sex ratio distortion

There are a couple of interesting papers (1, 2) over at PLoS biology (also discussed here ) studying a sex ratio distortion system. This is a type of mutation, which causes the sex ratio of the bearer's offspring to differ from the usual 50:50. If the mutation occurs on a sex chromosome such a mutation can be spread through the population. For example, a mutation on the X chromosome, which causes XY father to have more female offspring (i.e. XX offspring) will benefit itself as the offspring will carry the X chromosome more often. This can lead to situations where the population is very strongly biased towards a particular sex, e.g. females.

Now, as this sex-ratio mutation spreads through the population, mutations that block this distorting mutation can arise on the autosomes, returning an individual's sex ratio (of their offspring) to 50:50. These mutations are in turn strongly favoured. For example, if the population is mostly female (due to the sex-ratio mutation) and you start having sons, your sons will father a lot of children.

These papers study such a system between two closely related species of Drosophila. One of these species has both the sex-ratio mutation and the autosomal restorer mutation, while the other have neither. By performing crosses between the two species the authors unmask the sex-ratio mutation and autosomal mutation, and so can map the location in the genome of both mutations. They have characterized both the sex-ratio mutation on the X chromosome and the mutation on the autosomes that masks the effect returning the sex ratio to normal. It is a really pretty system.

The authors of the paper point to a number of other examples of sex-ratio distortion loci (and their suppressors) in Drosophila. These kind of conflicts are happening all of the time.
Another example of sex-ratio distortion are parasites (e.g. Wolbachia) that are transmitted only to daughters. These parasites often kill the male offspring to further their own transmission, and the host then evolves mechanisms to suppress these effects.

The rate at which these sex-ratio distorters evolve and are suppressed can be phenomenal. Here for example is a paper, which reports on a population of butterflies where the sex-ratio was distorted 100:1 and then is returned to 50:50 within 10 generations.

In fact one wonders whether evolution to resolve these conflicts is as common or more common than evolution in response the external environment. A sex-ratio distorting mutation can actually drive a population extinct if a suppressor does not arise in time.

I love these sex-ratio distortion systems as they are a wonderful example of evolution in action. They also show that evolution does not always act to improve an organism or a population, in fact the initial distorter is deleterious for the population.