The evolutionary ecology of aphids' immunity

Abstract

Aphids comprise 4,400 species that live in close interactions with their host-plants, the parasitoid wasps and fungi they encounter, as well as several bacteria including Buchnera aphidicola, an obligatory, nutrient-providing symbiont. Aphids also interact with a cohort of facultative secondary symbionts that strongly interfere with their major life history traits such as host-plant specialization, heat tolerance and resistance to natural enemies. Here, we present some evolutionary and ecologically-relevant aspects of these interactions, focusing on aphid defenses to parasitism, and considering aphids either as "extended organisms" comprising aphid and symbionts' genomes, or as "single-genome" organisms whose immune components are still poorly known. We highlight the complexity of predicting evolution of aphid immune resistance in the field, due to variable selection pressures, short-term costs, and cross-talk between symbionts. Finally, we present perspectives to strongly improve our understanding of the "aphid-symbiont-bacteriophage" meta-organism defenses and to elucidate the interactions between immunity, pathogenicity and symbiosis.