Balancing costs and benefits in a mutualism: Conditionality in the interaction between the grass, Hystrix patula Moench (Poaceae: Triticeae), and its fungal endophyte.

Abstract

The symbiosis between grasses and seed-borne fungal endophytes (Acremonium sp.) is considered a mutualism; the fungi receive nutrients from the grass and produce alkaloids that are toxic or deterrent to grass herbivores. Grass individuals infected by endophytes are assumed to consistently have higher fitness than non-infected individuals. However, if costs or benefits vary with the environment, the fungus-plant interaction may be conditional, shifting from mutualism (+,+) to parasitism (+,$-$). I found that natural grass populations of Hystrix patula contained both infected (I) and non-infected (NI) individuals, suggesting that the interaction might be conditional. I tested whether the host benefited or was harmed by the presence of the fungus by examining the relative fitness of I and NI individuals under different environmental conditions. The relative performance of I and NI plants depended on the availability of nitrogen and light to the plant, and herbivore pressure. Reducing nitrogen and light reduced survival, growth rates and biomass of infected plants more than non-infected plants. Reducing nitrogen resulted in higher survival of phloem-feeding insects on infected plants; infected plants were not as well protected from herbivores at low nitrogen. Reducing nitrogen reduced fungal biomass, suggesting that fungus and plant compete for limited nutrients. Infected planes did not compensate (reduced biomass/increased fruit abortion) to the same degree as non-infected plants for defoliation, especially at low resources. Under defoliation, infected plants maintained a constant fruit mass, while non-infected plants reduced fruit mass. In NI plants recruitment was seed-limited due to high seed predation. In contrast, seed predation was low in I plants, but seed production was resource-limited. Recruitment of I seedlings was largely limited to light gaps ($>$50% full sun). In light gaps, recruitment of I seedlings depended on the number of seeds, while under low light ($<$20% full sun), compensatory seedling mortality limited recruitment. Differences in recruitment limitation may explain the coexistence of both infected and non-infected plants in the same population. These results indicate that costs and benefits in this interaction vary with environmental conditions, and this variation changes the outcome for the host plant. Outcomes for the host range from positive to negative; in contrast with results in previous studies, costs for the host can be significant, and outcomes are often negative.