Author(s): Louhi, K, Karvonen A, Rellstab C, Jokela J
Understanding the processes that maintain genetic variation in transmission traits of parasites is important as they are directly related to evolution of transmission efficiency and virulence. In addition to variation among parasite genotypes, environmental factors such as the condition of the previous host may also affect infection success of the parasite in the next host. The life cycle of many macroparasites includes multiple consecutive hosts and clonal stages that replicate within intermediate hosts. Parasites that have a clonal reproduction stage in their life cycle provide interesting opportunities to test to what extent variation in life-history traits has a genetic basis. This is because both environmental and genetic sources of variation are present in life-history traits among individual clones whereas variation in life-history traits of one clone should result only from environmental effects. In this study we measured transmission traits (i.e. production, activity, infection success and survival of the transmission stages) of parasite clones (Diplostomum pseudospathaceum), and tested experimentally whether manipulation of the external environment (food deprivation of the snail host) could explain variability in these traits. All measured traits were highly variable among the parasite clones prior to the food-treatment indicating significant genetic variation in these traits. Importantly, the traits were also phenotypically variable as there was marked temporal variation in the traits within each individual clone. However, food deprivation of the snail hosts had little effect on the parasite traits suggesting that a short-term reduction in host resources was not limiting the production rate or performance of the parasite clones . Overall, these results suggest significant interclonal and phenotypic variation in parasite transmission traits that are not affected by short changes in host nutritional status.