Genetics - Theses

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    Insecticide resistance in Lucilla cuprina and Drosophila melanogaster
    Magoc, Lorin. (University of Melbourne, 2001)
    Insecticide resistance is a major challenge not only for agricultural production, but also in terms of environmental preservation and human health. Intensive pesticide use for the last half century has resulted in the rapid evolution of resistance not only in pest species, but also in beneficial and non-targeted insects. The options for resistance in insect species have been well investigated, and two major types of resistance - metabolic and target-site resistance have been identified. EMS mutagenesis and selection represent tools for generating resistant mutants in the laboratory, enabling us to predict the evolution of resistance to particular chemicals before it evolves in the field. This may prevent the use of ineffective chemicals and the evolution of resistance in other species that may have been targeted inadvertently. Laboratory studies in the Australian sheep blowfly, Lucilia cuprina have been shown to parallel the field situation. This study investigates the possibility of the existence of a resistance option for the organophosphorous compound, diazinon, other than the known Rop-1/E3-mediated resistance. Only one resistant mutant was generated after EMS-mutagenesis of the susceptible strain. Genetic mapping of the resistant mutation and molecular analysis showed that the mutation is Rop-1. Explanation for Rop-1/E3 bias in L. cuprina response to organophosphates is discussed. Furthermore, options for resistance to novel insecticides - Insect Growth Regulators (IGRs) were investigated, using Drosophila melanogaster as a model. IGRs were introduced in late 1970s as a group of highly selective insecticides with low toxicity for vertebrates and the environment. It was thought that resistance to chemicals with similarities to the insect hormones (juvenile hormone or ecdysone) would be unlikely to evolve. However, it did evolve and very rapidly to some of these chemicals. Resistance to lufenuron has been found in non-targeted species D. melanogaster, in strains isolated from the East coast of Australia. This resistance was mapped and its position within a chromosome determined. Another example of lufenuron resistance was found in EMS-induced cyromazine resistant strain, Cyr1b. Lufenuron resistance was mapped, to determine whether the same gene confers resistance to both chemicals. Phenotypic correlation of survivorship in Cyr1b cross-resistant strain was shown to be result of a high genotypic correlation, which suggests that single mechanism confers resistance to both, lufenuron and cyromazine. Cross-resistance studies were performed with dicyclanil, a recently introduced IGR, on all cyromazine-resistant mutants generated by EMS-mutagenesis. Resistance to dicyclanil in Cyr2 strain was mapped to the same region as Rst(2)Cyr. A field lufenuron-resistant strain was tested for cross-resistance to cyromazine, diflubenzuron and dicyclanil and a very unpredictable cross-resistance pattern among IGRs was found. This thesis highlights the importance of using the prediction of resistance, positional cloning and cross-resistance studies for insecticide resistance management in the field.
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    Insecticide resistance in Lucilla cuprina and Drosophila melanogaster
    Magoc, Lorin. (University of Melbourne, 2001)
    Insecticide resistance is a major challenge not only for agricultural production, but also in terms of environmental preservation and human health. Intensive pesticide use for the last half century has resulted in the rapid evolution of resistance not only in pest species, but also in beneficial and non-targeted insects. The options for resistance in insect species have been well investigated, and two major types of resistance - metabolic and target-site resistance have been identified. EMS mutagenesis and selection represent tools for generating resistant mutants in the laboratory, enabling us to predict the evolution of resistance to particular chemicals before it evolves in the field. This may prevent the use of ineffective chemicals and the evolution of resistance in other species that may have been targeted inadvertently. Laboratory studies in the Australian sheep blowfly, Lucilia cuprina have been shown to parallel the field situation. This study investigates the possibility of the existence of a resistance option for the organophosphorous compound, diazinon, other than the known Rop-1/E3-mediated resistance. Only one resistant mutant was generated after EMS-mutagenesis of the susceptible strain. Genetic mapping of the resistant mutation and molecular analysis showed that the mutation is Rop-1. Explanation for Rop-1/E3 bias in L. cuprina response to organophosphates is discussed. Furthermore, options for resistance to novel insecticides - Insect Growth Regulators (IGRs) were investigated, using Drosophila melanogaster as a model. IGRs were introduced in late 1970s as a group of highly selective insecticides with low toxicity for vertebrates and the environment. It was thought that resistance to chemicals with similarities to the insect hormones (juvenile hormone or ecdysone) would be unlikely to evolve. However, it did evolve and very rapidly to some of these chemicals. Resistance to lufenuron has been found in non-targeted species D. melanogaster, in strains isolated from the East coast of Australia. This resistance was mapped and its position within a chromosome determined. Another example of lufenuron resistance was found in EMS-induced cyromazine resistant strain, Cyr1b. Lufenuron resistance was mapped, to determine whether the same gene confers resistance to both chemicals. Phenotypic correlation of survivorship in Cyr1b cross-resistant strain was shown to be result of a high genotypic correlation, which suggests that single mechanism confers resistance to both, lufenuron and cyromazine. Cross-resistance studies were performed with dicyclanil, a recently introduced IGR, on all cyromazine-resistant mutants generated by EMS-mutagenesis. Resistance to dicyclanil in Cyr2 strain was mapped to the same region as Rst(2)Cyr. A field lufenuron-resistant strain was tested for cross-resistance to cyromazine, diflubenzuron and dicyclanil and a very unpredictable cross-resistance pattern among IGRs was found. This thesis highlights the importance of using the prediction of resistance, positional cloning and cross-resistance studies for insecticide resistance management in the field.