Gene content evolution on the X chromosome

Abstract

Compared with autosomes, the X chromosome shows different patterns of evolution as a result of its hemizygosity in males. Additionally, inactivation of the X during spermatogenesis can make the X chromosome an unfavorable location for male-specific genes. These factors can help to explain why in many species gene content of the X chromosome differs from that of autosomes. Indeed, the X chromosome in mouse is enriched for male-specific genes while they are depleted on the X in Drosophila but show neither of these trends in mosquito. Here, we will discuss recent findings on the ancestral and neo-X chromosomes in Drosophila that support sexual antagonism as a force shaping gene content evolution of sex chromosomes and suggest that selection could be driving male-biased genes off the X.

Figure 1

Karyotypes for D. melanogaster, D. pseudoobscura, and D. willistoni (males are shown). Chromosomes from which movement of male-biased genes has been observed are in gray (ancestral X chromosome or Muller element A) and in purple (the neo-X chromosome in D. pseudoobscura and D. willistoni, which evolved when Muller element D fused to the ancestral X chromosome).

Figure 2

Trends in sex-biased gene content on the X chromosome in different species. Karyotypes for the heterogametic sex are shown for each species. Blue arrows indicate the overall trend for male-specific genes; red arrows indicate the overall trend for female-specific genes. In the genus Drosophila, male-biased genes are depleted on the X chromosome while female-biased genes are enriched. C. elegans shows the same overall trend as Drosophila. Chicken is a species with ZW sex-determination system where the Z chromosome is enriched for male-biased genes and depleted for female-biased genes. A. gambiae has an even distribution of male-biased genes among the chromosomes. In the mouse, both male-biased and female-biased genes are enriched on the X.