Molecular population genetics of X-linked genes in Drosophila pseudoobscura

This article presents a nucleotide sequence analysis of 500 bp determined i n each of five X-linked genes, runt, sisterlessA, period, esterase 5, and H eat-shock protein 83, in 40 Drosophila pseudoobscura strains collected from two populations. Estimates of the neutral migration parameter for the five loci show that gene flow among D. pseudoobscura populations is sufficient to homogenize inversion frequencies across the range of the species. Nucleo tide diversity at each locus fails to reject a neutral model of molecular e volution. The sample of 40 chromosomes included six Sex-ratio inversions, a series of three nonoverlapping inversions that are associated with a stron g meiotic drive phenotype. The selection driven by the Sex-ratio meiotic dr ive element has not fixed variation across the X chromosome of D. pseudoobs cura because, while significant linkage disequilibrium was observed within the sisterlessA, period, and esterase 5 genes, we did not find evidence for nonrandom association among loci. The Sex-ratio chromosome was estimated t o be 25,000 years old based on the decomposition of linkage disequilibrium between esterase 5 and Heat-shock protein 83 or 1 million years old based o n the net divergence of esterase 5 between Standard and Sex-ratio chromosom es. Genetic diversity was depressed within esterase 5 within Sex-ratio chro mosomes, while the four other genes failed to show a reduction in heterozyg osity in the Sex-ratio background. The reduced heterogeneity in esterase 5 is due either to its location near one of the Sex-ratio inversion breakpoin ts or that it is closely linked to a gene or genes responsible for the Sex- ratio meiotic drive system.