Genetic Flexibility and Inflexibility

Ecologists are familiar with the concept of genetic flexibility, in which an ecotype responds to either positive or negative selection pressures, and changes accordingly, from generation to generation (see 1.15). Crop scientists, on the other hand, are more familiar with genetic inflexibility, with which a cultivar, which is usually a clone or a pure line, can be relied on to keep its agriculturally valuable characteristics, without significant genetic change, during many years of propagation and cultivation.

Genetic flexibility is a feature of heterogeneous, wild populations, and wild plant pathosystems. Genetic inflexibility is a feature of the vegetative propagation of clones such as potatoes, citrus, stone and pome fruits, olives, sugarcane, bananas, strawberries, sweet potatoes, and cassava. It is also a feature of the seed propagation of homozygous pure lines of autogamous crops such as wheat, rice, and beans. And it is a feature of hybrid varieties of allogamous (i.e., cross-pollinating) crops such as maize, sorghum, onions, and cucumbers that are cultivated as hybrid varieties that exhibit heterosis (hybrid vigour).

If a wild host population has too little horizontal resistance, it will respond to positive selection pressure and gain resistance, because it is genetically flexible. Equally, if it has too much resistance, it will respond to negative selection pressure and lose resistance. In complete contrast, most cultivated host populations, being genetically inflexible, do not respond in this way.

A notable exception to this rule of genetic inflexibility is the open-pollinated maize of subsistence farmers in the tropics. Highly susceptible, but genetically flexible, maize crops in tropical Africa responded dramatically to the newly introduced, re-encounter, tropical rust parasite, (Pucciniapolysora). And this response (see 7) has taught us much about a possible new approach to the management of our crop pathosystems.

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