Mutator factors and genetic variance components of viability in Drosophila melanogaster

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 randomyl sampled, inversion-free second chromosomes. Mutations were accumulated only through males heterozygous for the Pm-carrying chromosome [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; translocation: 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 crossover) 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.