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ItemNo Preview AvailableInsecticide resistance in the Australian sheep blowfly, Lucilia cuprina : selection, chance and response(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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ItemNo Preview AvailableInsecticide resistance in the Australian sheep blowfly, Lucilia cuprina : selection, chance and response(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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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.
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ItemInvestigations of Drosophila melanogaster as a model for insecticide resistance studies(University of Melbourne, 1991)Standard techniques were defined for using Drosophila melanogaster as a model organism for insecticide-resistance studies. The dosage mortality curves for sensitve (wild-type (WT)) strains (CS and attached-X) challenged with dieldrin, diazinon and cyromazine were defined to choose a concentration to screen for monogenically resistant survivors. Ethylmethanosulphonate mutagenesis was used to create diversity within the sensitive populations prior to screening. From a screen of 600,000 embryos, and the rescreening of 160 survivors, two stabley cyromazine resistant strains were isolated. By standard mapping techniques the resistance in these two strains was found to be conferred by different genes (or gene complexes), designated Cyr-1 and Cyr-2. These mapped respectively within 1 m.u. of ri on chromosome III, and in the region of vg on chromosome II. The dosage mortality for various combinations of the resistance alleles showed that Cyr-1 confers 3xWT tolerance when heterozygous and homozygous, while Cyr-2 confers 2xWT when heterozygous and 4xWT when homozygous. Fitness tests on the resistance alleles showed that Cyr-1 causes a significant decrease in developmental time and survival compared with CS. Neither of the resistance alleles, however, appear to affect developmental homeostasis since the asymmetry values of resistant strains are similar to that of WT. It is argued on the basis of these results, with some modification to the techniques, that Drosophila melanogaster can be a useful model for studying the inheritance of resistance.