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-infected Daphnia hosts experienced a reproductive advantage when virulent parasites were common and a reproductive disadvantage when resources were scarce and virulent parasites were rare. Our findings highlight the importance of considering multiparasite community context and resource availability in host-parasite studies and open the door for future research into conditions driving shifts along parasitism to mutualism gradients.

Two data sets (MicG_benefit_Indiana and MicG_benefit_Michigan) share results of field sampling of Daphnia dentifera populations in lakes in Indiana and Michigan, respectively, from 2017 and 2016, respectively. We measured the prevalence of several parasites of D. dentifera on each sampling date and also recorded the egg counts in MicG-infected and uninfected adult asexual D. dentifera. MicG is an unidentified species of microsporidian, infecting the gut ephthelial cells of Daphnia.

Data have been processed to calculate disease prevalence and mean egg counts for each date. In addition, we calculated the "MicG Benefit" by comparing mean egg counts for MicG-infected/ uninfected Daphnia - 1. 

The third data set (GP_resources_infectionstatus) shows data from an experiment on field collected D. dentifera that were either uninfected or infected with MicG. We exposed these individuals to the fungal parasite Metschnikowia, observing the number of spores that attacked and successfully penetrated the gut wall of these animals. 

Data processing included identifying mean egg counts for uninfected animals from the field data (described above). We calculated gut penetrability as the fraction of spores that infected the gut/ the number that attacked the gut.