The recombination gender gap
Males and females of the same species can be strikingly different. Even the amount of genetic reshuffling that goes on during egg and sperm production differs between males and females in most species. An evolutionary reason for this has eluded researchers since the phenomenon was originally discovered in fruitflies, Chinese silk worms, and amphipods almost 100 years ago. In a new paper in PLoS Biology, Thomas Lenormand and Julien Dutheil provide evidence that sexual selection may provide the answer.
An early 20th century hypothesis to explain the sex difference in recombination proposed that recombination is restrained within a pair of unlike sex chromosomes (X and Y, for example) and that the suppression spills over to the rest of the chromosomes. In this case, the sex with dissimilar sex chromosomes (XY instead of XX, for example) should be the one with the least amount of recombination in all chromosomes. But this doesn't actually explain all the examples of sex-biased recombination. For example, some hermaphroditic species of flatworms lack sex chromosomes altogether but still display marked differences in male and female recombination rates. Lenormand and Dutheil realized that selection was not necessarily limited to the adult stage and that differences in selection among gametes - eggs or sperm - might help account for recombination differences between the sexes.
By analyzing a dataset of 107 plants and animals, they showed that the opportunity for selection to act on a gamete can influence the rate of recombination. For example, in some plants, such as pines, where females have lower recombination rates than males, the female gametes can experience strong selection because the ovules compete with each other for resources over an entire year before being fertilized. The authors thus provide the first empirical evidence for a theory explaining male and female differences in the amount of recombination and help to solve contradictory observations that have puzzled geneticists for almost a century.
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