Four Evolutionary Strata on the Human X Chromosome

TLDR

Human sex chromosomes evolved from autosomes, retaining 19 ancestral autosomal genes as differentiated X‑Y homologs. The ages of X‑Y gene pairs correlate with their X chromosomal positions, revealing at least four evolutionary strata where recombination suppression occurred, with the earliest event ~240–320 Myr after the mammalian–avian split.

Abstract

Human sex chromosomes evolved from autosomes. Nineteen ancestral autosomal genes persist as differentiated homologs on the X and Y chromosomes. The ages of individual X-Y gene pairs (measured by nucleotide divergence) and the locations of their X members on the X chromosome were found to be highly correlated. Age decreased in stepwise fashion from the distal long arm to the distal short arm in at least four “evolutionary strata.” Human sex chromosome evolution was probably punctuated by at least four events, each suppressing X-Y recombination in one stratum, without disturbing gene order on the X chromosome. The first event, which marked the beginnings of X-Y differentiation, occurred about 240 to 320 million years ago, shortly after divergence of the mammalian and avian lineages.