Parasites keep things sexy in 'hotspots'

Evolutionarily speaking, parasites make sex a worthwhile thing to do, according to a study published online on July 23rd in Current Biology, a Cell Press publication. The researchers report that freshwater snails, which can reproduce either sexually or asexually, are more likely to have sex if they live in the shallows, where the pressure from trematode parasites is more intense than it is in deeper waters. The findings lend support for the Red Queen hypothesis, which says that coevolving parasites reduce the reproductive advantage of asexual reproduction by adapting to infect clonal individuals after they become locally common. The results also show that the intensity of selection driven by interactions among species can vary sharply over relatively small spatial scales – lending support to another evolutionary idea known as the Geographic Mosaic Theory.

"Because the Red Queen hypothesis is based on species interactions, it follows that the Red Queen may operate within a geographic mosaic, resulting in sexual reproduction in coevolutionary hotspots, and asexual reproduction in coevolutionary coldspots," said Kayla King of Indiana University.

If all else were equal, sexual lineages would reproduce at half the rate of asexual ones, the researchers explained. That considerable cost of sex has led to lively debate among scientists about the reasons sex persists when sexual individuals face direct competition from asexual clones.

In the current study, the research team led by Curtis Lively wanted to see whether the Red Queen, acting within a geographic mosaic, could explain the maintenance of sex in the case of the snails, which are common to lakes in New Zealand. Both asexual and sexual snails can inhabit single lakes, wherein sexual snails dominate in shallow zones and asexual snails dominate in deeper ones. Interestingly, King said, there is also a higher infection rate of a trematode parasite (Microphallus sp.) in the shallow water.

They suspected that the shallow water might represent a coevolutionary hotspot because of the foraging behavior of ducks. Ducks are the parasite's final host and they feed only in shallow water. As a result, shallow snails are continuously exposed to infection, and successful parasites here more often reproduce. This provides the perfect scenario for coevolution, King said, because there is strong selection on both species as a result of their interaction.

The team exposed snails taken from shallow and deep habitats in two lakes to parasites derived from those lakes or one other. In theory, King explained, if parasites and snails are coevolving, then they should be more infective to same-lake snails than different-lake snails. And that's exactly what they found but on an even smaller scale: parasites from the same lake were significantly more infective to shallow-water snails than to deep-water snails, they show. Thus, it appears that snails living in deep water are completely removed from the coevolutionary interactions taking place in the shallows.

King sums the findings up this way: "Parasites play a role in the evolutionary maintenance of sex, and this can be seen on small spatial scales."

Source: Cell Press