Cost of immunity in pea aphids associated with host plant interactions
Following parasitic infection, immune mechanisms are energetically costly to activate and maintain in an invertebrate host. The tradeoff between immunological investment and host fecundity has been proposed as a mechanism for host life history traits in nature. For example, certain morphs of pea aphids (Acyrthosiphon pisum) have been shown to produce fewer offspring when exposed to a heat-killed fungal pathogen (Zoophthora occidentalis). Furthermore, previous studies have shown that costs of immunity are context-dependent when placed in sub-optimal energy conditions. For example, starved, bacteria-infected bumblebee workers have been shown to have lower survival rates than nourished, bacteria-infected bumblebees. Here we combine the immunological investment-fecundity tradeoff and sub-optimal energy conditions by placing the idea of context-dependency to immune costs within a natural, ecological context. We look for immune costs on two host plants. Vicia fava (fava bean) is an ideal host plant for pea aphids. Trifolium pratense (red clover) is a non-ideal host plant on which aphids have been shown to have lower fitness. We used four genotypes, two of which are better adapted to living on the non-ideal host plant, and measured the lifetime fecundity of control and fungus-exposed aphids. We predicted that fungus-infected pea aphids that feed on clover will show a cost of immunity on clover (non-ideal host plant), but not on fava (ideal host plant). We also predicted that differently adapted genotypes will perform differently on the two host plants.
After 22 days spanning the lifetime of each aphid host, we found that fungus-infected pea aphids produced significantly more offspring on fava than on clover, suggesting that fava is a more optimal energy source than clover. Genotypes significantly influenced fecundity on clover, but not on fava, supporting the hypothesis that fungus-infected aphids of the genotypes better adapted to clover would be able to produce more offspring on clover than the two genotypes better adapted to fava. Interestingly, pathogen exposure imposed no costs on either host plant. This was expected for fava since it is an optimal energy source, but not for clover. These data suggest that fungus-infected aphids living on clover are not mounting an immune response either because they are too energy limited or that the clover is protective.