Phenotype diversity in pea aphids carrying the Hamiltonella defensa bacteria

A majority of species interacts, closely and durably, with symbiotic microorganisms. These latter can modify the ecology and evolution of their hosts by inducing a variety of effects on their phenotype. However, the extended phenotype associated with a symbiotic organism can vary largely depending on the host species. For example, the symbiont Wolbachia can manipulate the reproduction of its host, protect it against natural enemies or enhance its nutrition. All these effects induced by Wolbachia, which can be deleterious or beneficial, depend on the host species. The aim of our study is to measure the variability of phenotypes that can be associated with a bacterial symbiont within a single host species. Our biological model is the pea aphid (Acyrthosiphon pisum). This particular aphid presents populations that have specialized on different legumes species (ie, biotypes). Among the symbiotic bacteria hosted by this insect, Hamiltonella defensa is highly prevalent in different biotypes but its phenotypic effects – altered behavior, reduced fitness and parasitic resistance - are known only in two of these biotypes. By analyzing the life history traits and the level of resistance of individuals carrying this symbiotic bacterium from four A. pisum biotypes, our study reveals that there is a high variability associated with this bacterium amongst inter and intrabiotype phenotypes. Therefore, a symbiotic microorganism/eukaryote relationship cannot be summarized by a single phenotypic effect.