Cooperation or conflict between facultative bacterial symbionts of insects and their effect on the ecology and evolution of their host

Researchers at the University of York are investigating how bacterial symbionts of insects are interacting inside the host and how these interactions shape the ecology of their hosts.   Many insects carry bacterial symbionts that are transmitted from mother to offspring and a large number of these insect species are agricultural pests or vectors of human diseases.  Aphids are one of the most important agricultural pests in temperate regions. They damage plants by feeding on plant sap and often transmit plant viruses.

A team of researchers led by Julia Ferrari is studying symbiotic bacteria in aphids in a project funded by BBSRC.  They are focusing on so-called secondary symbionts, bacteria that cannot live outside the insects, but that are not required for the host’s survival and reproduction.  Within a population of insects they are usually found in some individuals but not others. These types of bacteria 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 pathogens or parasitoids, or reduce the detrimental effects of high temperatures.   An individual aphid tends to carry between zero and three of these bacterial species, with multiple infections being common.

The research undertaken at York seeks to understand the interactions in this menagerie of symbionts.  We have a good understanding of many of the effects of the 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. 

One of the aims of the BBSRC-funded work is to investigate whether this competition limits the bacteria’s ability to benefit the host. However, because two symbionts are transmitted from mother to offspring they likely coexist over many generations and therefore share interests with their host.  Thus, they should be selected to cooperate and maximise insect fitness. The ongoing 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.  It might also provide avenues towards new pest control strategies through a better understanding of aphid biology.

Pea aphids feeding on a bean leaf Left: Pea aphids feeding on a bean leaf.