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Genetic degeneration of old and young Y chromosomes in the flowering plant Rumex hastatulus
- Hough, Josh, Hollister, Jesse D., Wang, Wei, Barrett, Spencer C. H., Wright, Stephen I.
- Proceedings of the National Academy of Sciences of the United States of America 2014 v.111 no.21 pp. 7713-7718
- Rumex, X chromosome, Y chromosome, alleles, amino acids, evolution, gene deletion, gene dosage, gene expression
- Heteromorphic sex chromosomes have originated independently in many species, and a common feature of their evolution is the degeneration of the Y chromosome, characterized by a loss of gene content and function. Despite being of broad significance to our understanding of sex chromosome evolution, the genetic changes that occur during the early stages of Y-chromosome degeneration are poorly understood, especially in plants. Here, we investigate sex chromosome evolution in the dioecious plant Rumex hastatulus , in which X and Y chromosomes have evolved relatively recently and occur in two distinct systems: an ancestral XX/XY system and a derived XX/XY ₁Y ₂ system. This polymorphism provides a unique opportunity to investigate the effect of sex chromosome age on patterns of divergence and gene degeneration within a species. Despite recent suppression of recombination and low X-Y divergence in both systems, we find evidence that Y-linked genes have started to undergo gene loss, causing ∼28% and ∼8% hemizygosity of the ancestral and derived X chromosomes, respectively. Furthermore, genes remaining on Y chromosomes have accumulated more amino acid replacements, contain more unpreferred changes in codon use, and exhibit significantly reduced gene expression compared with their X-linked alleles, with the magnitude of these effects being greatest for older sex-linked genes. Our results provide evidence for reduced selection efficiency and ongoing Y-chromosome degeneration in a flowering plant, and indicate that Y degeneration can occur soon after recombination suppression between sex chromosomes.