Anderson 9729144 The research goals of this project are to test hypotheses about the genetic forces that modulate variation of chromosomal alterations that modify gene order known as inversions. The chromosomal inversions on the third chromosome of Drosophila pseudoobscura have figured prominently in the literature of evolutionary biology for more than fifty years. They have proven to be a very useful model system for studies of geographic variation, natural selection, gene flow, and genetic drift. The inversions seem to represent a clear case of adaptive selection because they vary in frequency with the environment, form classical clines, and in some populations there are seasonal cycles of inversion frequency. The objective of this proposal is to determine how selection acts on the genetic contents of inversions within and among D. pseudoobscura populations. This research will discriminate between various models of selection by analyzing nucleotide diversity of nine PCR amplified markers uniformly distributed on the third chromosome of D. pseudoobscura. The nine markers will be sequenced in D. pseudoobscura strains collected from four populations that span an inversion frequency cline. Studies of molecular evolution over the past two decades have focussed on the genetic forces that shape diversity of single genes and have not considered if selection maintains assemblages of linked genes on chromosomes. It is an open question whether the association of genes on particular chromosomes within the genome happened due to selection or accident. Inversions provide an important mechanism to maintain the association of genetic diversity distributed across long distances of chromosomes. This project will provide valuable data on the genetic forces that modulate the evolution of large segments of the genome in natural populations.
