Parasites give snails an incentive for sex


P. antipodarum snail. Image via Wikipedia.

Among the New Zealand mud snail (Potamopyrgus antipodarum), some do it, some don’t. Have sex, that is.

Just how many snails live the sexual life is determined, in large part, by how many snails in the population are infected by parasites, according to new research by Kayla King, Jukka Jokela, and Curtis Lively. The findings are published in the current issue of the journal Evolution (See the abstract here).

Most mud snails are asexual and produce clones of themselves. Even though all the children of a single snail will be genetically identical, there is still a lot of genetic diversity  in the population. This means there is no winner as to who has the best set of genes (or, genotype).

One possible reason for this diversity is something called the “Red Queen Hypothesis.” Under this hypothesis, parasites would adapt to the most common snail genotype. This is bad news for the snail, because once it’s infected by a trematode parasite, the snail becomes sterile. So, there should be diversity in the population to guard against the parasite killing everyone off. It will take the parasite time to adapt to a new common genotype. In the meantime, hopefully the snails will find a way to get some more genetic diversity. (By the way, it’s called the Red Queen hypothesis because of the famous line by the Red Queen in ‘Alice in Wonderland’: “It takes all the running you can do, to keep in the same place.”)

So, where does the sex come in? The researchers collected snails from 17 isolated populations in New Zealand. What they found was that populations with higher levels of infection (especially infection of asexual snails) had higher percentages of sexual snails.

This suggests that there is some advantage to being sexual when there is a high frequency of infection. When individuals reproduce asexually, they pass on 100% of their genes to their offspring. From an evolutionary standpoint, this is a total win. But because of things like parasites and disease, most animals reproduce through sex, creating new combinations of genes that haven’t been seen yet by those nasty parasites.

The exact advantage that sex gives these snails is still a mystery. Even though parasite levels could explain both genetic diversity and frequency of sex, the frequency of sex couldn’t explain any of the genetic diversity (I know, it’s confusing). You might expect the new genetic combinations to come from the sexual snails, but this doesn’t seem to be directly the case (maybe they’re a backup plan?).

I guess more research is needed to really figure out why these snails keep sex alive at all, and why parasites give the snails an incentive for sex.

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