Aphids harbour symbiotic bacteria that can have positive, negative or neutral effects on their survival and performance. These bacteria are split into two groups: the primary obligate endosymbiont, Buchnera aphidicola, and the secondary ‘facultative’ bacteria. In pea aphid (Acyrthosiphon pisum) three vertically transmitted secondary facultative bacteria have been shown to influence various fitness traits in their aphid hosts, including susceptibility to natural enemies. Very little is known however, about the fitness effects of bacterial associations in other aphid species. The aim of this study was to characterise bacterial diversity in a Scottish arable pest, the cabbage aphid (Brevicoryne brassicae), and investigate whether bacterial composition influences trophic interactions by testing their impact on aphid–parasitoid interactions. The bacterial community of cabbage aphid was quite different to pea aphid, both in terms of the density of the primary symbiont, Buchnera, but also in secondary bacterial complement. There was a wide diversity of bacteria associated with the cabbage aphid although these were not the three commonly studied pea aphid secondary symbionts and were likely to be a different type of symbiont, relying primarily on horizontal transmission. Phylogenetic analysis of 16S sequence revealed that the majority of bacterial types could be split into two groups: Group 1 Pseudomonas type bacteria and Group 2 Erwinia type bacteria. A real-time (Taqman®) qPCR assay was used to determine the infection status of the cabbage aphid lines and indicated there were four different community types in cabbage aphid: (1) aphid lines dominated by Group 1 bacteria; (2) aphid lines dominated by Group 2 bacteria; (3) aphid lines that harboured large approximately equal amounts of each group of bacteria; (4) aphid lines that harboured relatively little of either group of bacteria. The molecular results provided a basis for aphid–parasitoid fitness experiments. Preliminary results, based on single aphid genotypes, indicated that harbouring Group 2 Erwinia bacteria had both direct and indirect fitness costs by reducing cabbage aphid fitness and positively influencing the fitness of emergent Diaeretiella rapae parasitoids. The extent to which bacterial associations can influence aphid fitness and the truly multitrophic nature of cabbage aphid population dynamics in arable systems are highlighted by this study.
|Date of Award
|Stephen Hubbard (Supervisor) & Alison Karley (Supervisor)