Host control of symbioses with bacteria

: symbiont density regulation and selection in insects

Abstract

The presence of bacteria within insect hosts can be important, or even essential, for host fitness. However, the extent to which insects benefit from such symbioses is context dependent, and many insects demonstrate traits and behaviours which allow them to maximise the benefit of their interactions. In this thesis, I explore two aspects of symbioses which are potentially under the control of the host: symbiont density regulation and symbiont selection. First, I review the literature which indicates that insects that harbour obligate, intracellular symbionts have tight control over the density of their symbionts, and conclude that hosts may be able to actively regulate symbiont density according to their requirements. Second, building on the assumption that hosts regulate symbiont density in order to maximise their own fitness, I use optimisation modelling methods to explore the ecological drivers and evolutionary outcomes of symbiont density regulation in a general host-symbiont system. Third, I use Glossina morsitans tsetse flies to compare symbiont density between an obligate and a facultative symbiont, under several ecological scenarios. I find no evidence to support the hypothesis that tsetse actively regulate the density of their obligate symbiont, Wigglesworthia, in response to diet, but suggest that the density of this symbiont is regulated to long-term requirements of the host. I find that the density of the facultative symbiont, Sodalis, is dependent on the ecological context, and attribute the independence of this symbiont from the control of its host to its relatively recent transition to host-associated living. Finally, I test the ability of hosts to select which symbionts their offspring form associations with using Aedes aegypti mosquitoes. I find that females demonstrate preferences for oviposition site according to the species of environmental bacteria, and find a correspondence between the bacteria species which are preferred and the ability of bacteria species to support juvenile mosquito development. Taken together, I demonstrate that the degree of control exerted by insects over their symbionts depends on the evolutionary history of the host-symbiont interaction, and that hosts use several strategies to maximise the fitness benefits of symbioses.

Date of Award	10 Dec 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Sinead English (Supervisor)