Low variability in a Y-linked plant gene and its implications for Y-chromosome evolution

Abstract

Sex chromosomes have evolved independently in several different groups of organisms, but they share common features, including genetic degeneration of the Y chromosome^1,2. Suppression of recombination between ancestral proto-X and proto-Y chromosomes is thought to have led to their gradual divergence, and to degeneration of the Y chromosome², but the evolutionary forces responsible are unknown. In non-recombining Y chromosomes, deleterious mutations may be carried to fixation by linked advantageous mutations (“selective sweeps”)³. Occurrence of deleterious mutations may drive “Muller's ratchet” (stochastic loss of chromosomes with the fewest mutations)^2,4. Selective elimination of deleterious mutations, causing “background selection”^5,6 may accelerate stochastic fixation of mildly detrimental mutations². All these processes lower effective population sizes, and therefore reduce variability of genes in evolving Y chromosomes. We have studied DNA diversity and divergence in a recently described X- and Y-linked gene pair⁷ (SLX-1 and SLY-1) of the plant Silene latifolia to obtain evidence about the early stages of Y degeneration. Here we show that DNA polymorphism in SLY-1 is 20-fold lower than in SLX-1, but the pattern of polymorphism does not suggest a selective sweep.