Transcriptional age-grading of the dengue vector, Aedes aegypti

Cook, Peter Edward (2007). Transcriptional age-grading of the dengue vector, Aedes aegypti PhD Thesis, School of Integrative Biology, The University of Queensland.

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Author Cook, Peter Edward
Thesis Title Transcriptional age-grading of the dengue vector, Aedes aegypti
School, Centre or Institute School of Integrative Biology
Institution The University of Queensland
Publication date 2007-08
Thesis type PhD Thesis
Supervisor Blows, Mark W.
Iturbe-Ormaetxe, Inaki
O'Neill, Scott L.
Subjects 270000 Biological Sciences
Formatted abstract Mosquito population age structure is a major determinant of the population’s capacity to transmit pathogens such as the dengue viruses. This relationship exists because the duration of the extrinsic incubation period for most pathogens is long relative to the expected lifespan of the mosquito vector. Given the epidemiological significance of vector longevity, numerous age-grading techniques have been developed to estimate the age of field-caught mosquitoes. Previous age-grading methods have provided limited information on mosquito survival rates and the demographic structure of the vector population. There is a need for new and informative mosquito age determination methods that are applicable across the entire mosquito lifespan and are amenable to a high-throughput workflow. Development of an age-grading method such as this may allow for large population samples to be accurately age-graded and mosquito population age structure to be inferred with greater precision.
Microarray studies have demonstrated that age-related transcription of a small proportion of the genome occurs in a range of insects. Here, we report the first application of gene expression measures for age determination in adult female Aedes aegypti, the primary vector of dengue viruses. Candidate genes, with age-related expression profiles, were identified from published Drosophila melanogaster microarray data. Aedes aegypti orthologues were isolated and their expression examined in mosquitoes reared under controlled laboratory conditions. Only two genes, Ae-8505 and Ae-15848, displayed distinct transcriptional changes with adult female age. However, when the relative gene expression measures were examined with multivariate statistics it became apparent that collectively there was sufficient information for accurate age prediction.
Following the positive results obtained in the preliminary studies on laboratory-reared mosquitoes a study was undertaken to determine whether transcriptional measures were influenced by environmental conditions. The field validation of the age-grading method involved the comparative analysis of age predictions made from gene expression and cuticular hydrocarbon data collected from the same individual. These results indicated that transcriptional age-grading gave more accurate and precise age predictions than the cuticular hydrocarbon method under the environmental conditions tested. A simplified assay, using the three most informative gene expression measures, also performed better than the cuticular hydrocarbon method.
In order to increase sample throughput and reduce costs, a multiplex assay was developed for the three gene transcriptional age-grading method. This new assay was used to investigate seasonal fluctuations in female Ae. aegypti population age structure in Cairns, Queensland, Australia. The transcriptional measures correlated to a lesser extent than in previous studies, which resulted in age predictions with large confidence intervals. Despite these disappointing results, a shift to older age classes was observed between the hot, dry season and the wet season. This change appeared to be driven primarily by increased female mosquito survivorship and not by increases in population size.

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