Natural selection with nuclear and cytoplasmic transmission. II. Tests with Drosophila from diverse populations.

Observations of intraspecific variation in organelle DNA have prompted a renewed interest in the evolutionary consequences of cytoplasmically transmitted factors. Attempts to quantify the significance of cytoplasmic effects are frequently limited by the difficulty in partitioning the cause of reciprocal cross differences among a series of possibilities. In the experiment reported here the nuclear genomes of a set of six lines of Drosophila melanogaster from diverse geographic locations were replaced in a series of cytoplasms. The segregation of the SM5 balancer chromosome was scored in a factorial design, and the data allowed a partitioning of variance such that cytoplasmic effects were distinguished from maternal effects and meiotic drive. An attempt was made to avoid the confounding problem of hybrid dysgenesis by performing the entire experiment (including chromosomal extractions) in a P cytotype. Results indicated a significant contribution of cytoplasm to the variance in SM5 segregation. Error variance showed an increasing trend as the experiment proceeded, and additional tests indicated that this was due to an accumulation of chromosomal mutations. These findings are interpreted in light of the population genetic theory that addresses the maintenance of cytoplasmic polymorphism.