Asymmetric interactions between species and invasion.

First of all I should start by saying that everything on this blog (about peer-reviewed research) comes with the disclaimers 'I may well have misunderstood the paper or be incorrect in my thinking' and 'I think that this is interesting and novel, but I may have missed a flaw in the author's logic or prior work on this topic'. This is especially true when I talk about papers outside my area. While I'm pointing out (fairly obvious/pointless) disclaimers, spelling and grammar are not my strengths (especially as I'm trying to minimize my time writing this blog), so I apologize in advance for that. One of the reasons for writing this blog is to improve (and speedup) my science writing skills, so hopefully I'll get better with time.

Anyway all of that aside, here's an interesting article on the pre-release server in Science .

I came across it in a short segment in the Science magazine podcast, this might be a good way of hearing the main results if you can't access the article (one of the annoying things about blogging on non-open-access research). The paper discusses the invasion/replacement of one species of White fly with another. Lets call the invading species A and the species being replaced B. This replacement has happened quickly (in the early 90's and 00's) and in at least two distinct locations (China and Australia). The authors investigate one cause of this replacement, asymmetric breeding behavior between the two species.

The species are haploid-diploid, males are born from unfertilized eggs (hence they are haploid) while the females are born from fertilized eggs (diploid). Thus for a mother to give birth to females she has to mate.

The authors note that the two species (A and B) don't produce viable (female) offspring, and while they perform courtship they do not copulate. The fact that cross species pairings court is key as it means that A and B waste each other's time (if they occur in the same location). The authors perform experiments that show that the A females cope with this by mating a lot more, while B females fail to do this. This reduces the number of matings between B males and B females if species A is present, which reduces the number of fertilizations of the eggs of B females. This in turn reduces the number of B females in the population, while the A females mate more and so produce more females. This allows species A to (potentially) expand more rapidly and so replace species B. The authors record this sex-ratio change in both species in both the wild (when the overlap during the invasion), suggesting that this process seems to occur.

We often expect mating interaction between closely related species to be asymmetric so this is a really fun example of how mating interactions between closely related species can lead to the differential success of species.

Also I think that the species don't have to be haploid-diploid for this to be a factor in the survival of the species (though this form of sex-ratio change does require it). Imagine two closely related species C and D. Further, imagine that C males waste D females reproductive opportunities more than D males waste C females reproductive opportunities. Then all else being equal, C females will have more successful matings than D females and so species C will potentially out compete species D. This form of between species sabotage could take many forms, from simply getting in the way of courtship to successfully mating with females of the opposite species and making them invest in sterile/inviable offspring.

I'm guessing that these competition ideas have been explored in depth (the authors of the Science paper cite a couple of book chapters that I intend to read someday).

I think that these ideas also link into while females might be the ones to initially evolve reinforcement, which is the evolution of behavior to avoid mating with closely related species (see Coyne and Orr's Chapter on reinforcement for a discussion on potential differences between the sexes).

Thus the process of speciation and the survival of recently diverged species can both depend on these concepts.

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.