Genetics - Theses
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ItemInsecticide resistance, development and fitness in the Australian sheep blowfly, Lucilia cuprina(University of Melbourne, 1993)Insecticide resistance offers an advantageous system for the study of evolutionary genetics. The transition from a susceptible population to a resistant population is an evolutionary event that can be studied at the ecological, genetic and molecular levels. This study investigates the inter-relationship between genotype, development and fitness within the context of the evolution of insecticide resistance in the Australian sheep blowfly, Lucilia cuprina. In the first part of this study, a phenomenon called fatal association is investigated that may have been a contributing factor to the rapid rate at which dieldrin resistance evolved. It is shown that dieldrin-resistant adult flies, which have been raised from larvae on media containing dieldrin, can cause the death of susceptible adult flies by inhabiting shared space. Chemical analyses show that resistant adult genotypes, which are raised on media containing dieldrin, have internalised significant quantities of dieldrin. The toxic effect on susceptible flies is the result of resistant flies excreting that internalised dieldrin in biologically significant quantities. The mechanism of resistance, reduced affinity of the dieldrin target site for the insecticide, makes the fatal association effect possible as high concentrations of non-metabolised dieldrin can be tolerated and stored by resistant flies. This phenomenon is hypothesised to have had an impact on the rate at which dieldrin resistance evolved by increasing the relative fitness of resistant phenotypes compared with susceptible phenotypes in a frequency-dependent manner. The effect of fatal association would be to reduce the refuge from insecticide that can maintain a level of susceptibility in a population. The pattern of diazinon use of continued treatment on populations where resistance was at a high frequency, resulted in modification of the fitness of diazinon-resistant phenotypes by co-adaptation. This modification of fitness is due to allelic substitution at a single gene. The second part of this study involves the candidate gene for this modifier, the Scalloped wings gene. Scalloped wings (Scl) is shown to be homologous at the genetic and molecular levels to the Drosophila melanogaster developmental gene Notch. This includes molecular homology by in situ hybridisation and similarities in the dominant adult phenotypes and recessive lethal phenotypes of mutations in Scl and Notch. A mutagenesis screen for novel alleles of Scl that correspond to different allele classes of Notch is performed with the isolation of an unusual Scl allele that does not conform to the Notch allele class system. The significance of the homology between Scl and Notch with regard to Scl being the candidate gene for the fitness modifier of diazinon resistance is discussed. As a test for the allelism of the fitness modifier of diazinon resistance with the Scl gene, the effect of the modifier allele on the incompletely penetrant adult phenotypes of two Scl alleles is examined in susceptible and resistant backgrounds. The modifier is shown to increase the penetrance of these phenotypes, an effect that can be interpreted as an allelic interaction. Interestingly, the presence of the resistance allele (Rop-1) also affects the penetrance of the Scl phenotypes. This implies that the Rop-1 gene may function in developmental pathways in common with the Scl gene. The nature of the interaction between the two genes is discussed in terms of the likely structure of their gene products and in light of the developmental asymmetry phenotype associated with the resistance allele.