Microbial symbiosis in insects : toward a better understanding of the mechanisms involved in host-bacteria associations

Abstract

Serratia symbiotica is a valuable model to understand the evolution of microbial symbiosis in insects. Indeed, it includes a wide variety of strains varying in their interdependence degree with aphid hosts, some of which have been cultivated in our laboratory. The main objective of this thesis was to investigate how symbionts form new symbioses and extend their influence on insect populations. We established artificial infections of cultivable S. symbiotica in aphid gut to decipher its infection processes, symbiotic status, transmission mode, as well as the way in which it infects aphids. Our results showed that cultivable S. symbiotica colonized and multiplied in aphid gut, inducing fitness costs. However, it can offer a protection against parasitoids that should in some conditions offset the costs. The bacteria seem also to be extracellularly transmitted in laboratory conditions. We showed that plants can mediate horizontal transfers of cultivable S. symbiotica between aphids, and bacteria can be absorbed by plant roots, inducing new infections in aphids, as well as positive effects on plant fitness. A field study further highlighted the great diversity of S. symbiotica, as well as its presence in plants and insects associated with aphid colonies, confirming the existence of new acquisition and transfer routes of S. symbiotica in nature. This thesis also examined another insect symbiosis, showing that Arsenophonus likely complements the role of the deficient obligate symbiont of psyllids. This thesis thus brought new insights into the nature of symbiosis in insects and the importance of considering symbiosis in complex systems.(SC - Sciences) -- UCL, 202