Parasites Explain Advantage of Sexual Reproduction

Scientists from Indiana University and the Swiss Federal Institute of Technology have provided a scientific rationale for the evolution of sex by studying the host-parasite relationship in the New Zealand snail. The unique ability of the snail, Potamopyrgus antipodarum, to hold asexual and sexual forms allowed scientists to confirm the strategic advantage of sexual reproduction over asexual reproduction. The results of this study were recently published in the July issue of the American Naturalist.

Many organisms within the animal kingdom display sexual reproduction. The theory to explain this trend was The Red Queen hypothesis, which explains that sexual reproduction allows for host species to develop new genetic defenses against parasites. Since asexual forms produce clones, all forms have the same genes. If the genotype of the clone exhibits vulnerability to a particular parasite, then the whole population could be wiped out. However, sexual forms are genetically unique. Therefore, one parasite cannot eliminate a population of sexual snails.

Scientists were able to use P. antipodarum to confirm this hypothesis. For ten years,

the scientists recorded the number of snails that exhibited either sexual or asexual reproduction, as well as the rate of parasite infection for a population of snails in New Zealand. The snails were then genotyped in order to distinguish the asexual clones from one another. The data show that under low parasitic pressure, the common asexual clones persisted while the sexual population of snails diminished.

However, under high parasitic pressure, the sexual population of snails had an advantage.

Asexual clones that were plentiful at the beginning of the study became more susceptible to the parasites over time. As parasite infections increased, the number of clones dramatically decreased. Meanwhile, the population of snails exhibiting sexual reproduction stayed relatively stable. Jukka Jokela of the Swiss Federal Institute of Aquatic Science and Technology said that, "These results suggest that sexual reproduction provides an evolutionary advantage in parasite rich environments." The host-parasite relationship and the sexual and asexual forms of the snail were key to proving that sexual reproduction is advantageous.


--Article revised 12/16/09

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