Sex biased gene expression and recent sex chromosome turnover
Cichlids are well known for their propensity to radiate generating arrays of morphologically and ecologically diverse species in short evolutionary time. Following this rapid evolutionary pace, cichlids show high rates of sex chromosome turnover. We here studied the evolution of sex-biased gene expression in recently diverged taxa of the Lake Tanganyika Tropheini cichlids, which show different XY sex chromosomes. Across species, sex chromosome sequence divergence predates divergence in expression between the sexes. Only one sex chromosome, the oldest, showed signs of demasculinisation in gene expression and potentially contribution to the resolution of sexual conflict. Sex-biased genes in general showed high rates of turnovers and evolved mostly under drift. Sexual selection did not shape the rapid evolutionary changes of sex-biased genes. Male-biased genes evolved faster than female-biased genes, which seem to be under more phylogenetic constraint. We found a relationship between the degree of sex-bias and sequence evolution driven by sequence differences among the sexes. Consistent with other species, strong sex-bias towards sex-limited expression contributes to resolving sexual conflict in cichlids.