School of BioSciences - Research Publications
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ItemTropical Drosophila pandora carry Wolbachia infections causing cytoplasmic incompatibility or male killing(OXFORD UNIV PRESS, 2016-08)Wolbachia infections have been described in several Drosophila species, but relatively few have been assessed for phenotypic effects. Cytoplasmic incompatibility (CI) is the most common phenotypic effect that has been detected, while some infections cause male killing or feminization, and many Wolbachia infections have few host effects. Here, we describe two new infections in a recently described species, Drosophila pandora, one of which causes near-complete CI and near-perfect maternal transmission (the "CI" strain). The other infection is a male killer (the "MK" strain), which we confirm by observing reinitiation of male production following tetracycline treatment. No incompatibility was detected in crosses between CI strain males and MK strain females, and rare MK males do not cause CI. Molecular analyses indicate that the CI and MK infections are distantly related and the CI infection is closely related to the wRi infection of Drosophila simulans. Two population surveys indicate that all individuals are infected with Wolbachia, but the MK infection is uncommon. Given patterns of incompatibility among the strains, the infection dynamics is expected to be governed by the relative fitness of the females, suggesting that the CI infection should have a higher fitness. This was evidenced by changes in infection frequencies and sex ratios in population cages initiated at different starting frequencies of the infections.
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ItemRapid Sequential Spread of Two Wolbachia Variants in Drosophila simulans(PUBLIC LIBRARY SCIENCE, 2013-09)The maternally inherited intracellular bacteria Wolbachia can manipulate host reproduction in various ways that foster frequency increases within and among host populations. Manipulations involving cytoplasmic incompatibility (CI), where matings between infected males and uninfected females produce non-viable embryos, are common in arthropods and produce a reproductive advantage for infected females. CI was associated with the spread of Wolbachia variant wRi in Californian populations of Drosophila simulans, which was interpreted as a bistable wave, in which local infection frequencies tend to increase only once the infection becomes sufficiently common to offset imperfect maternal transmission and infection costs. However, maternally inherited Wolbachia are expected to evolve towards mutualism, and they are known to increase host fitness by protecting against infectious microbes or increasing fecundity. We describe the sequential spread over approximately 20 years in natural populations of D. simulans on the east coast of Australia of two Wolbachia variants (wAu and wRi), only one of which causes significant CI, with wRi displacing wAu since 2004. Wolbachia and mtDNA frequency data and analyses suggest that these dynamics, as well as the earlier spread in California, are best understood as Fisherian waves of favourable variants, in which local spread tends to occur from arbitrarily low frequencies. We discuss implications for Wolbachia-host dynamics and coevolution and for applications of Wolbachia to disease control.
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ItemGenome-Wide Transcription Analysis of Clinal Genetic Variation in Drosophila(PUBLIC LIBRARY SCIENCE, 2012-04-13)Clinal variation in quantitative traits is widespread, but its genetic basis awaits identification. Drosophila melanogaster shows adaptive, clinal variation in traits such as body size along latitudinal gradients on multiple continents. To investigate genome wide transcription differentiation between North and South that might contribute to the clinal phenotypic variation, we compared RNA expression patterns during development of D. melanogaster from tropical northern and temperate southern populations using whole genome tiling arrays. We found that genes that were differentially expressed between the cline ends were generally associated with metabolism and growth, and experimental alteration of expression of a sample of them generally resulted in altered body size in the predicted direction, sometimes significantly so. We further identified the serpent (srp) transcription factor binding sites to be enriched near genes up-regulated in expression in the south. Analysis of clinal populations revealed a significant cline in the expression level of srp. Experimental over-expression of srp increased body size, as predicted from its clinal expression pattern, suggesting that it may be involved in regulating adaptive clinal variation in Drosophila. This study identified a handful of genes that contributed to clinal phenotypic variation through altered gene expression level, yet misexpression of individual gene led to modest body size change.
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ItemFunctional Characterization of the Frost Gene in Drosophila melanogaster: Importance for Recovery from Chill Coma(PUBLIC LIBRARY SCIENCE, 2010-06-02)BACKGROUND: Almost all animals, including insects, need to adapt to temperature fluctuations. The molecular basis of thermal adaptation is not well understood, although a number of candidate genes have been proposed. However, a functional link between candidate genes and thermal tolerance has rarely been established. The gene Frost (Fst) was first discovered when Drosophila flies were exposed to cold stress, but the biological function(s) of Fst has so far not been characterized. Because Fst is up-regulated after a cold stress, we tested whether it was essential for chill-coma recovery. METHODOLOGY/PRINCIPAL FINDINGS: A marked increase in Fst expression was detected (by RT-PCR) during recovery from cold stress, peaking at 42-fold after 2 h. The GAL4/UAS system was used to knock down expression of Fst and recovery ability was assessed in transgenic adults following 12 h of chill coma at 0 degrees C. The ability to recover from cold stress (short-, medium- and long-term) was significantly altered in the transgenic adults that had Fst silenced. These findings show that Fst plays an essential role in the recovery from chill coma in both males and females. CONCLUSIONS/SIGNIFICANCE: The Frost gene is essential for cold tolerance in Drosophila melanogaster and may play an important role in thermal adaptation.
