A set of 1,000 "mutation accumulation" lines of Drosophila melanogaster, which originated from two different wild-type, lethal-bearing second chromosomes (Yamaguchi and Mukai 1974; Mukai and Cockerham 1977), was examined for evidence of a mutator factor by using the occurrence of recessive visible mutations and male recombination to identify its presence. The 1,000 lines were screened at approximately generation 240 for the presence of recessive visible mutations at twelve loci, by outcrossing to a balanced multiply marked second chromosome stock (Muller's "12ple" Bowling Green). Twenty-three lines were found to carry a visible mutation at one of the loci. Seventeen of these lines carried a mutation of either the dp or the vg locus. Mutations found in three lines, two at the dp locus and one at the vg locus, demonstrated instability as revertants to the wild type and were recovered and verified in these three cases. The three revertant lines, and three lines showing no reversion, were tested for their ability to induce male recombination. Male recombination was observed in the three lines in which revertants were recovered. Male and female sterility assays indicated conclusively that these "hybrid dysgenic" characteristics could not be used to identify lines potentially carrying mutator factors, whereas the consistent ability of the lines to induce high rates of reversion and male recombination was successful in determining that the "mutation accumulation lines" do possess mutator factors.
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