Browsing by Subject "Spiroplasma"
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Item Insights into Endosymbiont-mediated Defense of Drosophila Flies Against Parasitoid Wasps(2013-08-27) Xie, JialeiMaternally-transmitted associations between endosymbiotic bacteria and insects are diverse and widespread in nature. To counter loss by imperfect vertical transmission, many heritable microbes have evolved compensational mechanisms, such as manipulating host reproduction and conferring fitness benefits to their hosts. Symbiont-mediated defense against natural enemies of hosts is increasingly recognized as an important mechanism by which endosymbionts enhance host fitness. Members of the genus Spiroplasma associated with distantly related Drosophila, are known to engage in either reproductive parasitism (i.e., male killing, MSRO strain) or defense against natural enemies (a parasitic wasp and a nematode). My previous studies indicate the Spiroplasma hy1 enhances survival of Drosophila hydei against the parasitoid wasp Leptopilina heterotoma, but whether this phenomenon can contribute to the long-term persistence of Spiroplasma is not clear. Here, I tracked Spiroplasma frequencies in fly lab populations repeatedly exposed to high or no wasp parasitism throughout ten generations. A dramatic increase of Spiroplasma prevalence was observed under high wasp pressure. In contrast, Spiroplasma prevalence in the absence of wasps did not change significantly over time; a pattern consistent with random drift. Thus, the defensive mechanism may contribute to the high prevalence of Spiroplasma in D. hydei populations despite imperfect vertical transmission. A male-killing strain of Spiroplasma (MSRO), closely related to strain hy1, associates with the model organism D. melanogaster, and co-occurs with Wolbachia (strain wMel) in certain wild populations. We examined the effects of Spiroplasma MSRO and Wolbachia wMel, on Drosophila survival against parasitism by two common wasps, L. heterotoma and L. boulardi, that differ in their host ranges and host evasion strategies. The results indicate that Spiroplasma MSRO prevents successful development of both wasps, and confers a small, albeit significant, increase in larva-to-adult survival of flies subjected to wasp attacks. We modeled the conditions under which defense can contribute to Spiroplasma persistence. Wolbachia also confers a weak, but significant, survival advantage to flies attacked by L. heterotoma. This additive protective effect of Spiroplasma and Wolbachia may provide conditions for such co-transmitted symbionts to become mutualists. Occurrence of Spiroplasma-mediated protection against distinct parasitoids in divergent Drosophila hosts implies a general protection mechanism.Item Maternally Inherited Endosymbiotic Bacteria Of Drosophila : Spiroplasma and Wolbachia(2013-12-02) Silva, Pinnaduwage OMaternally inherited endosymbionts are ubiquitous among insects and are known to influence the ecology and evolution of a host species. As such, burgeoning evidence exemplifies their effect on host biology through reproduction, nutrition, resistance to pathogens, and heat tolerance. Many inherited symbionts are not guaranteed passage to a new host generation, and therefore can be lost from a host population. To circumvent loss, certain inherited symbionts have evolved the ability to manipulate host reproduction to enhance their transmission. Two common strategies used to do so are: male-killing, where sons of infected females die during embryogenesis; and cytoplasmic incompatibility (CI), which leads to conditional male sterility that can be reversed by mating with a female of the same infection type. Both phenotypes selectively favor female lineages, as heritable symbionts are only transmitted through them. Many heritable symbionts from the bacterial genera Spiroplasma and Wolbachia infect Drosophila. The model organism D. melanogaster, in particular, is naturally infected by a male-killing Spiroplasma strain and weak CI-inducing Wolbachia strain. Unlike other bacteria, infections of inherited symbionts remain during the life span of the host. This is energetically costly to the host, and yet these well adapted microbes persist in host populations. Many inherited symbionts provide condition dependent benefits (i.e., resistance to pathogens) to the host, to alleviate the cost of infection. Heritable symbionts typically transmit through the egg cytoplasm, and therefore rely on the female host to replicate and enter an egg. I performed quantitative PCR on Spiroplasma strains with and without the male-killing phenotype, to trace bacterial replication in reproductive females. My results suggest, in contrast to previous studies, that there is no evidence for the correlation between the male-killing phenotype and densities. I also compared maternal mRNA found in eggs of Spiroplasma and Wolbachia infected females to determine whether these symbionts alter gene expression of the female during oogenesis. I used RNA-sequencing and bioinformatics tools to determine differential maternal gene expression due to infection. The results suggest that Spiroplasma causes expression changes in genes, that code for a protein incorporated into the vitelline membrane of the oocyte, involved in pre-mRNA splicing, and a candidate gene for the mechanism of male-killing, involved in the sex determination pathway of Drosophila. Wolbachia infection had a minimal effect on maternal gene expression.