Selection for parthenogenesis in Drosophila mercatorum.

PARTHENOGENESIS, as a substitute for syngamy, is widespread in nature. This is especially true for thelytoky, a type of parthenogenesis in which virgin females produce almost exclusively female offspring. The evidence is ovenvhelming that this condition has arisen independently in dozens of species of organisms. Each case may indeed be regarded as a special evolutionary event in the sense that natural selection has favored the accumulation of genes which provide a genetic basis for parthenogenesis. Little attention has been given to experimental study of the origin of the thelytokus state in the laboratory. This is probably due largely to the fact that most organisms cannot be reared sufficiently easily and in large enough numbers to make such a study feasible. After STALKER'S (1954) discovery of low-rate parthenogenesis in Drosophila and his success in selecting for increased rate in D. parthenogenetica and D. polymorpha, the writer has sought a species of Drosophila well adapted to an extended study of this phenomenon. Drosophila mercatorum has met the requirements; several wild strains show very low unselected rates of parthenogenetic development. Impaternates are mostly diploid females; they are highly vigorous and viable and are fully fertile when crossed to sexually produced males. Selection has resulted in an approximately 60-fold increase in the rate of parthenogenesis.