Cooperation or conflict in coexisting aphid symbionts?

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Many insects carry bacterial symbionts that are transmitted from mother to offspring. These symbionts have diverse roles in their hosts' life-histories. The best-known group of symbionts are bacteria that provide nutrients to their hosts, which the host cannot obtain from its diet. These types of bacteria typically occur in insects that feed on very imbalanced types of food such as plant saps, cellulose or blood and are usually found in every individual of the insect species. A second group of symbionts has a much more variable distribution; within a population of insects they are usually found in some individuals but not others. However, these types of bacteria, called secondary symbionts, can provide important benefits to their hosts and often help the insect to cope with environmental challenges. They can, for example, protect the insect from attack by natural enemies such as viruses and other pathogens or reduce the detrimental effects of high temperatures.Many of the insects that carry such symbionts are agricultural pests or vectors of human diseases. Understanding the biology of these interactions is therefore important and might lead to novel control strategies. For example, there are attempts of using symbiotic bacteria to inhibit the transmission of insect-borne pathogens that cause diseases such as malaria, dengue virus or Chagas disease. Aphids are one of the most important agricultural pests in temperate regions. They damage plants by feeding on plant sap and simultaneously often transmit plant viruses. All aphids host a nutritional symbiont and many also carry one or more secondary symbionts.The proposed project seeks to understand the interactions in this menagerie of symbionts. Often several secondary symbiont species are found in one host individual. We have a good understanding of many of the effects of these symbionts on their own, but know very little of how they act in concert. For example, one of the symbiont species increases the aphid's resistance to parasitic wasps, while another increases resistance to pathogenic fungi. When they co-occur, it is likely that the two species compete with each other inside the insect and we seek to investigate whether this competition limits their ability to benefit the host. At the same time, the two symbionts are likely to coexist over many generations since they are transmitted from mother to offspring. They therefore share interests with their host and should be selected so that the insect fitness is maximised. The proposed work aims at understanding these interacting processes. One prediction is that the negative effects of competition are minimised when symbionts that have coexisted for a long time interact, but that competition is more severe and might carry higher costs for the host when symbionts form novel associations. This work will advance our understanding how ecological communities evolve and might through a better understanding of aphid biology provide avenues towards new pest control strategies.