Sex to keep parasites at bay
31 July 2009
One of the most fascinating questions in biology is why large parts of the animal and plant kingdom go to the trouble of reproducing sexually. A study led by Eawag has now shown that, in the long run, sexual reproduction offers advantages in protecting populations against parasites.
Although sexual reproduction plays a central role in biology, the motivation for sex remains one of the most puzzling aspects of evolution. Many plants, microbes and even some reptiles can also reproduce asexually. At first glance, this would seem to be more efficient, as it does not require the conjunction of two individuals, only one of which then produces offspring. Nonetheless, sexual reproduction is the dominant strategy. Why should this be so, when asexual reproduction permits a reproduction rate that is twice as high, as well as avoiding other complications?
Jukka Jokela, an evolutionary biologist at Eawag, has been studying various aquatic snails for the best part of 20 years. One freshwater species, the New Zealand mud snail Potamopyrgus antipodarum, was introduced to Europe with shipments of live fish around 1880 and has subsequently spread across the entire continent. What is peculiar about this snail (which measures a mere 5 mm) is that – at least in its country of origin – it can reproduce either sexually or asexually. Together with researchers from Washington State University and Indiana University, Jokela monitored various populations of the mud snail, paying special attention to parasite infection rates. They found that the different snail populations do not cope equally well with parasites. Among the sexual types, no major fluctuations were seen in the number of individuals over a period of several years. Among the asexual organisms, meanwhile, initially plentiful types were superseded by other clonal lineages within just a few years: the formerly dominant lines had become particularly vulnerable to parasites. Put differently, the parasites targeted the most abundant host.
Although the pattern had already been predicted by mathematical models, this is the first time it has been demonstrated in nature. According to Jokela, “These results suggest that sexual reproduction provides an evolutionary advantage, especially in parasite-rich environments.”
Parthenogenesis
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The minute freshwater snail Potamopyrgus antipodarum can reproduce either sexually or asexually. In New Zealand, “normal” males and females live alongside triploid females (i.e. having three sets of chromosomes). The latter reproduce solely by parthenogenesis (“virgin birth”) – cloning themselves to produce offspring. All forms of this snail are infected by a large number of parasitic trematodes. In Europe, only asexual reproduction has been observed to date. This could also provide an explanation for the mass occurrence and subsequent collapse of populations of this species. In the 1970s, densities of up to 100,000 per square metre were recorded in Lake Constance. Today, this snail occurs almost everywhere, but is not dominant anywhere. |
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The roughly 5 mm long New Zealand mud snail Potamopyrgus antipodarum (left) and cysts of the parasitic trematode Microphallus spp.
- Original publication in the July issue of The American Naturalist, Vol. 174; Jukka Jokela, Mark F. Dybdahl, Curtis M. Lively: The Maintenance of Sex, Clonal Dynamics, and Host-Parasite Coevolution in a Mixed Population of Sexual and Asexual Snails; [pdf]
- Further information: Professor Jukka Jokela: Tel. 044 823 51 71, jukka.jokela@eawag.ch