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 treatments. 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.