Pesticide Resistance Strategies and Tactics for Management (1986) / Chapter Skim
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Pleiotropy and the Evolution of Genetic Systems Conferring Resistance to Pesticides
Pleiotropy and the Evolution of Genetic Systems Conferring Resistance to Pesticides
Pages 207-221
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From page 207... ...
In this paper I consider the evolutionary process at the earlier stage, in which qualitative improvement of the expression of resistance arises as an adaptation both to the pesticide and to natural selection. In this discussion I consider pesticide resistance as an expression of an entire genetic system and examine the implications of this multilocus perspective with respect to the optimal conditions for its evolution.
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From page 208... ...
The processes of adaptation occurring in microbes in the laboratory and in pests of commercial crops in the field share two characteristics: the extraordinary intensity of selection imposed and the sophistication of the genetic mechanisms for the coordinated induction and repression of catabolic enzymes that respond. Responses of modern microbes to laboratory selection may in fact reveal more about the evolution of pesticide resistance than the evolution of primitive microorganisms.
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From page 209... ...
Costs Associated with Pleiotropy If the modification of normal regulation or specificity of the key enzyme favored under artificial selection interferes with its original function, then the mutant form may suffer a disadvantage relative to the wild type in the absence of artificial selection. This disadvantage under natural selection may be regarded as the cost of pleiotropy.
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From page 210... ...
Reduction in pleiotropic costs associated with new functions permits adaptation by the population to both environments. MECHANISMS OF PESTICIDE RESISTANCE The effective, highly evolved mechanisms for tolerating or detoxifying pesticides possessed by laboratory strains derived from resistant populations are not very likely to be representative of the rudimentary resistance mechanisms that were marshaled on initial exposure to the pesticides.
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From page 211... ...
He hypothesized that epistatic networks can evolve under close inbreeding or asexual reproduction, but that selection in outcrossing, genetically heterogeneous populations produces nonepistatic mechanisms of resistance. If elements of rudimentary resistance mechanisms evolving in nature contribute nonepistatically to fitness in both treated and untreated environments, then the characterization of resistance as the response of a genetic network is inappropriate.
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From page 212... ...
Evolution of Epistatic Resistance The question of fashioning resistance to pesticides from the components of normal metabolism centers on the evolutionary process by which an integrated genetic network controlling normal metabolism transforms into another genetic network capable of responding to both treated and untreated environments. Known single-locus determinants of resistance may represent highly evolved mechanisms, the products of the evolutionary process discussed here.
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From page 213... ...
In the imagery of the adaptive topography, valleys may be temporarily uplifted, permitting the population to wander into the domain of attraction of a new peak by means of a wholly adaptive process. THE EVOLUTION OF PESTICIDE RESISTANCE In its simplest form the evolution of a rudimentary resistance mechanism and the reduction of pleiotropic costs through the separation of incipient detoxification pathways from metabolic pathways represents a peak shift under fluctuating selection.
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From page 214... ...
All individuals possess the bifunctional structural locus; the sole genetic difference between susceptible and resistant individuals at this stage lies at the regulatory locus. Temporary suspension of pesticide treatments tends to reduce the level of resistance in the population by restoring the original selective regime, which favors a lower rate of production.
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From page 216... ...
assessment of the effects of the key mutations on normal metabolism. Direct estimates of pleiotropic costs associated with poorly formed resistance mechanisms could be obtained by comparing the levels of additive genetic variance in fitness in experimental populations before and after exposure to a novel pesticide.
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From page 217... ...
A Model of Epistatic Resistance In its simplest form the peak shift required for the evolution of resistance mechanisms that incur low pleiotropic costs entails genetic changes at two loci: the regulatory locus controlling the level of synthesis at the key structural locus and a modifier locus permitting separation of the two pathways. The effects of migration and population size on the refinement of resistance in a population that exchanges migrants with untreated populations could be investigated through the analysis of the two-locus model described in this section.
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From page 218... ...
CONCLUSION The central concern of this discussion has been to suggest that empirical and theoretical investigation be directed toward the elucidation of the process under which primitive responses to pesticides develop into highly effective mechanisms of resistance. The bifunctionality of components of primitive resistance mechanisms suggests that in the early evolutionary stages the defense against pesticides involves some disruption of normal physiological processes.
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From page 219... ...
Of particular relevance to the development of effective control policies is the question of whether migration between treated and untreated regions promotes the reduction of pleiotropic costs and the rate of preadaptation to the pesticide by untreated populations. The confrontation of theoretical population genetics with the practical problems of the control of pesticide resistance enriches both fields by revealing new perspectives on old problems and by provoking the development of new questions.
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From page 220... ...
1982. Managing pesticide resistance in crop-arthropod complexes: Interactions between biological and operational factors.
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From page 221... ...
PLEIOTROPY AND GENETIC SYSTEMS CONFERRING RESISTANCE 221 Wright, S
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