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Multiple infections within the same host are now understood to be common and important determinants of the outcomes of disease processes. Multiple infections are particularly important in insects, which are often infected by vertically transmitted symbionts that are passed from the mother to her offspring. In many cases, these symbionts have evolved to confer high levels of protection against co-infecting parasites, pathogens, or other natural enemies. Despite widespread examples of symbiont-mediated defense, there are key outstanding questions in the ecology and evolution of defensive symbiosis. These include the mechanisms through which protection is conferred, the specificity of defensive effects against different parasites and pathogens, and the overall roles of defensive and other symbioses in host communities and ecosystems. To address these questions, I used a model of defensive symbiosis in which the bacterium Spiroplasma protects the woodland fly Drosophila neotestacea from the nematode parasite Howardula aoronymphium. First, I conducted a series of experiments that included transcriptome sequencing of \textit{D. neotestacea} infected by Howardula and Spiroplasma to uncover the mechanistic basis of defense in this symbiosis. Through these experiments, I found evidence of a putative protein toxin encoded by Spiroplasma that might contribute to defense. Following this, we characterized the protein as a novel member of a class of toxins known as ribosome-inactivating proteins (RIPs). RIPs are important virulence factors in bacteria such as enterohemorrhagic E. coli; I exploited recent approaches for quantifying RIP activity to design sensitive assays that demonstrate that Howardula suffers a high degree of ribosome cleavage specific to RIP attack during Spiroplasma-mediated defense. This is among the first demonstrations of a mechanism of defense against a specific enemy in an insect defensive symbiosis. I next worked with collaborators to culture and characterize a novel trypanosomatid parasite of Drosophila that I uncovered during the above transcriptome sequencing. Trypanosomatids are protist parasites that are common in insects, and the causes of important human diseases that include Chagas disease and African sleeping sickness. Despite Drosophila's history as an important model of infection and immunity, little is known of its trypanosomatid parasites, and we describe this parasite as a new genus and species: Jaenimonas drosophilae, the first trypanosomatid formally described from a Drosophila host. We conduct a series of experiments to understand infection dynamics, immune responses and interactions with other parasites and symbionts within the host, beginning to establish Drosophila-Jaenimonas infections as a tractable model of trypanosomatid infection in insects. Finally, though examples of ecologically important defensive symbioses accumulate, an understanding of their overall roles in ecosystems is lacking. I close with a synthesis of the ways in which symbioses - defensive or otherwise - can affect ecosystem structure and function through their effects on food webs. This work will help to develop a conceptual framework to link reductionist findings on specific symbioses to larger scale processes. / Graduate

This work concerns itself with essentially four experiments: (1) the cecum-injective-infection experiment; (2) the anus-injective-infection experiment; (3) the mouth ingestive-infection with larvae, and (4) the mouth ingestive-infection with the flesh of infected rats.

A research report submitted to the Faculty of Health sciences, University of the Witwatersrand, Johannesburg, in partial fulfillment of the requirements for the degree of Master of Science in Medicine in Epidemiology and Biostatistics 30th May 2012 / Introduction The burden of HIV/AIDS is higher in the sub-Saharan region and multiple sexual partnerships are among the sexual behaviors that put people at risk of HIV transmission. The main aim of this study was to determine the prevalence and associated demographic, socio-economic and behavioral factors of multiple sexual partnerships among men in Zimbabwe. Materials and Methods This was an analytical cross-sectional study that used data from the Zimbabwe Demographic Health Survey 2005-06. Negative binomial regressions were fitted to identify factors associated with multiple sexual partnerships among men in Zimbabwe. Results The prevalence of multiple sexual partnerships was 13.5 %, 12.9%, and 11.2% among the formerly, never and currently married men respectively. Among the formerly married men, the risk factors significantly associated with multiple sexual partnerships included access to newspapers (RR= 1.28; 95% CI:1.02 , 1.60). Formerly married men aged 35-44 years had lower risk of engaging in multiple sex partnerships (RR = 0.59, 95%CI: 0.42, 0.83) than the other age groups. Relative to formerly married men in Manicaland, formerly married men in Mashonaland East region had lower risks of having more sexual partners. Age at first intercourse and sexual attitude were also significantly associated with multiple sexual partnerships among the formerly married men iv Among the never married men, the risk factors associated with multiple sexual partnerships included employment status (RR = 1.33%; 95%CI: 1.17, 1.52), Watching TV (RR = 1.33%; 95%CI: 1.05, 1.69) and sexual attitude (RR = 1.37%; 95% CI: 1.05, 1.79). Relative to never married men in Manicaland, the never married men in Mashonaland East and Mashonaland West had lower risks of having multiple sexual partners. Among the married men, the risk factors associated with multiple sexual partnerships included first intercourse at the age of 19 years or below (RR = 1.07%; 95% CI: 1.04, 1.11) and sexual attitude (RR = 1.16%; 95% CI: 1.09, 1.23). Protective factors included higher level of education (RR = 0.87%; 95% CI: 0.77, 0.98), being 35-44 (RR = 0.94%; 95% CI: 0.89, 0.99) or 45-54 years old (RR = 0.93%; 95% CI: 0.88, 0.99) and being from Mashonaland East (RR= 0.89%; 95%CI: 0.85, 0.93) region. Discussion and Conclusions The results show that after adjusting for potential confounders in the multivariate negative binomial regression analysis; age, geographical region, education, working status, frequency of reading newspapers/magazines/TV, age at first intercourse and sexual attitude remained significantly associated with MSP. However, the extent to which each of these factors was associated with multiple sex partnership varied among marital status groups, therefore HIV/AIDS intervention programs should be designed accordingly.