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Title: Incidence, diversity and evolution of Rickettsia and other endosymbionts that infect arthropods
Author: Weinert, Lucy A.
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2008
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The biology and evolution of many arthropod species cannot be understood without also considering their bacterial symbionts. In order to proliferate, many endosymbionts manipulate the reproduction of their hosts, and do so in a multitude of ways. The phenotypes that result from this bacterial manipulation have profound implications for the evolution of their hosts. These interactions will both influence, and be influenced by the abundance of the bacterial symbiont in arthropods. One of the best-studied bacterial manipulations is male-killing, where endosymbionts kill their male hosts in order to distribute resources towards infected females (the only individuals who will transmit the infection). Ladybirds beetles are known to harbour male-killing symbionts, and so the incidence of bacteria was investigated in the whole of this family. It was found that endosymbionts infected over half of the host species, mainly at low prevalence, which indicates that symbiont incidence and diversity may be currently under-estimated. In addition, multiple symbionts were found in the same population, lending strength to the hypothesis that they are being maintained by balancing selection with host resistance genes. The data was combined with a world-wide screen and other data from the literature, and used to estimate the distribution of across-species prevalences of the bacteria Wolbachia, Rickettsia and Cardinium in wild arthropod populations. A newly developed likelihood approach was used to find a best fit distribution, and properties of the distribution then used to predict how these symbionts manage to invade and spread through populations. The analysis revealed that the skew toward low prevalence infections may apply quite generally, suggesting that much of the diversity of endosymbionts will be missed from screens that test only a few individuals. In addition, the analysis highlighted differences in the incidence levels of different bacteria, and heterogeneity in prevalence distributions between clades of host species. Contemporary patterns of endosymbiont abundance must also be understood within the long-term evolutionary context, best investigated with a phylogenetic approach. Comparison of Rickettsia and Wolbachia phylogenies with those of their hosts indicate that these symbionts frequently switch horizontally between related hosts. Rickettsia have been less thoroughly investigated than Wolbachia, but these arthropod endosymbionts can also infect and cause serious diseases in humans and other mammals. In this study, 20 new strains of arthropod Rickettsia are identified and multiple genes sequenced to produce a robust phylogeny of the whole genus. Rickettsia are devised of two main clades, one of which primarily infects arthropods and the other infect a diverse range of protists, leeches, unidentified hosts from metagenomic samples, and some arthropods. Strategies such as male-killing and parthenogenesis induction appear to be recent innovations. Arthropod Rickettsia generally group basal to medically important strains, but some also cluster within the strains that infect vertebrates. There is increasing evidence against the traditional view that intracellular symbionts are refractory to recombination. Recombination does occur in Rickettsia but seems to be uncommon. However, there is strong evidence of large scale horizontal gene transfer events. Numerous conjugation genes were also discovered which indicates that plasmids may be common throughout the whole genus.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available