1,720,986 research outputs found
Larval ecological adaptations, water quality and aquaticbacterial communities in Anopheles gambiae s.l. :prospects for improved rearing techniques towards releaseprogrammes
Get PDFThe sibling species Anopheles gambiae s.s. and Anopheles coluzzii are the most important vectors of human malaria in sub-Saharan Africa. They are thought to be undergoing speciation with gene flow with rare viable hybrids but are reproductively isolated by assortative mating and ecological divergence. As such important vectors, they are the focus of novel control strategies based on mosquito releases. One of the known drivers of their ecological speciation is their divergent larval ecological adaptation that is possibly linked to rice domestication in Africa. The potential impact of such divergence has never been integrated into mosquito rearing to improve alternative vector control methods such as Sterile Insect Technique (SIT) and Genetically Modified Mosquitoes (GMM), that are needed to accelerate the progress towards malaria elimination. These innovative vector control methods depend on mass rearing of millions of mosquitoes in a manner that is both efficient and economic, to produce mosquitoes that are of adequate quality, able to favourably compete with wild populations. In this thesis, we investigated the phenotypic plasticity of these sibling species to typical stressors in the rice field ecosystem with a focus on ammonia in their larval habitat. Experiments were conducted in small containers and in contrasted microcosms to test the direct effects of mineral water and increasing ammonia concentrations on larval development and to highlight divergent reaction norms between the sibling species. We also evaluated the use of zeolite to improve larval water quality management in An. gambiae s.l. insectary. To further understand the dynamics of the nitrogen cycle in larval rearing trays that led to larval mortality, we characterised their bacteria communities using 16S rRNA gene sequencing. Functional filters were applied to identify candidate bacteria species beneficial and detrimental to larval development and these were validated by qPCR. Our results suggest that genotype-by-environment interactions associated with rice domestication event in Africa are indeed an important driver of the eco-speciation between the sibling species. An. coluzzii was more tolerant to ammonia and rice-field like conditions supporting the idea that this may have driven its speciation from the ancestral An. gambiae s.s. We show that mineral water is beneficial for improved mosquito yield and phenotypic quality of adult mosquitoes in the insectary and this can be used to improve rearing protocols for these species. For the first time, we demonstrated that zeolite can be used to improve rearing results for An. gambiae s.l., providing a water conserving alternative for rearing mosquitoes for mass release programmes, especially in arid regions. Furthermore, the ensuing analyses of bacterial communities larval trays is also a novel endeavour which led to the identification of 1031 bacteria species and of several key species with various opportunities for further improvement of larval rearing towards mass release purposes and/or for novel direct vector control. In conclusion, we have made modest contributions towards the control of these malaria vectors and the fight to eliminate this multifaceted disease. It is therefore important that policy makers in malaria endemic countries ensure that policy reformations in irrigational agriculture and urbanization consider the impact of policy on these disease vectors that are of immense public health importance
Genetic and ecological processes of speciation in the Anopheles coluzzii and Anopheles gambiae sibling species
Get PDFMalaria is a leading cause of death, killing thousands every year. Sub-Saharan Africa has the highest mortality rate, with pregnant women and children under the age of five most affected. The culprits responsible for transmission of the disease are mosquitoes of the genus Anopheles. Preventative measures using vector control are mainly insecticide based, however, resistance threatens the efficacy of these methods. The spreading of transgenes is considered an alternative vector control method but the success of this depends on knowledge of the Anopheles population and cryptic taxa in the wild. The genetic and ecological differences that exist between the populations and how reproductive isolation occurs between the vectors is of great importance in prospective vector control programs which rely on the release of transgenic mosquitoes.Focusing on Anopheles coluzzii and Anopheles gambiae, the recently diverged members of the Anopheles gambiae complex, morphological and molecular techniques were used to study genetic and ecological differences between the sibling species. Egg morphological differences facilitated by ecological divergences between the two sibling species were studied. Results showed a difference in egg shape and size between the two species and populations within species.The genetic studies focused on the identification of assortative mating genes. In order to identify candidate mate choice genes, expression levels in 27 putative genes located on the X-island of speciation were investigated. A majority of the genes were over-expressed in virgin males in the samples. An attempt to silence the two top candidate putative assortative mating genes through RNAI using injection as the method of dsRNA delivery led to a surprising outcome, as the mechanical impact of the injections appeared to disrupt the assortative mating pattern
