The human X and Y chromosomes evolved from a pair of autosomes approximately 180 million years ago. Despite their shared evolutionary origin, extensive genetic decay has resulted in the human Y chromosome losing 97% of its ancestral genes while gene content and order remain highly conserved on the X chromosome. Five 'stratification' events, most likely inversions, reduced the Y chromosome's ability to recombine with the X chromosome across the majority of its length and subjected its genes to the erosive forces associated with reduced recombination. The remaining functional genes are ubiquitously expressed, functionally coherent, dosage-sensitive genes, or have evolved male-specific functionality. It is unknown, however, whether functional specialization is a degenerative phenomenon unique to sex chromosomes, or if it conveys a potential selective advantage aside from sexual antagonism. We examined the evolution of mammalian orthologs to determine if the selective forces that led to the degeneration of the Y chromosome are unique in the genome. The results of our study suggest these forces are not exclusive to the Y chromosome, and chromosomal degeneration may have occurred throughout our evolutionary history. The reduction of recombination could additionally result in rapid fixation through isolation of specialized functions resulting in a cost-benefit relationship during times of intense selective pressure.
Conflict of interest statement
The authors declare no competing financial interests.
Strict evolutionary conservation followed rapid gene loss on human and rhesus Y chromosomes.Nature. 2012 Feb 22;483(7387):82-6. doi: 10.1038/nature10843. Nature. 2012. PMID: 22367542 Free PMC article.
The Biology and Evolution of Mammalian Y Chromosomes.Annu Rev Genet. 2015;49:507-27. doi: 10.1146/annurev-genet-112414-055311. Epub 2015 Oct 6. Annu Rev Genet. 2015. PMID: 26442847 Review.
Ancestral Chromatin Configuration Constrains Chromatin Evolution on Differentiating Sex Chromosomes in Drosophila.PLoS Genet. 2015 Jun 26;11(6):e1005331. doi: 10.1371/journal.pgen.1005331. eCollection 2015 Jun. PLoS Genet. 2015. PMID: 26114585 Free PMC article.
Evolution and survival on eutherian sex chromosomes.PLoS Genet. 2009 Jul;5(7):e1000568. doi: 10.1371/journal.pgen.1000568. Epub 2009 Jul 17. PLoS Genet. 2009. PMID: 19609352 Free PMC article.
The evolution of heteromorphic sex chromosomes.Biol Rev Camb Philos Soc. 1990 Aug;65(3):249-76. doi: 10.1111/j.1469-185x.1990.tb01426.x. Biol Rev Camb Philos Soc. 1990. PMID: 2205302 Review.