Steps in the evolution of heteromorphic sex chromosomes

Abstract

We review some recently published results on sex chromosomes in a diversity of species. We focus on several fish and some plants whose sex chromosomes appear to be 'young', as only parts of the chromosome are nonrecombining, while the rest is pseudoautosomal. However, the age of these systems is not yet very clear. Even without knowing what proportions of their genes are genetically degenerate, these cases are of great interest, as they may offer opportunities to study in detail how sex chromosomes evolve. In particular, we review evidence that recombination suppression occurs progressively in evolutionarily independent cases, suggesting that selection drives loss of recombination over increasingly large regions. We discuss how selection during the period when a chromosome is adapting to its role as a Y chromosome might drive such a process.

Patterns of Neutral Genetic Variation on Recombining Sex Chromosomes

“…That constraint could be offset, however, if regions of sex chromosomes close to the SDR show reduced recombination per DNA base. Even limited recombination may protect these regions flanking the SDR from the rapid accumulation of rearrangements and repetitive sequences often seen within the nonrecombining regions of Y chromosomes (Charlesworth et al 2005). If so, these flanking regions may be a fruitful focus for empirical research.…”

No amicable divorce? Challenging the notion that sexual antagonism drives sex chromosome evolution

“…In flowering plants, dioecy appears to have evolved from hermaphroditism on many separate occasions via gynodioecy or, more rarely, via androdioecy [4]. The existence of these transitional states implies that alleles causing male sterility and female sterility occur at separate loci [78].…”

“…In mammals, for example, recombination between the sex chromosomes appears to have been suppressed through a combination of inversions, translocations and suppression of crossing over by local modifiers [4][5][6]. In mammals, birds and some flowering plants, genetic divergence between the sex chromosomes decreases with distance from the sex-determination locus, with discontinuities producing evolutionary strata [5,7,8].…”

“…In mammals, birds and some flowering plants, genetic divergence between the sex chromosomes decreases with distance from the sex-determination locus, with discontinuities producing evolutionary strata [5,7,8]. Although strata may be caused by variation in the rate of gene conversion along the sex chromosomes [4], they are usually interpreted as evidence that recombination has been restricted in several stages [5,6]. This suggests that non-recombining regions have gradually expanded through cumulative recombination-suppressing events.…”

The Y chromosome as a target for acquired and amplified genetic material in evolution

“…(1 -3) To account for its evolution, several alternative hypotheses have been proposed to explain the driving forces of degeneration of the Y chromosomes that can occur as recombination with the X chromosome is increasingly blocked. (1,2,(4)(5)(6)(7) This problem has been discussed extensively by population geneticists and was not included in this review.…”