Exploring the use of attractive sugar bait stations to deliver antiplasmodial compounds to Anopheles mosquitoes
Get PDFBetween 2000 and 2015, Sub-Saharan Africa made historic process against the scourge of malaria. Since this time there has been a plateau in the number of malaria cases and mortalities. The decreasing efficacy of the current methods used against this parasitic disease, such as IRS and bednets, is associated with a multitude of interspersed factors; such factors may be overcome with the incorporation of novel strategies. Furthermore, the re-emerging interest in attractive toxic sugar bait stations has been negatively impacted by the suggestion that the toxic compounds used could cause issues for non-target organisms. In this study the aim was to identify a series of methods through which the viability of anti-plasmodial compounds can be investigated for their application into a mosquito bait station. To do so, the potencies of two known sexual-stage active compounds were evaluated, alongside two control compounds, before and after their prolonged exposure to the environmental conditions of an attractive bait station in sub-Saharan Africa. The conditions used in this assessment included pH, temperature, time and light. The inhibition (%) of the compounds against the asexual and sexual stages of the P. falciparum (Dd2 strain) lifecycle were determined using luciferase assays and mosquito feeding assays respectively. Promisingly, the compounds maintained their stability within the relevant temperature cycles, however, they were unable to provide the necessary pan-activity against the oocyst and sporozoite stages of the plasmodia lifecycle. MMV011895 displayed some activity against the oocyst development, but not against sporozoite formation whereas MMV666060 displayed some activity against the sporozoite formation but not against oocyst development. Although a compound with the required characteristics has yet to be found, the in vitro methods used do provide a way in which the stability of candidate compounds can be screened without the use of more expensive, timely and labourintensive methods. With the identification of a suitable compound, the exposure of infected Anopheles to an anti-plasmodial using bait stations could be a climacteric strategy in the bid for malaria elimination
Natural variations in egg hatching behaviour and selection of novel slowhatching strains for male anopheline deployment
No full textGenetic Mosquito mass release is a promising tool in the suppression of wild Anopheles gambiae sensu lato mosquitoes as part of malaria control in Africa. This tool requires a plethora number of eggs and a strain with good fitness. Thus, the success greatly depends on how the strain can survive and successfully introgress with the wild (target) population. Given the geographical distribution of Anopheles gambiae complex in sub-Sahara Africa, this technique may also require shipment of eggs from egg production facilities to egg rearing and deployment locations. Because colonised mosquito strains are inadvertently selected for early hatching, due to the simplified rearing protocols in insectaries, shipment of eggs of the prolonged established strains is therefore constrained particularly to remote locations. The production of novel late hatchers with good fitness should be readily available for this purpose to compensate for the early time of hatching of Anopheles eggs. It was hypothesised that egg hatching time was under genetic control and the study was designed to test that hypothesis.Using prolonged and recently colonised strains of three sibling species of Anopheles gambiae complex: An. gambiae s.s, (Kisumu strain 44 years old); An. arabiensis, (Senn strain 50 years old and Rufisque strain 2 years old) and An. coluzzii, (Mopti strain 16 years old and VK strain 1 year), egg-hatching biology was explored to determine variation in egg hatching distribution of the five strains colonised at different times and effect of the time of colonisation on hatch rate. The effect of simulating rainfall by splashing water of different temperatures (Still-non agitation at 26 C, 4 C spray, and 15 C spray) on eggs every day for 7 days was explored. It was observed that Anopheles eggs colonised at different times exhibited distinct hatching patterns in relation to water treatments. No eggs hatched within 24 hours (day 0) post oviposition in both agitated and non-agitated groups. However, on the 2nd day (day1), the hatch rate started increasing, with the older strains exhibiting outstanding higher hatch rates compared to their recently colonised counterparts. Overall, the older strains (Senn, Kisumu, and Mopti) hatched within days 1 and 2. In the five strains, hatching spread up to a span of 5 days with only the recently colonised Rufisque and VK strains hatching on the 7th day (day 6)- the last day of the experiment. Across all water conditions, the older strains had highest hatching success compared to their recently colonised counterparts.The recently colonised VK strain was subjected to an evolutionary study by employing a bidirectional (artificial) selection to investigate if egg-hatching time has a genetic component that could be selected and passed down from the parents to their offspring over many generations. Fast (Early1 and Early2) and Slow (Late1 and Late2) hatching strains were established from a parental strain. At the end of the study, the eggs of the Early hatchers hatched significantly faster than their parental strain, while Late hatchers on the other hand took a significantly longer time to hatch than the parental strain in the second trial at generation 