MUTATOR FACTORS AND GENETIC VARIANCE COMPONENTS OF VIABILITY IN DROSOPHILA MELANOGASTER

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Abstract
In the process of testing whether or not the independent-locus selection model holds true with previously estimated genetic parameters (cf. Mukai and Maruyama 1971) in D. melanogaster collected near Raleigh, North Carolina, we found an abnormal phenomenon: an unusually large increase in dominance variance for viability in comparison with additive variance with the accumulation of mutations on 140 randomly sampled, inversion-free second chromosomes. Mutations were accumulated only through males heterozygous for the Pm-carrying chromosomes [In(2LR)bwV1] and the extracted second chromosomes, and the genetic variance components were estimated by using a partial diallel cross method.—Further investigations clarified that chromosome abberations occurred at a frequency of 0.0114 per second chromosome per generation (inversions: 0.0098; transpositions: 0.0011; translocations: 0.0004), and recessive lethal mutations occurred at an average rate of 0.031 per second chromosome per generation.—From these results and from the amount of change in the homozygous load, it was speculated that about 60-70% of the second chromosomes used had a kind of mutator which induced chromosome and/or chromatid breaks at a minimum rate of 0.18 per second chromosome per generation. These breaks resulted in recessive lethal mutations at a rate more than ten times higher than the normal rate. Also these breaks were most probably the cause of male recombination.—The above unusual increase in dominance variance can be explained by assuming that chromosome segments, introduced into the extracted "wild" chromosomes by male recombinations (double cross-over) from the marker chromosomes [In(2LR)bwV1], showed heterosis and linkage disequilibria with deleterious mutations and possibly with other introduced segments.—Finally, the nature and possible significance of mutator factors are discussed.