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Abstract
The Sex-Ratio X chromosomes of Drosophila pseudoobscura and D.persimilis are subject to drive,resulting in an excess of female offspring from Sex-Ratio fathers. The several linked loci that cause the Sex-Ratio trait are held together by inversions greatly reducing recombination between the Standard and Sex-Ratio X chromosomes. The Esterase-5 gene region,tightly linked to the Sex-Ratio complex,was used to infer Sex-Ratio molecular evolution in the sibling species D.pseudoobscura and D.persimilis . We propose that the Sex-Ratio complex in D.persimilis was stabilized when an inversion swept to fixation on the Standard X chromosome.Our data support the hypothesis that the Sex-Ratio chromosome in D.persimilis has a monophyletic origin and diverged from the Standard X chromosome approximately 400,000 years ago.This is particularly striking as the Sex-Ratio chromosome of its sibling species,D.pseudoobscura ,diverged almost a million years earlier.