Abstract
Y chromosomes often degenerate via the accumulation of pseudogenes and transposable elements. By contrast, little is known about X-chromosome degeneration. Here we compare the pseudogenization process between genes on the neo-sex chromosomes in Drosophila miranda and their autosomal orthologues in closely related species. The pseudogenization rate on the neo-X is much lower than the rate on the neo-Y, but appears to be higher than the rate on the orthologous autosome in D. pseudoobscura. Genes under less functional constraint and/or genes with male-biased expression tend to become pseudogenes on the neo-X, indicating the accumulation of slightly deleterious mutations and the feminization of the neo-X. We also find a weak trend that the genes with female-benefit/male-detriment effects identified in D. melanogaster are pseudogenized on the neo-X, implying the masculinization of the neo-X. These observations suggest that both X and Y chromosomes can degenerate due to a complex suite of evolutionary forces.
Highlights
Y chromosomes often degenerate via the accumulation of pseudogenes and transposable elements
Since neo-sex chromosomes are in an early stage of sex chromosome evolution[3], they are ideal materials to trace pseudogenization events on the neo-X and neo-Y compared with an orthologous autosome in the closely related species D. pseudoobscura, with the autosome in D. obscura as an outgroup
To investigate the detailed process of pseudogenization of the neo-sex chromosomes in D. miranda, we focused on the three species, D. miranda, D. pseudoobscura and D. obscura
Summary
Y chromosomes often degenerate via the accumulation of pseudogenes and transposable elements. We find a weak trend that the genes with female-benefit/male-detriment effects identified in D. melanogaster are pseudogenized on the neo-X, implying the masculinization of the neo-X These observations suggest that both X and Y chromosomes can degenerate due to a complex suite of evolutionary forces. The degeneration process of the X as well as of the Y is essential for understanding the evolution of sex chromosomes after their emergence, it has been, in general, very difficult to trace the steps of degeneration evolutionarily This is because the sex chromosomes are ancient in many species, including in model organisms such as humans, mice and Drosophila melanogaster, which has hindered reconstruction of the ancestral states of sex chromosomes at their emergence. We here report that the rate of pseudogenization on the neo-X is higher than that on autosomes due to a complex suite of evolutionary forces including feminization and possibly masculinization
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