Short-term fitness consequences of parasitism depend on host genotype and within-host parasite community.

Multi-parasite communities inhabiting individual hosts are common and often consist of parasites from multiple taxa. The effects of parasite community composition and complexity on host fitness are critical for understanding how host-parasite coevolution is affected by parasite diversity. To test how naturally-occurring parasites affect host fitness of multiple host genotypes, we performed a common-garden experiment where we inoculated four genotypes of host plant Plantago lanceolata with six microbial parasite treatments: three single parasite treatments, a fungal mixture, a viral mixture, and a cross-kingdom treatment. Seed production was affected by both host genotype and parasite treatment, and their interaction jointly determined growth of the hosts. Fungal parasites had more consistent negative effects than viruses in both single mixed parasite treatment. These results demonstrate that parasite communities have the potential to affect the evolution and ecology of host populations through their effects on host growth and reproduction. Moreover, the results highlight the importance of accounting for the diversity of parasites as well as host genotypes when aiming to predict the consequences of parasites for epidemics as the effects of multi-parasitism are not necessarily additive of single parasite effects, nor uniform across all host genotypes.