Pesticide Resistance Strategies and Tactics for Management (1986) / Chapter Skim
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Quantitative Genetic Models and the Evolution of Pesticide Resistance
Quantitative Genetic Models and the Evolution of Pesticide Resistance
Pages 222-235
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From page 222... ...
Quantitative Genetic Models and the Evolution of Pesticide Resistance SARA VIA When tolerance to pesticides varies continuously among individuals, a quantitative genetic approach to resistance evolution is more useful than is the usual single-locus view. Relative characteristics of polygenic and single-gene resistance are described; then the evolution of polygenic resistance is discussed in terms of basic quantitative genetics principles.
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From page 223... ...
Because pesticides are agents of selection, pesticide resistance can be studied by using the same theoretical frameworks as have been applied to other types of evolutionary change. Previous population genetic models have considered that resistance is determined by a single gene.
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From page 224... ...
In polygenic resistance a continuous dose-response relationship results from the combination of environmental and genetic factors. No distinct genotypic classes can be identified because classes overlap when several loci determine a trait; polygenic characters thus are also called "continuous characters" (Falconer, 19811.
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From page 225... ...
Genetic Correlations Among Traits The univariate formulation presented in equation 2 applies only when selection acts on a single character, such as tolerance to a particular pesticide.
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From page 226... ...
Correlated characters cannot evolve independently: if two traits are negatively correlated, selection for one to increase may result in a correlated decrease in the other even if this is disadvantageous. Therefore, genetic correlations can constrain the evolution of the whole phenotype and can cause maladaptation of some traits within a correlated suite.
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From page 227... ...
Because an intermediate optimum is assumed, the model does not apply to characters like total fitness or survival, which are assumed to be under continual directional selection to increase. Pesticide tolerance may have an intermediate optimum: individuals with high membrane impermeability or excessive behavioral avoidance of chemicals that they could metabolize may be at a disadvantage relative to individuals with more intermediate values of the features that confer tolerance.
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From page 228... ...
Until they are tested or proved, the models function primarily to introduce hypotheses about what can happen in the course of evolution of pesticide resistance. The mode of selection that seems most realistic here is so-called hard selection, in which the contribution of each patch to the mating pool after selection is proportional to both q and to the relative mean fitness of individuals selected in that patch (WilW, where W = qW~ + (1 - qjW2~.
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From page 229... ...
If, however, the genetic correlation is low, evolution of resistance to the rarer compound will be slow to occur; most of the population experiences the other pesticide. For strongly negative genetic correlations, Figure 2 illustrates that tolerance to the rare compound can actually decrease as the evolution of resistance to the common pesticide occurs.
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From page 230... ...
to the old pesticide (Figure 3~. This example requires that an intermediate optimum tolerance actually exists, so that a positive genetic correlation in tolerances will cause an overshoot of the optimum tolerance to pesticide 1 and a corresponding decrease in mean fitness.
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Other Approaches As seen in Figures 2 and 3, the effect of the genetic correlation in tolerance on resistance evolution depends on the initial mean tolerance to each compound relative to the optimum level of tolerance. Within the context of the basic model described here and its attendant assumptions, several alternative strategies of pesticide application could be investigated.
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From page 232... ...
Simultaneous application of multiple pesticides is not the answer, since it could cause an increase in selection intensity and thus would probably speed rather than retard evolution of resistance. To improve the descriptive power of a quantitative genetic model of pesticide resistance, a model of directional selection that is not tied to the weak selection requirement is necessary.
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in tolerance to different pesticides are virtually unknown. The quantitative genetic variance in tolerance can be estimated by breeding individuals to generate families and then exposing some siblings from each family to the different compounds in replicate groups.
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1982. The effect of genetic background on the fitness of diazinon resistance genotypes of the Australian sheep blowfly, Lucilia cuprina.
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II. Genetic correlations in larval performance within and among host plants.
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