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Changes in the genetic structure of the Katsunuma natural population of Drosophila melanogaster have been examined during the past 35 years. The frequency of recessive lethal genes on the second chromosome once increased from 15% to 30% in the early 1970s, then decreased to about 24% in the late 1970s, and thereafter showed no significant changes. Sterility genes, the frequency of which is always less than the lethals, showed a similar tendency. The SD (segregation distorter) mutant gene disappeared but some others such as rbl (reduced bristle) and bw (brown) persisted in the population. The frequency of inversion-carrying chromosomes gradually decreased in the period, such that the standard chromosome frequency in the second and third chromosomes increased from about 40% to more than 80%. Coincident with these frequency changes is the invasion of a transposable element P into the Katsunuma population. The P element should have invaded into Katsunuma in the late 1960s. It spread over the population apparently inducing deleterious mutations, causing the decrease in the allelism rate, and hence increasing the effective population size. Soon, however, most flies became resistant to the P element-mediated transposition as they began to harbor defective P elements. During the course of spreading, the P element must also have induced deleterious mutations on the polymorphic inversions, breaking up the heterotic gene complexes along the chromosomes, which probably caused the reduction in the frequency of inversion chromosomes. Temporal invasion of D. simulans, a sibling species of D. melanogaster, into Katsunuma occurred several times after 1978, and the species seems to have been settled since 1990. This, however, did not have any effect on the genetic structure of D. melanogaster population.

Further genetic studies on the Katsunuma population of Drosophila melanogaster.