Aphids, Bacteria and Fungal Pathogens; the Ecology of a Complex Symbiosis

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The easy availability of molecular techniques over the last two decades has revolutionised our understanding of how animals and plants interact with micro-organisms. Many previously unsuspected symbioses have been discovered and many new issues for our understanding of biodiversity and community ecology have arisen. This proposal is part of a long-term project to understand the dynamics and persistence of a complex mutualistic symbiosis involving an aphid and no less than eight bacterial partners. This has become a model system used by laboratories around the world exploring the ecology and evolution of symbiosis.Aphids are familiar insects in temperate regions and in these areas are the most important pests of arable crops. Despite being closely studied since the dawn of the scientific age the last 15 years have seen a complete reappraisal of many aspects of their biology as the importance of their bacterial symbionts has become apparent. Aphids feed on plant sap which is nutritionally imbalanced and it has been known for 50 years that they carry an obligate (or primary) bacterial symbiont called Buchnera which synthesises essential nutrients missing in their diet. But we now know that in addition to Buchnera there are at least seven other facultative or secondary symbionts, present in some aphids but not others. Moreover, these facultative symbionts have many effects on their host's biology including conferring resistance to parasites and pathogens, enabling their hosts to withstand heat shock or use different host plants, and influencing life history strategy.Our laboratory has particularly worked on how symbionts influence the aphid's ability to withstand attack by fungal pathogens. We discovered that one bacteria called Regiella insecticola markedly increased resistance to the most common fungal disease. Curiously this symbiont species is particularly associated with aphids feeding on clover (pea aphid has a complex population structure comprised of genetically differentiated "biotypes" associated with different host plants within the pea family). Very recently our laboratory has mapped the genetic structure of Regiella showing that isolates from pea aphid are organised into two major genetic groups (or clades). We have also found that other recently discovered secondary symbionts can impart fungal resistance which suggests that this might be a general strategy that symbionts use to spread through host populations.The proposal is to support continuing work on pea aphid symbionts in our laboratory focussing on the ecology of the interaction between pea aphid, their host plants and fungal pathogens, and the facultative symbionts that confer resistance. We shall test the hypothesis that the aphid biotype on clover suffers particularly from fungal pathogens and hence needs to carry Regiella. Using reciprocal introductions of bacteria we shall ask why Regiella from the two major clades infect different biotypes and explore whether all clades provide resistance. Fungus can still kill aphids carrying Regiella (though with lower probability) but they are then less likely to produce infectious spores and we shall test the hypothesis that this has evolved through kin selection. All work to date has involved a single fungal pathogen though we know from our community studies that other pathogens are present in the field. We shall investigate the specificity of fungal resistance. We shall build on our recent pilot study demonstrating that some isolates of other symbionts confer resistance to establish the extent to which this occurs in natural populations. Finally, we shall use modern DNA sequencing techniques to test the hypothesis that fungal resistance is caused by a common mechanism that has been transferred horizontally amongst these unrelated bacteria.