Rapid change in mutation rate in a local population of Drosophila melanogaster.

The lethal and detrimental loads per second chromosome rapidly increased from 1968 to 1970 in a local population of Drosophila melanogaster in Japan (lethal load, from about 0.16 to 0.38; detrimental load, from 0.125 to 0.231 [Watanabe, T. K., Watanabe, T. & Oshima, C. (1976) Evolution 30, 109-118]). When the homozygous loads were measured in 1983, the lethal load had decreased to approximately the original amount (0.19) but the detrimental load had stayed high (0.241). The rise and fall of the lethal load can be accounted for by a P-type element that invaded a population with M cytotype, producing a high mutation rate. The mutation rate fell back to the earlier value after the cytotype became P. That the detrimental load did not decrease can be explained by assuming a longer persistence for detrimental mutations in the population. Evidence for a P-type mutator factor is that the mutation rate of the wild-type chromosomes differs between the different cytoplasmic and chromosomal backgrounds, being lower in the background from which the chromosomes were taken.