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, Université 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 naïve (= 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 naïve 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.