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Abstract / Description of output
In the putatively ancestral population of Drosophila melanogaster, the ratio of silent DNA sequence diversity for X-linked loci to that for autosomal loci is approximately one, instead of the expected value of three-quarters. One possible explanation for this result is that background selection (the hitchhiking effect of deleterious mutations) is more effective on autosomes than on the X chromosome, because of the lack of crossing over in male Drosophila. The expected effects of background selection on neutral variability at sites in the middle of an X chromosome or an autosomal arm were calculated for different models of chromosome organization and methods of approximation, using current estimates of the deleterious mutation rate and the distribution of the fitness effects of deleterious mutations. The robustness of the results to different distributions of fitness effects, dominance coefficients, mutation rates, mapping functions and chromosome size were investigated. It was found that the predicted ratio of X-linked to autosomal variability is relatively insensitive to these variables, except for the mutation rate. Provided that the deleterious mutation rate per genome is sufficiently large, it seems likely that background selection can account for the observed X to autosome ratio of variability in the ancestral population of D. melanogaster. The fact that this ratio is much less than one in D. pseudoobscura is also consistent with the model’s predictions, since this species has a high rate of crossing over. The results suggest that background selection may play a major role in shaping patterns of molecular evolution and variation.
Keywords / Materials (for Non-textual outputs)
- Background selection, Drosophila, effective population size, genetic recombination, neutral diversity