Complex evolutionary history of the Y chromosome in flies of theDrosophila obscuraspecies group

Abstract

TheDrosophila obscuraspecies group shows dramatic variation in karyotype, including transitions among sex chromosomes. Members of theaffinisandpseudoobscurasubgroups contain a neo-X chromosome (a fusion of the X with an autosome), and it was shown that ancestral Y genes of Drosophila have become autosomal in species that contain the neo-X. Detailed analysis in species of thepseudoobscurasubgroup revealed a translocation of ancestral Y genes to the small dot chromosome of that group. Here, we show that the Y-dot translocation is restricted to thepseudoobscurasubgroup, and translocation of Y genes in theaffinissubgroup followed a different route. We find that most ancestral Y genes moved independently to autosomal or X-linked locations in different taxa of theaffinissubgroup, and we propose a dynamic model of sex chromosome formation and turnover in theobscuraspecies group. Our results show that Y genes can find unique paths to escape an unfavorable genomic environment.