Context dependent host-symbiont interactions: shifts along the parasitism-mutualism continuum

Symbiotic interactions can shift along a mutualism to parasitism continuum. While there are many studies examining dynamics typically considered to be mutualistic that sometimes shift towards parasitism, little is known about conditions underlying shifts from parasitism towards mutualism. In lake populations, we observed that infection by a microsporidian gut symbiont sometimes conferred a reproductive advantage and other times a disadvantage to its Daphnia host. We hypothesized that the microsporidian might benefit its host by reducing infection by more virulent parasites, which attack via the gut. In a lab study using field-collected animals, we found that spores of a virulent fungal parasite were much less capable of penetrating the guts of Daphnia harboring the microsporidian gut symbiont. We predicted that this altered gut penetrability could cause differential impacts on host fitness depending on ecological context. Using data from field surveys, we found that microsporidian-infec...