Protective symbiosis: distribution of Hamiltonella defensa in natural populations of pea aphid

Symbiotic associations between microorganisms and eukaryotes are common in nature. These microorganisms can play important role of the evolution and ecology of their hosts by modifying their phenotype like conferring protection against parasitic wasps. This symbiotic protection is encountered in the pea aphid, Acyrthosiphon pisum, where the bacterial symbiont Hamiltonella defensa confers protection against parasitoid wasp, Aphidius ervi. Despite this protective benefit, this symbiont occurs only at intermediate frequencies in natural populations. To identify factors influencing the frequency of H. defensa in natural population, we measured pea aphid life history traits and the protective effect of this symbiont. We found that protective symbiont doesn’t influence the host life history traits and the protection conferred by this symbiont varies according the biotype of pea aphid and the bacterial strain. However, the multiple infections with facultative symbionts reduce the fitness of the host while increasing protection against parasitoids. We also observed the positive correlation between aphid reproduction in the absence of parasitoid and the level of resistance: higher the symbiotic protection higher the aphid fecundity. This finding has important implications for the evolution of defensive symbiosis and allows evidence the need for better understanding of how these protective symbionts is maintained in the population.