The degeneration of Y chromosomes

Y chromosomes are genetically degenerate, having lost most of the active ge nes that were present in their ancestors. The causes of this degeneration h ave attracted much attention from evolutionary theorists. Four major theori es are reviewed here: Muller's ratchet, background selection, the Hill-Robe rtson effect with weak selection, and the 'hitchhiking' of deleterious alle les by favourable mutations. All of these involve a reduction in effective population size as a result of selective events occurring in a non-recombin ing genome, and the consequent weakening of the efficacy of selection. Wie review the consequences of these processes for patterns of molecular evolut ion and variation at loci on Y chromosomes, and discuss the results of empi rical studies of these patterns for some evolving Y-chromosome and neo-Y-ch romosome systems. These results suggest that the effective population sizes of evolving Y or neo-Y chromosomes are severely reduced, as expected if so me or all of the hypothesized processes leading to degeneration are operati ve. It is, however, currently unclear which of the various processes is mos t important; some directions for future work to help to resolve this questi on are discussed.