Journal
NATURE
Volume 416, Issue 6878, Pages 323-326Publisher
NATURE PUBLISHING GROUP
DOI: 10.1038/416323a
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Sex chromosomes are generally believed to have descended from a pair of homologous autosomes. Suppression of recombination between the ancestral sex chromosomes led to the genetic degeneration of the Y chromosome(1). In response, the X chromosome may become dosage-compensated(1,2). Most proposed mechanisms for the degeneration of Y chromosomes involve the rapid fixation of deleterious mutations on the Y-1. Alternatively, Y-chromosome degeneration might be a response to a slower rate of adaptive evolution, caused by its lack of recombination(3). Here we report patterns of DNA polymorphism and divergence at four genes located on the neo-sex chromosomes of Drosophila miranda. We show that a higher rate of protein sequence evolution of the neo-X-linked copy of Cyclin B relative to the neo-Y copy is driven by positive selection, which is consistent with the adaptive hypothesis for the evolution of the Y chromosome(3). In contrast, the neo-Y-linked copies of even-skipped and roundabout show an elevated rate of protein evolution relative to their neo-X homologues, probably reflecting the reduced effectiveness of selection against deleterious mutations in a non-recombining genome(1). Our results provide evidence for the importance of sexual recombination for increasing and maintaining the level of adaptation of a population.
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