Measuring parasite fitness under genetic and thermal variation.

Accurate measures of parasite fitness are essential to study host-parasite evolution. Parasite fitness depends on several traits involved in establishing infection, growth and transmission. Individually, these traits provide a reasonable approximation of fitness, but they may also be under the shared control of both host and parasite genetics (G(H) x G(P) interactions), or be differentially sensitive to environmental variation. Using the natural host-parasite system Daphnia magna-Pasteuria ramosa, we performed experimental infections that incorporated host and parasite genetic variation at three different temperatures, and compared the measures of parasite fitness based only on growth rate, or incorporating the ability to infect. We found that infectivity was most important for parasite fitness and depended mainly on the combination of host and parasite genotypes. Variation in post-infection parasite growth and killing time depended on the parasite genotype and its interaction with temperature. These results highlight the merits of studies that can incorporate natural infection routes and emphasize that accurate measures of parasite fitness require knowledge of the genetic control and environmental sensitivity of more than one trait. In addition, no G(H) x G(P) x E interactions were present, suggesting that the potential for genetic specificities to drive frequency-dependent coevolution in this system is robust to thermal variation.