Abstract

Genetic variation for fitness-relevant traits may be maintained in natural populations by fitness differences that depend on environmental conditions.
For herbivores, plant quality and variation in chemical plant defences can maintain genetic variation in performance. Apart from plant secondary
compounds, symbiosis between plants and endosymbiotic fungi (endophytes) can produce herbivore-toxic compounds. We show that there is significant variation among aphid genotypes in response to endophytes by comparing life-history traits of 37 clones of the bird cherry-oat aphid Rhopalosiphum padi feeding on endophyte-free and endophyte-infected tall fescue Lolium arundinaceum.
Clonal variation for life-history traits was large, and most clones performed better on endophyte-free plants. However, the clones differed in
the relative performance across the two environments, resulting in significant
genotype · environment interactions for all reproductive traits. These findings suggest that natural variation in prevalence of endophyte infection can contribute to the maintenance of genetic diversity in aphid populations.