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ItemEffects of small Hsp genes on developmental stability and microenvironmental canalization(BIOMED CENTRAL LTD, 2010-09-16)BACKGROUND: Progression of development has to be insulated from the damaging impacts of environmental and genetic perturbations to produce highly predictable phenotypes. Molecular chaperones, such as the heat shock proteins (HSPs), are known to buffer various environmental stresses, and are deeply involved in protein homeostasis. These characteristics of HSPs imply that they might affect developmental buffering and canalization. RESULTS: We examined the role of nine Hsp genes using the GAL4/UAS-RNAi system on phenotypic variation of various morphological traits in Drosophila melanogaster. The stability of bristle number, wing size and wing shape was characterized through fluctuating asymmetry (FA) and the coefficient of variation (CV), or among-individual variation. Progeny of the GAL4/Hsp-RNAi crosses tended to have reduced trait means for both wing size and wing shape. Transcriptional knockdown of Hsp67Bc and Hsp22 significantly increased FA of bristle number, while knockdown of Hsp67Ba significantly increased FA and among-individual variation of wing shape but only in males. Suppression of Hsp67Bb expression significantly increased among-individual variation of bristle number. The knockdown of gene expression was confirmed for Hsp67Ba, Hsp67Bc, Hsp22, and Hsp67Bb. Correlation between FA and CV or among-individual variation of each trait is weak and not significant except for the case of male wing shape. CONCLUSION: Four small Hsp genes (Hsp22, Hsp67Ba, Hsp67Bb and Hsp67Bc) showed involvement in the processes of morphogenesis and developmental stability. Due to possible different functions in terms of developmental buffering of these small Hsps, phenotypic stability of an organism is probably maintained by multiple mechanisms triggered by different environmental and genetic stresses on different traits. This novel finding may lead to a better understanding of non-Hsp90 molecular mechanisms controlling variability in morphological traits.
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ItemMitochondrial DNA variants help monitor the dynamics of Wolbachia invasion into host populations(SPRINGERNATURE, 2016-03)Wolbachia is the most widespread endosymbiotic bacterium of insects and other arthropods that can rapidly invade host populations. Deliberate releases of Wolbachia into natural populations of the dengue fever mosquito, Aedes aegypti, are used as a novel biocontrol strategy for dengue suppression. Invasion of Wolbachia through the host population relies on factors such as high fidelity of the endosymbiont transmission and limited immigration of uninfected individuals, but these factors can be difficult to measure. One way of acquiring relevant information is to consider mitochondrial DNA (mtDNA) variation alongside Wolbachia in field-caught mosquitoes. Here we used diagnostic mtDNA markers to differentiate infection-associated mtDNA haplotypes from those of the uninfected mosquitoes at release sites. Unique haplotypes associated with Wolbachia were found at locations outside Australia. We also performed mathematical and qualitative analyses including modelling the expected dynamics of the Wolbachia and mtDNA variants during and after a release. Our analyses identified key features in haplotype frequency patterns to infer the presence of imperfect maternal transmission of Wolbachia, presence of immigration and possibly incomplete cytoplasmic incompatibility. We demonstrate that ongoing screening of the mtDNA variants should provide information on maternal leakage and immigration, particularly in releases outside Australia. As we demonstrate in a case study, our models to track the Wolbachia dynamics can be successfully applied to temporal studies in natural populations or Wolbachia release programs, as long as there is co-occurring mtDNA variation that differentiates infected and uninfected populations.
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ItemAssociation between Three Mutations, F1565C, V1023G and S996P, in the Voltage-Sensitive Sodium Channel Gene and Knockdown Resistance in Aedes aegypti from Yogyakarta, Indonesia(MDPI, 2015-09)Mutations in the voltage-sensitive sodium channel gene (Vssc) have been identified in Aedes aegypti and some have been associated with pyrethroid insecticide resistance. Whether these mutations cause resistance, alone or in combination with other alleles, remains unclear, but must be understood if mutations are to become markers for resistance monitoring. We describe High Resolution Melt (HRM) genotyping assays for assessing mutations found in Ae. aegypti in Indonesia (F1565C, V1023G, S996P) and use them to test for associations with pyrethroid resistance in mosquitoes from Yogyakarta, a city where insecticide use is widespread. Such knowledge is important because Yogyakarta is a target area for releases of Wolbachia-infected mosquitoes with virus-blocking traits for dengue suppression. We identify three alleles across Yogyakarta putatively linked to resistance in previous research. By comparing resistant and susceptible mosquitoes from bioassays, we show that the 1023G allele is associated with resistance to type I and type II pyrethroids. In contrast, F1565C homozygotes were rare and there was only a weak association between individuals heterozygous for the mutation and resistance to a type I pyrethroid. As the heterozygote is expected to be incompletely recessive, it is likely that this association was due to a different resistance mechanism being present. A resistance advantage conferred to V1023G homozygotes through addition of the S996P allele in the homozygous form was suggested for the Type II pyrethroid, deltamethrin. Screening of V1023G and S996P should assist resistance monitoring in Ae. aegypti from Yogyakarta, and these mutations should be maintained in Wolbachia strains destined for release in this city to ensure that these virus-blocking strains of mosquitoes are not disadvantaged, relative to resident populations.