Synchronous attack is advantageous: mixed genotype infections lead to higher infection success in trematode parasites.

Co-infecting parasite genotypes typically compete for host resources limiting their fitness. The intensity of such competition depends on whether parasites are reproducing in a host, or using it primarily as a transmission vehicle while not multiplying in host tissues (referred to as 'competition hypothesis'). Alternatively, simultaneous attack and co-infection by several parasite genotypes might facilitate parasite infection because such a diverse attack could present an additional challenge to host immune defence (referred to as 'facilitation hypothesis'). We tested the competition hypothesis by comparing the production of transmission stages (cercariae) from snails infected with one or two genotypes of the trematode Diplostomum pseudospathaceum. We found that cercarial production did not differ between the two groups of snails, suggesting lower per genotype production in double infections, and competition for host resources. Second, we tested the facilitation hypothesis by comparing parasite infection success on fishes (proportion of parasites establishing in the host) using cercariae originating from single-infected snails, double-infected snails and artificial mixtures of the single genotypes. In both cases, we found higher infection success when fishes were challenged with two parasite genotypes instead of one, supporting the facilitation hypothesis. Our results suggest that constraints defining the success of multiple genotype infections in parasites with multiple host life cycles include both between-genotype resource competition in the host and performance of host immune defences against a diverse parasite challenge.