BEHAVIOURAL ECOLOGY

Abstract Mixed infections, where more than one para-site genotype is present in a single host, have been sug-gested to be an important factor in host–parasite interactions. As the host represents a limited resource, co-infecting parasite genotypes are expected to be under resource competition. Competition will not only modify the survival of the co-infecting genotypes, but is also likely to affect total within-host parasite growth as well as host survival and reproduction. We measured parasite infectivity and spore production of seven isolates of Octosporea bayeri (Microsporidia) and their effect on the reproduction and longevity of its host Daphnia magna (Cladocera), after single- or double-isolate inoculations through vertical and horizontal transmission. Double-isolate inoculations increased parasite infectivity and total spore production in horizontal infections, but had no significant effect on host reproduction or longevity. The increase in spore production in double-isolate inoculations was not found when infections occurred vertically. Our results suggest that, depending on the way the infection was acquired, within-host reproduc-tion can increase as a result of parasite genetic diversity, without necessarily affecting the host’s fitness. Whether this challenges the current views of virulence evolution depends on the definitions used