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Reduced adaptation of a non-recombining neo-Y chromosome


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 chromosome1. In response, the X chromosome may become dosage-compensated1,2. Most proposed mechanisms for the degeneration of Y chromosomes involve the rapid fixation of deleterious mutations on the Y1. Alternatively, Y-chromosome degeneration might be a response to a slower rate of adaptive evolution, caused by its lack of recombination3. 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 chromosome3. 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 genome1. 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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Figure 1: Phylogenetic relationships between the species investigated, constructed from the coding region of CycB.
Figure 2: Plots of the relative values of the log-likelihood functions ln L(kV), ln L(kS) and ln L(kV, ΔS) as a function of k, the reduction in the effective population size of the neo-Y chromosome relative to the neo-X.


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We thank P. Andolfatto, N. Barton, D. Charlesworth, I. Gordo, P. Keightley and S. Wright for helpful comments on the manuscript. D.B. is supported by a Marie Curie fellowship and B.C. by the Royal Society.

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Correspondence to Doris Bachtrog.

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Bachtrog, D., Charlesworth, B. Reduced adaptation of a non-recombining neo-Y chromosome. Nature 416, 323–326 (2002).

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