Jones, Jordan 
ORCID: 0000-0002-3484-4450
(2022)
The evolutionary ecology of a defensive symbiont in a host-parasitoid interaction.
PhD thesis, University of Liverpool.
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Abstract
All insects are subject to attack from natural enemies. Classically, the outcome of natural enemy attack was considered to be dependent on factors encoded within the host and parasite genomes. However, it is now widely appreciated that defensive bacterial symbionts residing within the host can dictate the outcome of natural enemy attack in insects. In this thesis, I utilise the Spiroplasma-Drosophila melanogaster system to evaluate the factors which determine the outcome of Spiroplasma-mediated defence against Leptopilina parasitoid wasps. I first examine whether the strength of protection conferred by Spiroplasma varies with the strain of Leptopilina heterotoma wasp attacking the host. This experiment was conducted in the presence and absence of ethanol, an environmental factor thought to impact the outcome of parasitism. The strength of protection conferred by Spiroplasma was observed to strongly depend on the strain of L. heterotoma. However, contrary to previous research, environmental ethanol did not substantially aid survival against parasitoid wasps, although it did contribute to a composite measure of protection which included the chance of fly surviving attack and the relative fertility of survivors. I then examine whether the strain of Spiroplasma alters the strength of protection conferred against Leptopilina wasps. I compared the response of flies carrying two strains of male-killing Spiroplasma to challenge by two strains of L. boulardi and two strains of L. heterotoma wasp. There was no evidence to suggest that the strength of protection conferred was dependent on the strain of Spiroplasma in any case. Finally, I consider the impact of the environment, examining the effect of temperature on the protection phenotype. I observed that Spiroplasma-mediated protection was weaker at cooler temperatures. However, this effect was only observed when flies were subject to cooler temperatures before, but not during or after wasp attack, suggesting that the thermal history of the fly determines the efficiency of Spiroplasma-mediated protection in this system. This effect appeared to be mediated at least partially through temperature effects on host Spiroplasma titre. Collectively, these results provide a more general understanding of defensive symbiont evolutionary ecology beyond the well studied aphid systems and demonstrate that the outcome of symbiont-mediated protection is much more complex than the mere presence or absence of the defensive symbiont. The results highlight the importance of host, symbiont and natural enemy genetics as well as the environment when considering the dynamics of a defensive symbiont in natural populations.
| Item Type: | Thesis (PhD) |
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| Divisions: | Faculty of Health and Life Sciences Faculty of Health and Life Sciences > Institute of Infection, Veterinary and Ecological Sciences |
| Depositing User: | Symplectic Admin |
| Date Deposited: | 09 Feb 2022 14:36 |
| Last Modified: | 18 Jan 2023 21:14 |
| DOI: | 10.17638/03147953 |
| Supervisors: |
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| URI: | https://livrepository.liverpool.ac.uk/id/eprint/3147953 |
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