unc-22 mutations derived in a Bergerac background and placed in a primarily Bristol background revert at frequencies of less than 10(-6). When reintroduced into a Bergerac/Bristol hybrid background the mutations once again become unstable. The mutator activity could not be localized to a discrete site in the Bergerac genome. Nor did mutator activity require the Bergerac
unc-22 gene as a target since the Bristol
unc-22 homolog placed in a Bergerac background also showed high mutation frequency. Intragenic mapping of two spontaneous
unc-22 alleles,
st136 and
st137, place both mutations in the central region of the known
unc-22 map. However, these mutations probably recombine with one another, suggesting that the unstable mutations can occur in more than one site in
unc-22. Examination of the phenotypic effect of these mutations on muscle structure indicates that they are less severe in their effect than a known amber allele. We suggest that this mutator system is polygenic and dispersed over the nematode genome and could represent activity of the transposable element Tc1.