Genetics - Theses

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Author: Scott, Megan. × End date: 2009 × Type: Masters Research thesis ×

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    Insecticide resistance in the Australian sheep blowfly, Lucilia cuprina : selection, chance and response
    Scott, Megan. (University of Melbourne, 2000)
    The influences of random genetic drift and selection by the insecticides dieldrin and diazinon on the initial establishment of the resistant allele (Rdl or Rop1) and subsequent allelic frequency changes within populations of L. cuprina were investigated. Discrete generation population cages were initiated with Rdl and Rop1 frequencies of 1 per cent or 5 per cent and maintained for 17 or 10 generations respectively on media with a concentration range that kills between 0 and 100 per cent of susceptible individuals. The resistant allele frequency was recorded at each generation. The probability of the initial establishment of the Rdl allele in a population was consistently greater at the 5 per cent frequency and dependent on the concentration of dieldrin in the medium for both starting frequencies. Once the resistant allele was established, responses to selection were concentration-dependent. In the absence of dieldrin the susceptible allele was selectively favoured, at concentration 0.00005 per cent (w/v) selection was approximately neutral and at concentrations above this the Rdl allele was at a selective advantage. Fixation of Rdl occurred at higher concentrations. The probability of the initial establishment of the Rop1 allele in a population was found to be independent of both founding allele frequency and the concentration of diazinon on which the population was maintained. Once the resistant allele was established, responses to selection were concentration-dependent. In the non-modifier populations, in the absence of diazinon, the susceptible allele was selectively favoured, at concentration 0.00002 per cent (w/v) selection was approximately neutral and at concentrations above this the Rop1 allele was at a selective advantage, with fixation occurring at higher concentrations. In populations where the modifier gene was present the resistant allele was maintained in all populations with selection generally favouring resistant phenotypes. In populations maintained on insecticide, that fixed for the susceptible allele neither resistance system provided evidence of a polygenic response. Computer simulations were conducted to estimate the relative genotypic fitness values that could give rise to the allele frequencies produced by the population cage experiments and then to extrapolate the results to field situations. Results support the hypothesis that in the absence of the chemical the susceptible allele is at an advantage and in the presence of the chemical the resistant allele is selected for in both the dieldrin and diazinon resistance systems. Population size and genetic drift size were found to have little effect on the development of insecticide resistance when founding allele frequencies were at 1 and 5 per cent. When founding allele frequencies representative of field situations were simulated population size and drift dramatically influenced the initial establishment, and thus the development of resistance, in both the dieldrin and diazinon resistance systems. The resistant allele failed to establish in the population on all experimental concentrations when a finite population size of 100 was analysed. Furthermore, the resistant allele failed to establish in an infinite population in the absence of the chemical and when selection coefficients were low. When selection coefficients were high, and population size was infinite, results obtained were basically the same as those achieved when the simulation was run using 1 and 5 per cent founding allele frequencies. The resistance system-specific impact of genetic drift and selection on the evolution of insecticide resistance is discussed in the context of the applicability of the results to resistance management strategies or for resistance systems as a general model of microevolution.
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    Insecticide resistance in the Australian sheep blowfly, Lucilia cuprina : selection, chance and response
    Scott, Megan. (University of Melbourne, 2000)
    The influences of random genetic drift and selection by the insecticides dieldrin and diazinon on the initial establishment of the resistant allele (Rdl or Rop1) and subsequent allelic frequency changes within populations of L. cuprina were investigated. Discrete generation population cages were initiated with Rdl and Rop1 frequencies of 1 per cent or 5 per cent and maintained for 17 or 10 generations respectively on media with a concentration range that kills between 0 and 100 per cent of susceptible individuals. The resistant allele frequency was recorded at each generation. The probability of the initial establishment of the Rdl allele in a population was consistently greater at the 5 per cent frequency and dependent on the concentration of dieldrin in the medium for both starting frequencies. Once the resistant allele was established, responses to selection were concentration-dependent. In the absence of dieldrin the susceptible allele was selectively favoured, at concentration 0.00005 per cent (w/v) selection was approximately neutral and at concentrations above this the Rdl allele was at a selective advantage. Fixation of Rdl occurred at higher concentrations. The probability of the initial establishment of the Rop1 allele in a population was found to be independent of both founding allele frequency and the concentration of diazinon on which the population was maintained. Once the resistant allele was established, responses to selection were concentration-dependent. In the non-modifier populations, in the absence of diazinon, the susceptible allele was selectively favoured, at concentration 0.00002 per cent (w/v) selection was approximately neutral and at concentrations above this the Rop1 allele was at a selective advantage, with fixation occurring at higher concentrations. In populations where the modifier gene was present the resistant allele was maintained in all populations with selection generally favouring resistant phenotypes. In populations maintained on insecticide, that fixed for the susceptible allele neither resistance system provided evidence of a polygenic response. Computer simulations were conducted to estimate the relative genotypic fitness values that could give rise to the allele frequencies produced by the population cage experiments and then to extrapolate the results to field situations. Results support the hypothesis that in the absence of the chemical the susceptible allele is at an advantage and in the presence of the chemical the resistant allele is selected for in both the dieldrin and diazinon resistance systems. Population size and genetic drift size were found to have little effect on the development of insecticide resistance when founding allele frequencies were at 1 and 5 per cent. When founding allele frequencies representative of field situations were simulated population size and drift dramatically influenced the initial establishment, and thus the development of resistance, in both the dieldrin and diazinon resistance systems. The resistant allele failed to establish in the population on all experimental concentrations when a finite population size of 100 was analysed. Furthermore, the resistant allele failed to establish in an infinite population in the absence of the chemical and when selection coefficients were low. When selection coefficients were high, and population size was infinite, results obtained were basically the same as those achieved when the simulation was run using 1 and 5 per cent founding allele frequencies. The resistance system-specific impact of genetic drift and selection on the evolution of insecticide resistance is discussed in the context of the applicability of the results to resistance management strategies or for resistance systems as a general model of microevolution.