séminaire du pôle évolution du vivant - vendredi 27 mars

Parasite coevolution and maintenance of sexual reproduction

Jukka Jokela, EAWAG, Swiss Federal Institute of Aquatic Sciences and Technology, Dübendorf (Suisse) et ETH-Zürich, Institute of Integrative Biology (Suisse)

vendredi 27 mars, 11 heures, amphi Mariotte

Sexual reproduction is a paradox because sexual populations should be vulnerable to invasion by ecologically similar asexual females. Asexual lineages have higher per-capita growth rates because they do not produce males, and do not need to invest in costly mating activities. Although in several groups of organisms occasional parthenogens are found, sexual reproduction dominates the world. One of the hypotheses to explain dominance of sex relates the advantage of sexual reproduction to avoidance of coevolving parasites. As asexual genotypes become common, they may also become disproportionately infected by parasites that have evolved to evade the host's genetically based self-nonself recognition system. Under this idea, high infection rates in the common asexual clones can periodically favor the genetically diverse sexual individuals (the Red Queen hypothesis), and may promote the short-term coexistence of sexual and asexual populations. Testing the idea requires comparison of sexual and asexual lineages, which are in direct competition. I will review results of a long term study that examines the clonal dynamics and parasite coevolution in a "mixed" (sexual and asexual) population of freshwater snails (Potamopyrgus antipodarum). We found that, within 7-10 years, the most-common clones were almost completely replaced by initially rare clones in two different habitats, while sexuals persisted throughout the study period. The common clones, which were initially more resistant to infection, also became more susceptible to infection to sympatric parasites over the course of the study. Overall, the results support the basic tenets of the Red Queen hypothesis for the maintenance of sex and show that the coevolutionary dynamics predicted by the theory may operate in mixed populations of sexuals and asexuals favoring sexual reproduction.