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 densi…