Abstract
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.
Highlights
The human X and Y chromosomes evolved from a pair of autosomes approximately 180 million years ago
This suggests that the majority of orthologous genes are under purifying selection and related to a small set of functions
In conjunction with existing literature, has shown that evolutionary trends believed to be unique to the Y chromosome are observed in the events following large-scale duplications and are still present in mammalian ortholog comparisons
Summary
The human X and Y chromosomes evolved from a pair of autosomes approximately 180 million years ago. The remaining functional genes are ubiquitously expressed, functionally coherent, dosage-sensitive genes, or have evolved male-specific functionality It is unknown, whether functional specialization is a degenerative phenomenon unique to sex chromosomes, or if it conveys a potential selective advantage aside from sexual antagonism. The factors that led to the biased retention of ubiquitously expressed single-copy genes, appear not to be restricted to the evolutionary history of the Y chromosome and have been observed in the events following large scale duplications. Subfunctionalization has been shown to increase the likelihood a gene will be preserved in duplicate due to partial loss of function mutations in both copies[46] This targeted divergence of the duplicates may lead to differential tissue expression of the paralogs[34,35,45,47,48,49] and has been proposed to occur frequently following WGD events[50]. The biased content of male reproductive genes on both sex chromosomes[4,56,57,58], suggests that subfunctionalization of Y-linked genes could explain the initial retention and accelerated divergence of male-advantage genes, as new evolutionary features typically bear marks of their ancestry[50]
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