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 and diversity of the symbiont in field-collected soybean aphids, and to determine whether Arsenophonus is defending soybean aphid against natural enemies. We performed diagnostic PCR and sequenced four Arsenophonus genes in soybean aphids from their native and introduced range to estimate infection frequency and genetic diversity, and found that Arsenophonus infection is highly prevalent and genetically uniform. To evaluate the defensive role of Arsenophonus, we cured two aphid genotypes 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 infection between our treatments. Our conclusion is that Arsenophonus does not defend its soybean aphid host against these major parasitoid and fungal natural enemies.
Highlights
Inherited bacterial endosymbionts are common in arthropods [1,2,3,4,5]
Using three parasitoid wasp species, we found no evidence that Arsenophonus provides this defense in either of two genotypes of soybean aphid
All three species of parasitoids were able to successfully attack soybean aphid, and there were no significant differences in successful parasitism of Arsenophonus-infected versus cured aphids in either cage or observation assays
Summary
Many insects are infected with obligate nutritional endosymbionts that are required for survival, e.g. Buchnera aphidicola in aphids [1,2,6]. Facultative endosymbionts are not strictly required for insect survival, but can provide a selective advantage in certain ecological contexts [7]. A subset of these facultative endosymbionts can defend their insect hosts against natural enemies such as parasitoids, entomopathogenic fungi, viruses, and nematodes [11,12,13,14]. Bacterial symbionts in the genus Arsenophonus are estimated to infect approximately 5% of arthropods [4,15]. In a geographic survey of the lerp psyllid, Glycaspis brimblecombei, Hansen et al (2007) found a positive correlation between parasitism and the frequency of Arsenophonus infection, potentially indicating that Arsenophonus provides the psyllid with a selective advantage in populations under heavy parasitism pressure [31]
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