A hundred years after it was found that male and female genomes differ in the rate at which they recombine, a cause has been found for this widespread phenomenon. The explanation lies not, as it was thought 80 years ago, in the nature of the sex chromosomes, but on the selective pressures acting on oocytes and sperm.

Sex differences in recombination rate are seen in animals and plants up and down the evolutionary scale — most famous, perhaps, is the lack of recombination in the male Drosophila melanogaster, which is used routinely to propagate mutations without the risk of them being shuffled about by recombination. Haldane and Huxley were the first to propose an explanation for this pattern: based on a small data sample they concluded that the heterogametic sex always recombines at a lower rate, and that this occurs because the lack of recombination between the sex chromosomes has spread to affect recombination between autosomes. However, this theory is lacking in many respects; perhaps the most convincing argument against it is that species that have no sex chromosomes also show differences in meiosis between males and females. Now, the availability of detailed linkage maps for a greater number of species has allowed us to revisit this long-standing enigma.

A dataset from 107 species of animals and plants reveals recombination patterns that would not have been available to Haldane or Huxley, and allow several new conclusions to be drawn. In the study, the rate of recombination in one species — as measured using chiasma number or map length — was correlated to the type of sex chromosomes present in that species and to the degree of competition that exists between gametes of the same sex (a measure of the selective force acting on gametes).

In plants at least, differences in the recombination rate between the sexes relate to the opportunity for selection on the haploid phase of an organism's life cycle. For example, in species where selection tends to be milder among male gametes than among female gametes (such as, in highly selfing compared with outcrossing species) then you would expect to see a higher recombination rate in males than in females. This is because of the advantage conferred by preserving, as much as possible, the existing gene combinations in both gametes. A second conclusion is that the lack of recombination seen in one sex of some species is not merely an extreme case of reduced recombination, but arises from qualitatively different evolutionary forces — this was deduced from the fact that absence of recombination in one sex is influenced by the nature of the sex chromosomes, whereas a lower rate of recombination is not.

The idea that differences in recombination rate between the sexes could be down to haploid selection is not new. However, this work has provided strong empirical evidence that — although it does not provide a causal link — substantiates this idea, and also ties together many apparently contradictory observations.