Natural Selection, Not Mutation: Recombination in Drosophila Increases Diversity

Plants do it. Animals do it. People do it, too. During meiosis—the cell division that creates eggs and sperm—all these organisms mix the genetic material inherited from their mother and father. This mixing is called recombination, and it results from crossover events, where pairs of chromosomes swap genetic material. Evolutionary biologists have long known that recombination seems to increase genetic diversity. That is, high rates of recombination in a species are correlated with high rates of genetic diversity between individuals in that species. What has been less clear is how recombination leads to increased diversity. Do crossover events predominantly cause mutations, or is their primary effect to break up linkage between genes? Genes that are linked—near each other on the same chromosome—are more likely to be inherited together. By breaking up linkage, recombination makes it easier for natural selection to target individual genes while avoiding the potentially disadvantageous effect of simultaneously reducing diversity at neighboring genes (a phenomenon known as ‘‘Hill-Robertson interference’’). To assess whether recombination causes mutations, researchers have previously looked at recombination rates at certain points on the same chromosome of closely related species and assessed whether there was increased genetic divergence—genetic variation between species—at those points. They found no increase in divergence, leading to the conclusion that the association between diversity and recombination was not predominantly caused by mutations. However, these analyses suffered from the implicit assumption that recombination rates were the same in related species. Recently, a number of studies have found that this is not always the case; even in closely related species, local recombination rates can vary greatly, raising questions about work that has not taken this variability into account and leaving room for the mutation hypothesis to resurface. In this issue of PLOS Biology, Suzanne McGaugh, Mohamed Noor, and colleagues at Duke University and the University of Wisconsin-Madison settle the question, at least for fruit flies. Using relatives of the well-known Drosophila melanogaster, the researchers compared the local genetic variation within and between species (i.e., diversity and divergence, respectively) at regions of the flies’ chromosomes that they knew to have similar recombination rates. In both D. pseudoobscura and D. miranda, local recombination rates vary throughout the genome. For this experiment, McGaugh and colleagues created finescale maps of recombination rates for both species, covering 43 percent of the D. pseudoobscura genome, and 31 percent of D. miranda. Using these maps, they located