9707676 Rand The goal of the research program is to understand the evolutionary forces acting on variation in mitochondrial DNA (mtDNA). MtDNA is the most widely used genetic marker in population and evolutionary biology and it is generally assumed that the variation in mtDNA sequence is neutral, i.e., is not altered by natural selection. However, a number of recent reports have shown that mtDNA evolution departs from various neutral models. While non-neutral mtDNA evolution can compromise specific cases of evolutionary study, departures from neutral evolution offer opportunities to examine the variety of general evolutionary forces that can act on mtDNA itself, and on nuclear-cytoplasmic interactions. The main question the investigator is studying is: Does natural selection operate differently on interactions between nuclear sex chromosomes and maternally inherited cytoplasmic genes (such as mitochondrial DNA) than on interactions between autosomes and mitochondrial DNA (where an autosome is a biparentally inherited non-sex chromosome). Theoretical models of cytonuclear population genetics show that selection on cytoplasmic chromosomes and autosomes cannot preserve genetic variation. A new model shows that selection on combinations of mtDNAs and sex chromosomes can preserve variation. This model will be tested with large samples of fruit flies (Drosophila melanogaster) from natural populations in distinct habitats. Multiple-marker DNA analysis will be performed on individual fruit flies using markers spread across the length of the sex (i.e., X-chromosome) and autosomes (i.e., the 2nd and 3rd chromosomes). Additional DNA sequence data will be collected to test other models of natural selection on mildly deleterious mutations in mtDNA. These studies will provide basic data bearing on how chromosomal inheritance and effective population size affects evolutionary change, and may lead to a greater understanding of the general principles of chromosomal coevolution.
