Location: Beneficial Insects Introduction ResearchTitle: Worldwide populations of APHIS CRACCIVORA have diverse facultative bacterial symbionts Author
Submitted to: Microbial Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/13/2013
Publication Date: 11/15/2013
Citation: Brady, C.M., Asplen, M.K., Heimpel, G.E., Hopper, K.R., Linnen, C.R., Oliver, K.M., Wulff, J.A., White, J.A., Desneux, N. 2013. Worldwide populations of APHIS CRACCIVORA have diverse facultative bacterial symbionts. Microbial Ecology. 67:195-204. Interpretive Summary: Aphids which are major crop pests often harbor inside their cells bacteria (called facultative bacterial endosymbionts) that may affect aphid traits, like ability to feed on a particular plant species or resistance to parasitic wasps. However, the distribution and abundance of these endosymbionts is known for only a few aphids. We studied the bacteria found in the cowpea aphid, APHIS CRACCIVORA, which is a pest of legumes. We found 11 strains of bacteria in 6 genera in 44 populations of the aphid. However, cowpea aphid had lower prevalence of these bacteria than other well-studied aphids. The almost complete lack of sexual reproduction in cowpea aphid may reduce transmission of bacteria between aphid strains, which would explain the lower prevalence of bacteria. Two bacteria showed strong host plant associations. ARSENOPHONUS was prevalent in cowpea aphid on locust but rare or absent on other plant species; HAMILTONELLA was prevalent in cowpea aphid on alfalfa but rare or absent on other plant species. Aphids from these two plant species also differed genetically, suggesting adaptation to these plant species. HAMITONELLA provides protection against a group of parasitic wasps, but ARSENOPHONUS does not, so that the aphids on different host plants are likely to experience different levels of parasitism.
Technical Abstract: Facultative bacterial endosymbionts can play an important role in the evolutionary trajectory of their hosts. Aphids are infected with a wide variety of facultative endosymbionts that can confer ecologically relevant traits, which in turn may drive microevolution in a dynamic selective environment. However, relatively little is known about how symbiont diversity is structured in most aphid species. Here, we investigate facultative symbiont species richness and prevalence among worldwide populations of the cowpea aphid, APHIS CRACCIVORA. We surveyed 44 populations of A. CRACCIVORA, and detected 11 strains of facultative symbiotic bacteria in 6 genera. There were two significant associations between facultative symbiont and aphid food plant: the symbiont ARSENOPHONUS was found at high prevalence in aphid populations collected from ROBINIA sp.(locust), whereas the symbiont HAMILTONELLA was almost exclusively found in aphid populations from MEDICAGO SATIVA (alfalfa). Aphids collected from these two food plants also had divergent mitochondrial haplotypes, potentially indicating the formation of specialized aphid lineages associated with food plant (host-associated differentiation). Overall, facultative symbiont prevalence in A. CRACCIVORA was lower than that observed in other well-studied aphids (e.g., ACYRTHOSIPHON PISUM), which we propose could be a consequence of A. craccivora’s almost purely parthenogenetic life history. Finally, most of the surveyed populations were polymorphic for facultative symbiont infection, symbiont-associated phenotypic variation may allow evolutionary responses to local selection.