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

Sex chromosomes have evolved independently in several different groups of o rganisms, but they share common features, including genetic degeneration of the Y chromosome(1,2). Suppression of recombination between ancestral prot o-X and proto-Y chromosomes is thought to have led to their gradual diverge nce, and to degeneration of the Y chromosome(2), but the evolutionary force s responsible are unknown. In non-recombining Y chromosomes, deleterious mu tations may be carried to fixation by linked advantageous mutations ("selec tive sweeps")(3). Occurrence of deleterious mutations may drive "Muller's r atchet" (stochastic loss of chromosomes with the fewest mutations)(2,4). Se lective elimination of deleterious mutations, causing "background selection "(5,6) may accelerate stochastic fixation of mildly detrimental mutations(2 ). All these processes lower effective population sizes, and therefore redu ce variability of genes in evolving Y chromosomes. We have studied DNA dive rsity and divergence in a recently described X- and Y-linked gene pair(7) ( 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.