A selective sweep in or near the Silene latifolia X-linked gene SlssX

The most prominent feature of Y chromosomes is that they do not recombine and are usually genetically degenerate, containing only a few genes. White campion Silene latifolia has evolved sex chromosomes relatively recently, probably within the last 10-15 million years. Perhaps due to its recent origin, the Y chromosome in this species has not completely degenerated and most isolated X-linked genes have intact Y-linked homologues. A gene encoding a protein with strong homology to spermidine synthases, Slss, is the exception to this rule, as the Y-linked copy of this gene has apparently lost its function. Here I report evidence for a recent selective sweep in the X-linked copy of this gene (SlssX) that could reflect compensatory evolution in an X-linked gene that has lost a functional Y-linked homologue. The spread and fixation of an advantageous mutation in SlssX has resulted in a dramatic loss of genetic diversity and an excess of high-frequency derived polymorphisms in this gene. As the sweep has not affected the closely linked DD44X gene, the selective advantage of the mutation that has driven the sweep in the SlssX gene might have been less than 1%.