Almost all aphid species harbour the primary bacterial endosymbiont
Buchnera aphidicola, which plays a vital role in essential nutrient provisioning. In the last two decades several additional ‘secondary’ bacterial endosymbionts have been detected across different aphid species, although not across all aphid populations. Recent research has revealed that secondary bacterial endosymbionts can affect several aspects of the morphology and ecology of their aphid host. However, many of these studies have been conducted using the pea aphid (Acyrthosiphon pisum), and it is not clear whether facultative bacteria confer the same fitness traits in other aphid species.
The potato aphid, Macrosiphum euphorbiae, is closely related to A. pisum and is also an agriculturally important pest, utilising several crops including potato, tomato and bean throughout the summer months and transmitting a range of plant viruses. More than half of the 19 clonal lines of M. euphorbiae established in culture for this study were found to be singly or doubly-infected with two secondary endosymbiont bacteria, Hamiltonella defensa and Regiella insecticola, previously characterised from A. pisum. However, only the H. defensa infections persisted in culture, and these were associated exclusively with two of the seven M. euphorbiae genotypes represented in the clonal lines. Under controlled experimental conditions, no inherent fitness costs to the aphid were identified for the two aphid genotypes in which H. defensa infections occurred naturally. Neither aphid reproduction and survival nor densities of B. aphidicola were detrimentally affected by the presence of the secondary endosymbiont, although this may not be true for other genotypes of M. euphorbiae. Despite an established role in conferring parasitoid resistance to A. pisum hosts, this study found very little evidence that H. defensa protects M. euphorbiae against one of its principal natural enemies, the parasitoid wasp Aphidius ervi. Instead the innate immunity of one specific genotype of M. euphorbiae dramatically reduced parasitoid susceptibility, regardless of secondary endosymbiont presence.
Which aphid genotypes will dominate in a population depends on the relative ability of aphid clones to locate and feed on suitable host plants, withstand abiotic stresses and escape natural enemies, and such knowledge is essential for the successful management of aphid infestations in agricultural systems. The research presented here has contributed to this knowledge by characterising M. euphorbiae genotypes and their associated facultative endosymbionts, and by quantifying genotypic differences in aphid intrinsic fitness and susceptibility to parasitoid wasps. Whilst secondary endosymbionts are recognized as a potential source of heritable traits that could influence the population dynamics of their aphid hosts, the selection pressures acting to favour persistence of the endosymbiotic association between M. euphorbiae and H. defensa have yet to be determined.
|Date of Award||2013|
|Supervisor||Alison Karley (Supervisor), Stephen Hubbard (Supervisor) & Danny Cullen (Supervisor)|