Question

For Plant Physiology.

Explain the following statement: “There is a fitness cost associated with an enhanced defense response.” In your answer you must do the following: A) Provide your opinion about what this statement means to you. B) Include and describe the experimental evidence that supports your opinion.

Answer 1

The study of fitness costs of disease resistance has a longhistory in plant pathology. Early researchwas stimulated by practical considerations of controlling disease in crops, particularly to understand how breeding for increased productivity might affect selection for disease resistance. Fifty years on,knowledge of fitness costs of disease resistance is morerelevant than ever to crop production. If disease resis-tance has a detrimental effect on the yield potential of acrop, a balance must be achieved between two desirablebut partly incompatible opposing goals, maximizing yieldand minimizing the cost of controlling pests and diseases.Understanding the nature of fitness costs and trade-offsof disease resistance and how they can be minimized inefficient farming systems will make progress towards thegoal of producing high yields of crops in ways that aresustainable both economically and environmentally.

Insecticide resistance mutations are widely assumed to carry fitness costs. However studies to measure such costs are rarely performed on genetically related strains and are often only done in the laboratory. Theory also suggests that once evolved the cost of resistance can be offset by the evolution of fitness modifiers. But for insecticide resistance only one such example is well documented. Here we critically examine the literature on fitness costs in the absence of pesticide and ask if our knowledge of molecular biology has helped us predict the costs associated with different resistance mechanisms. Even the most simplistic consideration of resistance mechanisms can give us a set of predictions about when and where mutation of a gene product might lead to a fitness cost. For target site resistance involving point mutations in so called ‘lethal’ genes encoding essential ion channel subunits, we would predict severe functional constraints on the nature and location of resistance associated mutations.