Submitted to: PLoS One
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/18/2013
Publication Date: 4/16/2013
Citation: Wulff, J.A., Buckman, K.A., Wu, K., Heimpel, G.E., White, J.A. 2013. The endosymbiont Arsenophonus is widespread in soybean aphid, Aphis glycines, but does not provide protection from parasitoids or a fungal pathogen. PLoS One. 8(4): e62145 7pp. http://dx.doi.org/10.1371/journal.pone.0062145. Interpretive Summary: The soybean aphid is an important insect pest of soybean in North America. Many times arthropods and pathogens can protect the soybean crop by controlling soybean aphid populations. However, aphids can also harbor symbiotic bacteria that live inside aphids and may protect aphids from attack by parasitic wasps and fungal pathogens. Previous studies have shown that symbiotic bacteria belonging to the genus Arsenophonus are present in soybean aphids. In this study, we documented how frequently the bacteria occur in soybean aphid and evaluated whether the bacteria defend the aphid against attack by parasitic wasps or fungal pathogens. We collected soybean aphids from their native range in Asia and from their introduced range in North America and tested multiple populations for the presence of symbiotic bacteria. Next, we compared the rates at which wasps and fungi attacked soybean aphids with the bacteria to aphids which had been cured of the bacteria by treatment with antibiotics. We included three species of parasitic wasps and one species of fungal pathogen. We found that the symbiotic bacteria are commonly found in soybean aphid populations in both Asia and North America. Furthermore, we found that aphids with the bacteria were just as likely to be attacked by all three parasitic wasp species and the fungal pathogen as aphids which did not have the bacteria. Thus, our results support the conclusion that while symbiotic bacteria are common in soybean aphid populations, the bacteria do not protect aphids from attack by the three parasitic wasp species or the fungal pathogen.
Technical Abstract: Aphids commonly harbor bacterial facultative symbionts that have a variety of effects upon their aphid hosts, including defense against hymenopteran parasitoids and fungal pathogens. The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is infected with the symbiont, Arsenophonus sp., which has an unknown role in its aphid host. Our research goals were to document the infection frequency of the symbiont in field collected soybean aphids, and to determine whether Arsenophonus is defending soybean aphid against natural enemies. We performed diagnostic PCR on soybean aphids from their native and introduced range to estimate infection frequency, and found that Arsenophonus infection is highly prevalent in soybean aphid populations within both regions. To evaluate the defensive role of Arsenophonus, we cured two genotypes of soybean aphid of their natural Arsenophonus infection through ampicillin microinjection, resulting in infected and uninfected isolines within the same genetic background. These isolines were subjected to parasitoid assays using a recently introduced biological control agent, Binodoxys communis [Braconidae], a naturally recruited parasitoid, Aphelinus certus [Aphelinidae], and a commercially available biological control agent, Aphidius colemani [Braconidae]. We also assayed the effect of the common aphid fungal pathogen, Pandora neoaphidis (Remaudiere & Hennebert) Humber (Entomophthorales: Entomophthoraceae), on the same aphid isolines. We did not find differences in successful parasitism for any of the parasitoid species, nor did we find differences in P. neoaphidis infections within our treatments. Our conclusion is that Arsenophonus does not defend its soybean aphid host against these major parasitoid and fungal natural enemies.