Sex chromosomes in vertebrates: XX/XY against ZZ/ZW.

It was noted only recently that the mammalian X/Y sex chromosome system originated late in our therian ancestors. The question is, what makes the X/Y sex chromosome system so special that it has replaced the original Z/W sex chromosome system? Two classes of genes are overrepresented on the X chromosome: sex and reproduction genes and brain-related genes. The X chromosome has acquired bursts of young male-biased genes engaged in sex and reproduction which exemplifies the dynamics of sex chromosome evolution. Brain genes are old genes and display the most conserved transcriptome. The new therian X chromosome was formed by fusion of chromosome building blocks already bearing the highest density of brain genes in the ancestral vertebrate karyotype. These building blocks constitute an X chromosome haplotype that undergoes strong selection and benefits both sexes by oscillating between males and females. We believe that this strategy is superior to male-driven Z/W sex chromosome evolution. Copyright (C) 2011 S. Karger AG, Basel