Patterns of local adaptation of a protozoan parasite to its bumblebee host

Typically, microparasites being short-lived and more numerous can evolve more quickly than their hosts. They are therefore expected to adapt rapidly to the prevailing host environments. Under certain conditions; this will be reflected in higher virulence expressed in sympatric as compared to allopatric hosts. We tested this prediction with cross-infection experiments of the trypanosome intestinal parasite Crithidia bombi. Individual workers of colonies of the bumblebee Bombus terrestris from three different geographic regions were infected with the respective sympatric and allopatric parasites. We used host mortality rate, host body mass difference, prevalence and intensity of infection as measures of virulence. Contrary to expectation, allopatric infections caused higher host mortality rates overall, but differences among regions existed. Corresponding changes in host body mass during infection were found, but no differences in prevalence or intensities of sympatric vs allopatric infections were observed. When the experiment was repealed at a smaller geographic scale, within a single region, the opposite tendency (sympatric infections more virulent) but no significant effects were found. We discuss the results in relation to the complexity of effects in host-parasite interactions, where migration rates of both host and parasites, as well as genotypic variation play a role.