Genetics - Theses

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    Dissection of the genetic and physiological basis of desiccation resistance in Drosophila melanogaster using laboratory selection
    de Garis, Sarah Elizabeth ( 2011)
    Desiccation stress is a major factor underlying the distribution, abundance and evolution of insect species. This study utilises laboratory selection to dissect the physiological and genetic basis of desiccation resistance in a population of D. melanogaster recently collected from the field. The first part of this thesis assays selected and control flies for a series of physiological traits. Selected flies were found to lose water at a slower rate than their controls when exposed to desiccation stress, in keeping with previous investigations. Another major physiological mechanism underlying the selection response was increased water storage, possibly mediated via enlarged glycogen reserves. This study is the first to demonstrate that these traits can evolve in response to desiccation selection in fly lines recently derived from nature. Dehydration tolerance and whole-body lipid content however did not contribute towards the selection response, a finding consistent with most prior studies. The second part of this study investigates whether desiccation selection produced correlated responses in other environmental stress traits. In keeping with previous experiments, a strong correlated response was observed between desiccation and starvation resistance, suggesting that the two traits are shaped by at least partially overlapping physiological mechanisms. In contrast this investigation found no correlated response between desiccation resistance and cold tolerance, as measured by both cold mortality and chill coma recovery. The data surrounding heat tolerance were more complex; while higher heat knockdown resistance failed to evolve in response to desiccation selection, increased heat mortality and, to a lesser extent, Critical Thermal Maximum (CTMAX) did. This investigation also aimed to assess whether flies selected for increased desiccation resistance in the laboratory showed an increase in an aspect of field fitness under dry conditions. Female selected flies were better able than their controls to reach food resources following release into a natural environment but this was not the case for males. This result indicates that though laboratory selection for desiccation resistance can result in ecologically relevant evolutionary change, levels of field fitness are not always accurately predicted by laboratory-based assays. Finally, tiling microarrays were used to detect sequence variation between selected and control populations. Six hundred and seventy one Single Feature Polymorphisms (SFPs), located across all chromosomes, were found between the two treatments, none of which correspond to known desiccation resistance candidates. Sequence divergence was validated via sequencing for eight genes: CG7084, beat-VII, CG7638, omega, Dystrophin, SNF4Agamma, mol and CG14304. These genes may become solid desiccation resistance candidate genes if they are found to be associated with the trait in independent, natural populations.