17. The fitness of both the selected (Early1, Late1, Early2, and Late2) strains and the longestablished unselected (Mopti) strain was assessed to determine if selection has negative impacts on mating success, fertility, and adult survival. Focusing on the late-selected strains, we observed significant improvement in all parameters tested compared to the well-established Mopti, which indicated a potential loss of fitness in their adult survival. There were no significant differences in mating success and fertility between Mopti and Late2. However, adults of the selected late hatching strains, Late1 and Late2 survived significantly longer compared to the long-established Mopti. The variations observed in these studies suggest that both genetic and environmental factors could be responsible for early or late hatching patterns in Anopheles eggs. Evidence from the study suggests that there are genes encoding egg hatch time as a heritable trait. Understanding these factors would help to improve egg hatching management and relieve constraints in Anopheles mass maintenance and egg shipment for male anopheline deployment
Evaluation of Nanopore sequencing for interrogating mosquito pools from surveillance systems
No full textVector-borne diseases are responsible for 17% of all infectious diseases worldwide and contribute to more than 700 000 deaths annually, mainly in sub-Saharan countries. Vector surveillance is a crucial element of vector control programmes to assure they remain effective. Genomic surveillance provides valuable information on species identification but is often neglected due to the associated costs and training demands. This thesis focused on the use of the MinION sequencer for interrogating mixed pools from surveillance systems, with minimal bioinformatic analysis requirements to empower African labs and push towards routine genomic analysis. Mosquito pools were designed using different species that act as vectors for main diseases, mimicking possible content of field traps. Some pools included DNA from pathogens carried by these vectors responsible for malaria and filariasis. It was observed that a targeted approach was preferable to whole genome sequencing in estimating species abundance, allowing for cost reduction and detection of target site mutations as well. Different bioinformatic pipelines were tested to verify the accuracy of multiple tools and the training level requirements. The use of the Minimap2 software coupled with Samtools provided the best results and it is suitable to deploy in low-income areas. The MinION device was shown to be effective for interrogating mosquito pools, offering a solution for challenges in vector surveillance in sub-Saharan Africa. Future studies should be done with field-collected mosquitoes and sequencing performed locally to evaluate feasibility. More species of mosquitoes and pathogens should be included as well as the use of RNA sequencing for viral detection
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Optimising the performance of genetically engineered Aedes aegypti
Get PDFThe mosquito Aedes aegypti is the primary worldwide vector of dengue fever, of which there are an estimated 390 million infections annually. Traditional vector control methods have been unsuccessful in preventing Ae. aegypti from posing a serious threat to human health worldwide, and as a result increasing emphasis is being placed on new control technologies. One of these is RIDL® (Release of Insects carrying a Dominant Lethal), a modified form of the sterile insect technique. Released RIDL males of strain OX513A are genetically engineered to possess a repressible lethal construct. Upon mating with a wild female, the construct is inherited by the offspring, causing their death.
The RIDL strategy depends on the mass production and release of vigorous male insects, capable of competing with wild males to mate with wild females. Chapter 2 investigates how rearing conditions can influence the size, asymmetry, quantity, and timing of OX513A mosquitoes produced. Chapter 3 describes the design and testing of devices for the release of these adult males, and their implementation during the worldâs first release of transgenic mosquitoes. Chapter 4 discusses how the modification of male size and nutritional status may affect subsequent performance, and how these changes to male vigour will be assessed in subsequent chapters. Chapter 5 measures changes in competitiveness by examining longevity. Larger males lived longer, and longevity was increased by increasing the duration of the initial sugar feed. Chapter 6 examines the potential improvements in male performance by measuring the flight capacity of OX513A males, using a purpose-built mosquito flight mill. Male size or nutritional status was not found to affect flight capacity, although light intensity was unexpectedly found to modulate the flight speed of the males. Finally, Chapter 7 examines potential improvements in the mating performance of male OX513A Ae. aegypti. When OX513A males and wild-type males were placed in direct competition for virgin females, smaller OX513A males were found to have a significant mating advantage over larger wild-type males. The results of all three of these performance measures are discussed with relevance to field releases.
The results presented in this thesis improve current knowledge of how the performance of male mosquitoes can be effectively measured and improved. This is a vital part of ensuring the success of male-release vector control strategies such as RIDL.</p
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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