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Title: The evolutionary ecology of an insect-bacterial mutualism
Author: Corbin, C.
ISNI:       0000 0004 7428 5985
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2018
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Heritable bacterial endosymbionts are responsible for much phenotypic diversity in insects. Mutualists drive large-scale processes such as niche invasion, speciation and mass resistance to natural enemies. However, to persist, mutualists need to be able to transmit with high fidelity from one generation to the next, to be able to express their beneficial phenotypes, and for the benefits they grant the host to outweigh their costs. The effect of ecologically-relevant environmental temperature variations upon transmission and phenotype is a poorly understood area of endosymbiont biology, as is how the symbiont’s cost varies under ecological stress. In this thesis, I examined these parameters for Spiroplasma strain hy1, a defensive mutualist which protects the cosmopolitan, temperate fruit fly Drosophila hydei from attack by a parasitoid wasp. I detected Spiroplasma hy1 in D. hydei individuals from the south of the U.K. The bacterium is at low prevalence compared to hy1 in other localities such as North America and Japan, but its presence in this temperate region conflicts with past studies indicating high sensitivity to low temperatures. I first demonstrate that the vertical transmission of Spiroplasma hy1 is more robust to the cool temperatures typical of temperate breeding seasons than previously considered, with transmission in a ‘permissive passage’ experiment occurring at high fidelity for two generations at a constant 18°C and in an alternating 18/15°C condition. Secondly, I demonstrate that the expression of the defensive phenotype is considerably more sensitive to cool temperatures than transmission. Spiroplasma hy1 protection ceases at 18°C, suggesting that for much of the D. hydei breeding season in areas such as the U.K., hy1 may be selectively neutral in many fly individuals. Finally, I show that hy1 has an unusually low standing cost to its host under starvation stress, contrasting with findings for the related MSRO strain in D. melanogaster. Measures of active cost – the fate of survivors of attack – were unclear. These results indicate that sensitivity to cold temperatures could account for hy1’s low U.K. prevalence. Small amounts of segregational loss could partially counteract selection upon natural enemy resistance, and loss of phenotypic expression at 18°C almost certainly causes hy1 to be neutral at best for parts of early summer and autumn. Future work should investigate the effects of different temperature on costs of symbiont carriage, and whether cool temperatures could push hy1 from mutualism and neutral commensalism to parasitism, as well as investigate how nuclear-mediated anti-wasp protection might interact and compete with hy1-mediated protection.
Supervisor: Hurst, Greg D. D. ; Fenton, Andy Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral