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Title: Studies of the spread and diversity of the insect symbiont Arsenophonus nasoniae
Author: Parratt, Steven R.
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2013
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Heritable bacterial endosymbionts are a diverse group of microbes, widespread across insect taxa. They have evolved numerous phenotypes that promote their own persistence through host generations, ranging from beneficial mutualisms to manipulations of their host’s reproduction. These phenotypes are often highly diverse within closely related groups of symbionts and can have profound effects upon their host’s biology. However, the impact of their phenotype on host populations is dependent upon their prevalence, a trait that is highly variable between symbiont strains and the causative factors of which remain enigmatic. In this thesis I address the factors affecting spread and persistence of the male-killing endosymbiont Arsenophonus nasoniae in populations of its host Nasonia vitripennis. I present a model of A. nasoniae dynamics in which I incorporate the capacity to infectiously transmit as well as direct costs of infection – factors often ignored in treaties on symbiont dynamics. I show that infectious transmission may play a vital role in the epidemiology of otherwise heritable microbes and allows costly symbionts to invade host populations. I then support these conclusions empirically by showing that: a) A. nasoniae exerts a tangible cost to female N. vitripennis it infects, b) it only invades, spreads and persists in populations that allow for both infectious and heritable transmission. I also show that, when allowed to reach high prevalence, male-killers can have terminal effects upon their host population. Secondly, I examine the phenotypic and genetic differences of a novel strain of Arsenophonus that has recently diverged from the male-killer following a host-shift event. I show that interspecific transmission can lead to rapid changes in symbiont biology, shifting away from reproductive parasitism and reliance upon mixed transmission towards mutualism, pure heritability and host-specialisation. I also show that these transitions are underpinned by specific genomic diversification. These findings have important implications for the way in which we view symbiont dynamics in nature and predict their outcome in terms of virulence and phenotype evolution. Further, it highlights the rapid directed selection pressures that symbionts are regularly exposed to following a host-shift and how this may be responsible for the diversity seen in nature.
Supervisor: Hurst, Greg Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available