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  • Title: Plant Y chromosome degeneration is retarded by haploid purifying selection.
    Author: Chibalina MV, Filatov DA.
    Journal: Curr Biol; 2011 Sep 13; 21(17):1475-9. PubMed ID: 21889890.
    Abstract:
    Sex chromosomes evolved many times independently in many different organisms [1]. According to the currently accepted model, X and Y chromosomes evolve from a pair of autosomes via a series of inversions leading to stepwise expansion of a nonrecombining region on the Y chromosome (NRY) and the consequential degeneration of genes trapped in the NRY [2]. Our results suggest that plants represent an exception to this rule as a result of their unique life-cycle that includes alteration of diploid and haploid generations and widespread haploid expression of genes in plant gametophytes [3]. Using a new high-throughput approach, we identified over 400 new genes expressed from X and Y chromosomes in Silene latifolia, a plant that evolved sex chromosomes about 10 million years ago. Y-linked genes show faster accumulation of amino-acid replacements and loss of expression, compared to X-linked genes. These degenerative processes are significantly less pronounced in more constrained genes and genes that are likely exposed to haploid-phase selection. This may explain why plants retain hundreds of expressed Y-linked genes despite millions of years of Y chromosome degeneration, whereas animal Y chromosomes are almost completely degenerate.
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