The Dynamics of Natural Enemy Resistance in Aphid Populations

Abstract

Many aphids harbour facultative bacterial endosymbionts that can influence aphid fitness. In Acyrthosiphon pisum (the pea aphid), the endosymbiont Hamiltonella defensa confers resistance to parasitism by parasitoid wasps due to the presence of the APSE bacteriophage in the Hamiltonella genome. Furthermore, pea aphids that harbour both H. defensa and PAXS endosymbionts are highly resistant to the parasitoid Aphidius ervi. Changes in the frequency of endosymbiont infection could compromise control of aphid populations by natural enemies with implications for bio-control. Sixteen pea aphid lines were genotyped and characterised for the presence of known facultative endosymbionts and the APSE bacteriophage and, of these lines, fourteen harboured at least one known secondary endosymbiont. Intriguingly, a pea aphid clonal line harbouring a single stable infection of PAXS was identified. Although pea aphids harbouring a single infection of PAXS have been previously noted in the published literature, this study is the first to explore the susceptibility of such aphids to parasitoid wasps. Experimental evidence suggests that the benefits of endosymbiont-conferred protection against parasitism come at a cost to aphid fitness. A mathematical model of the population dynamics of A.ervi parasitoids and endosymbiont-infected and endosymbiont-uninfected pea aphids based on published literature was further developed and refined to investigate the balance between the costs of infection and the beneficial strength of the protection and additionally to study the implications of this trade-off for endosymbiont infection frequencies in pea aphid populations. Host and parasitoid population dynamics were explored and stability boundaries between stable and oscillating populations identified. The degree of suppression of host populations by parasitoids was determined. Contrary to conclusions reported in the published literature, endosymbiont-infected and endosymbiont-uninfected aphid hosts were shown to coexist in a stable manner over a range of biologically realistic parameter values. Further simulations were carried out using the mathematical model to explore the effects of parasitoid choice and a strength of protection that could be overcome by superparasitism. Preliminary results suggest that this increases the potential for stable host coexistence. Using findings from the molecular characterisation of the pea aphid lines, future refinements to the mathematical model are proposed based on the potential for single protective endosymbiont infections, dual protective infections and genotypic variation in the strength of protection.

Date of Award	2016
Original language	English
Sponsors	The James Hutton Institute
Supervisor	Mark Chaplain (Supervisor), Stephen Hubbard (Supervisor) & Alison Jane Karley (Supervisor)