Extensive Gene Traffic on the Mammalian X Chromosome

TLDR

Mammalian sex chromosomes evolved from ancestral autosomes and have undergone profound changes. Retroposed genes on the mammalian X chromosome are disproportionately abundant and preferentially recruited, with many autosomal copies showing testis‑biased expression, indicating natural selection for male germline function alongside mutational bias.

Abstract

Mammalian sex chromosomes have undergone profound changes since evolving from ancestral autosomes. By examining retroposed genes in the human and mouse genomes, we demonstrate that, during evolution, the mammalian X chromosome has generated and recruited a disproportionately high number of functional retroposed genes, whereas the autosomes experienced lower gene turnover. Most autosomal copies originating from X-linked genes exhibited testis-biased expression. Such export is incompatible with mutational bias and is likely driven by natural selection to attain male germline function. However, the excess recruitment is consistent with a combination of both natural selection and mutational bias.