Selection driven differentiation between the neo-sex chromosomes in Drosophila americana. B.F. McAllister. Dept Biological Sci, Univ Iowa, Iowa City, IA.

   Differentiated sex chromosomes are present in a broad range of organisms. Universal mechanisms presumably shaped the formation of sex chromosomes in these independent lineages, because of the consistent differences observed between these pairs of chromosomes. A chromosomal translocation has generated a new sex chromosome pair in Drosophila americana. To examine factors influencing the initial differentiation between sex chromosomes, patterns of DNA sequence variation were examined at three loci distributed along this neo-sex-chromosome pair. Sequences were obtained from the Alcohol dehydrogenase (Adh), big brain (bib), and timeless (tim) gene regions. In the northwestern species range of D. americana, the bib gene region on neo-X chromosomes is significantly differentiated relative to neo-Y chromosomes from the same geographic region, and other chromosomal samples of D. americana. The differentiated region exhibits low nucleotide variation, and the existing variants are non-neutrally distributed. Measurements of DNA polymorphism indicate an excess of low frequency variants in the bib gene region, and an excess of these variants represent the ancestral nucleotide state. These observations are consistent with a selective sweep occurring on the neo-X chromosome and affecting nucleotide variation in the bib gene region. Adaptive evolution limited to the neo-X chromosome is apparently driving the initial differentiation between the neo-sex chromosomes in D. americana. Models of sex-chromosome evolution based on sexual antagonism predict such a pattern.