What parasites tell us about sex

The mud snail is no model of romance but it has helped scientists learn more about sex eawag.ch

Swiss-led scientists may be one step closer to finding the answer to a big evolutionary mystery – why do most species have sex to reproduce?

From his studies of snails, Jukka Jokela, from the Swiss Federal Institute of Aquatic Science and Technology near Zurich, has found evidence to suggest that sex may have partly evolved as a defence against parasites.

“It’s not clear what the advantage is of sexual reproduction,” evolutionary biologist Jokela told swissinfo.ch.

“Imagine you have a good genome and are successful in your environment. But then for some reason, at the point of reproduction, you decide to mix your genome by accepting some genes from someone else, basically random genes from the environment. It’s not clear why this is advantageous.”

Added to this is the fact that reproducing without sex – like microbes, some plants and even some fish and reptiles – would seem to be more efficient.

After all, every individual in an asexual species can reproduce on its own. Two – including a seemingly superfluous non-reproducing male – are needed in sexual reproduction.

Sex dominant

“The question is, why don’t these clones win? Why don’t they just reproduce more efficiently than the sexual lineages and what prevents them from taking over those populations where you have both asexual and sexual lineages?” Jokela said.

To help explain why sex remains so dominant in the living world, Jokela turned to the minute freshwater New Zealand mud snail Potamopyrgus antipodarum. This common snail is an ideal candidate as it has both asexual and sexual versions co-existing in the same environment.

Working with researchers from Washington State University and Indiana University, Jokela monitored various populations of the mud snail, paying special attention to parasite infection rates, over a period of ten years.

At first the clones were very successful, and seemed “to be winning the game”, Jokela said. Then, suddenly, they tended to decline, before disappearing from the natural system.

Clones’ decline

“What we found was that it was actually parasites, which were specialising through natural selection on these very common genotypes, that were driving the decline of these common clones,” he explained.

“So it appears that the sexually reproducing lineages are on average better at resisting this type of parasite which is detrimental to these clonal genotypes.”

Sexual snail populations had remained stable during this time.

The authors of the study, which appeared in the July edition of the American Naturalist journal, suggest that sexual reproduction therefore provides an evolutionary advantage in parasite-rich environments. In other words, mixing genes up via sex is a good defence mechanism.

Although the hypothesis that parasites keep asexual organisms from getting too plentiful had already been predicted by mathematical models, Jokela’s study was the first time it had ever been demonstrated in nature.

Pesky parasites

He said that studies in other systems would now be needed, as well as a better understanding of what happens at a genetic level when parasites invade an organism.

The research also gives food for thought. The importance of genetic diversity when it comes to disease interactions is something to bear in mind in our agricultural systems, which are very monoclonal and monocultural, the scientist added.

Most living organisms on the planet have to deal with some sort of detrimental disease or parasite. “So maybe one of the common denominators for why sexual reproduction is so widespread is that these types of pathogens, which can specialise at the genetic level, are a common phenomenon,” he said.

So there would seem to be some advantage to having these pesky parasites after all.

Isobel Leybold-Johnson in Zurich, swissinfo.ch

Parthenogenesis

The freshwater New Zealand mud snail (Potamopyrgus antipodarum) is about 5mm long and can reproduce either sexually or asexually.

“Normal” males and females live with triploid females (having three sets of chromosomes). The triploid females reproduce by parthenogenesis (“virgin birth”), cloning themselves to produce offspring.

In Europe, where the snail was introduced through shipments of live fish in the 1880s, only asexual reproduction has been observed to date. This could explain the mass occurrence and subsequent collapse of populations of this species on the continent.

In the 1970s, population densities of up to 100,000 snails per square metre were recorded in Lake Constance, which borders Switzerland. Today this snail occurs almost everywhere, but is not dominant in any place.