The maintenance of intraspecific biodiversity: the interplay of selection on resource use and on natural enemy resistance in the pea aphid

Secondary Symbionts Affect Foraging Capacities of Plant-Specialized Genotypes of the Pea Aphid

Ecological specialization is widespread in animals, especially in phytophagous insects, which have often a limited range of host plant species. This host plant specialization results from divergent selection on insect populations, which differ consequently in traits like behaviors involved in plant use. Although recent studies highlighted the influence of symbionts on dietary breadth of their insect hosts, whether these microbial partners influence the foraging capacities of plant-specialized insects has received little attention. In this study, we used the pea aphid Acyrthosiphon pisum, which presents distinct plant-specialized lineages and several secondary bacterial symbionts, to examine the possible effects of symbionts on the different foraging steps from plant searching to host plant selection. In particular, we tested the effect of secondary symbionts on the aphid capacity (1) to explore habitat at long distance (estimated through the production of winged offspring), (2) to explore habitat at short distance, and (3) to select its host plant. We found that secondary symbionts had a variable influence on the production of winged offspring in some genotypes, with potential consequences on dispersal and survival. By contrast, symbionts influenced both short-distance exploration and host plant selection only marginally. The implication of symbionts' influence on insect foraging capacities is discussed.

Predation determines different selective pressure on pea aphid host races in a complex agricultural mosaic

Field assessments were conducted to examine the interplay between host plant and predation in complex agricultural mosaic on pea aphid clover and alfalfa races. In one experiment, we examined the relative fitness on clover race (CR) and alfalfa race (AR) pea aphids on broad bean, red clover and alfalfa alone. But because clover is typically grown in a more complex agricultural mosaic with alfalfa and broad bean, a second experiment was conducted to assess the fitness consequences under predation in a more complex agricultural field setting that also included potential apparent competition with AR pea aphids. In a third experiment we tested for the effect of differential host race density on the fitness of the other host race mediated by a predator effect. CR pea aphids always had fitness losses when on broad bean (had lower fitness on broad bean relative to red clover) and fitness benefits when on red clover (higher fitness on red clover relative to broad bean), whether or not in apparent competition with alfalfa race aphids on bean and alfalfa. AR suffered fitness loss on both alfalfa and bean in apparent competition with CR on clover. Therefore we can conclude that the predation rate between host races was highly asymmetrical. The complexity of the agricultural mosaic thus can influence prey selection by predators on different host plants. These may have evolutionary consequences through context dependent fitness benefits on particular host plants.

Consequences of variation in plant defense for biodiversity at higher trophic levels

Antagonistic interactions between insect herbivores and plants impose selection on plants to defend themselves against these attackers. Although selection on plant defense traits has typically been studied for pairwise plant-attacker interactions, other community members of plant-based food webs are unavoidably affected by these traits as well. A plant trait might, for example, affect parasitoids and predators feeding on the herbivore. Consequently, defensive plant traits structure the diversity and composition of the complex community associated with the plant, and communities as a whole also feed back to selection on plant traits. Here, we review recent developments in our understanding of how plant defense traits structure insect communities and discuss how molecular mechanisms might drive community-wide effects.

Genetic variation in the effect of a facultative symbiont on host-plant use by pea aphids

Ecological specialisation on different host plants occurs frequently among phytophagous insects and is normally assumed to have a genetic basis. However, insects often carry microbial symbionts, which may play a role in the evolution of specialisation. The bacterium Regiella insecticola is a facultative symbiont of pea aphids (Acyrthosiphon pisum) where it is found most frequently in aphid clones feeding on Trifolium giving rise to the hypothesis that it may improve aphid performance on this plant. A study in which R. insecticola was eliminated from a single naturally infected aphid clone supported the hypothesis, but a second involving two aphid clones did not find the same effect. We created a series of new pea aphid-R. insecticola associations by injecting different strains of bacteria into five aphid clones uninfected by symbionts. For all aphid clones, the bacteria decreased the rate at which aphids accepted Vicia faba as a food plant and reduced performance on this plant. Their effect on aphids given Trifolium pratense was more complex: R. insecticola negatively affected acceptance by all aphid clones, had no effect on the performance of four aphid clones, but increased performance of a fifth, thus demonstrating genetic variation in the effect of R. insecticola on pea aphid host use. We discuss how these results may explain the distribution and frequency of this symbiont across different aphid populations.