The influence of the endosymbiont, Arsenophonus, on soybean aphid (Aphis glycines) performance on resistant and susceptible plants

Recent evidence suggests bacterial endosymbionts can mediate the intimate relationships between some insects and their host plants. However, deciphering the role of the symbiont is often confounded by complex interactions between the interwoven genotypes of plant, insect, and bacteria. The endosymbiont Candidatus Arsenophonus infects a wide taxonomic range of arthropod hosts, and is suspected of an uncharacterized mutualistic role in hemipterous insects. In the soybean aphid, Aphis glycines, a notorious invasive pest of soybeans in the United States, Arsenophonus is the sole facultative endosymbiont. Recently, resistant soybean genotypes, Rag plants, were developed to minimize aphid outbreaks. However, soybean aphid “biotypes” virulent to Rag plants have since been discovered. We hypothesized that Arsenophonus is partially responsible for overcoming the antibiosis effect of the resistant plants. This hypothesis was predicated on another system in which a Rickettsia-like symbiont was associated with virulence of the potato aphid, Macrosiphum euphorbiae, on tomato plants protected with a resistance gene. We cured five soybean aphid biotypes, virulent and avirulent, of their natural Arsenophonus infection through ampicillin microinjection, resulting in infected and uninfected isolines. These isolines were subjected to growth rate assays on resistant Rag versus susceptible soybean. Our preliminary results indicate that Arsenophonus-infected avirulent biotypes perform better than cured on both plant types. This trend was evident in virulent biotypes, but less pronounced.  In contrast to the M. euphorbiae system, Arsenophonus may provide certain biotypes of soybean aphids a general fitness benefit regardless of the presence of the resistance gene in the host plant.