Experimental coevolution in a microbial host-parasite system: direct and indirect costs of resistance
O. Kaltz (1), K. Lohse (2), A. Gutierrez (1)
(1) Laboratoire de Parasitologie Evolutive, CNRS-UMR 7103, UniversitÚ Pierre et Marie Curie, 75252 Paris Cedex 05
(2) Environmental and Evolutionary Biology, School of Biology, University of St.Andrews, St. Andrews, Fife, KY16 9TH, UNITED KINGDOM

Host-parasite coevolution is thought to consist of cycles of adaptation and counter-adaptation, driven by frequency-dependent selection. This requires that different parasite genotypes perform differently on different host genotypes. Such genotype-by-genotype interactions arise if adaptation to one host (parasite) genotype reduces performance on others. These direct costs of adaptation can maintain genetic polymorphism and generate geographic patterns of local host or parasite adaptation. Fixation of all-resistant (or all-infective) genotypes is further prevented if adaptation trades offwith other host (or parasite) life history traits. For the host, such indirect costs of resistance refer to reduced fitness of resistant genotypes in the absence of parasites. We studied (co)evolution in experimental microcosms of several clones of the freshwater protozoan Paramecium caudatum, infected with the bacterial parasite Holospora undulata. After 2 1/2 years of culture, inoculation of evolved and na´ve (= never exposed to the parasite) hosts with evolved and founder parasites revealed an increase in host resistance, but not in parasite infectivity. A cross-infection experiment showed significant host clone x parasite isolate interactions, and evolved hosts tended to be more resistant to their own (local) parasites than to parasites from other hosts. Compared to na´ve clones, evolved host clones had lower division rates in the absence of the parasite. Thus, our study indicates de novo evolution of host resistance, associated with both direct and indirect costs. This illustrates how interactions with parasites can lead to the genetic divergence of initially identical populations.