Parasite-mediated selection in a natural metapopulation of Daphnia magna

Parasite-mediated selection varying across time and space in
metapopulations is expected to result in host local adaptation and the
maintenance of genetic diversity in disease-related traits. However,
non-adaptive processes like migration and extinction-(re)colonization
dynamics might interfere with adaptive evolution. Understanding how
adaptive and non-adaptive processes interact to shape genetic variability
in life-history and disease-related traits can provide important insights
into their evolution in subdivided populations. Here we investigate
signatures of spatially fluctuating, parasite-mediated selection in a
natural metapopulation of Daphnia magna. Host genotypes from infected and
uninfected populations were genotyped at microsatellite markers, and
phenotyped for life-history and disease traits in common garden
experiments. Combining phenotypic and genotypic data a QST-FST-like
analysis was conducted to test for signatures of parasite mediated
selection. We observed high variation within and among populations for
phenotypic traits, but neither an indication of host local adaptation nor
a cost of resistance. Infected populations have a higher gene diversity
(Hs) than uninfected populations and Hs is strongly positively correlated
with fitness. These results suggest a strong parasite effect on reducing
population level inbreeding. We discuss how stochastic processes related
to frequent extinction-(re)colonization dynamics as well as host and
parasite migration impede the evolution of resistance in the infected
populations. We suggest that the genetic and phenotypic patterns of
variation are a product of dynamic changes in the host gene pool caused by
the interaction of colonization bottlenecks, inbreeding, immigration,
hybrid vigor, rare host genotype advantage and parasitism. Our study
highlights the effect of the parasite in ameliorating the negative fitness
consequences caused by the high drift load in this metapopulation.