Natural selection with nuclear and cytoplasmic transmission. III. Joint analysis of segregation and mtDNA in Drosophila melanogaster.

Despite the widespread use of mitochondrial DNA by evolutionary geneticists, relatively little effort has been spent assessing the magnitude of forces maintaining mtDNA sequence diversity. In this study the influence of cytoplasmic variation on viability in Drosophila was examined by analysis of second chromosome segregation. A factorial experiment with balancer chromosomes permitted the effects of cytoplasma and reciprocal crosses to be individually distinguished. The first test used six lines of diverse geographic origin, testing the segregation of all six second chromosomes in all six cytoplasms. The second and third tests were also factorial designs, but used flies from one population in central Pennsylvania. The fourth test was a large chain cross, using 28 lines from the same Pennsylvania population. Only the first test detected a significant nuclear-cytoplasmic effect. Restriction site variation in the mtDNA of all of these lines was assayed by Southern blotting, and statistical tests were performed in an effort to detect an influence of mtDNA type on fitness components. Posterior linear contrasts revealed an effect of mtDNA on segregation only among lines of diverse geographic origin. Within a population, no such influence was detected, even though the experiment was sufficiently large to have revealed statistical significance of a 0.5% segregation difference with a 57% probability.