Genetic variation within a crop gene pool can be found within and among professionally bred varieties, landraces or farmers' varieties, and nondomesticated relatives. In addition, new genetic variation can be introduced through mutations and the transfer of genes from different gene pools. Commercially released varieties aim to combine genes for high productivity with those required to meet different needs and environments. They contain a wealth of useful genes and gene combinations and normally form the basis for further professional plant-breeding efforts.
Landraces and farmers' varieties tend to be genetically heterogeneous and have proved to be an excellent source of genes for, inter alia, adaptive characters and disease and pest resistance. They are still widely grown, especially in marginal environments where they may be more stable, and even more productive, than many modern varieties. Landraces of many minor crop species are also still commonly grown as, in general, they have received relatively little attention from plant breeders and have been less subject to replacement by modern varieties.
Wild, nondomesticated relatives of crops frequently provide useful sources of genes. For example, a wild rice, Oryza nivara, was used to introduce resistance to grassy stunt virus in cultivated rice (Khush and Beachell, 1972). In Africa and India, cassava (Manihot esculenta) yields increased up to 18 times after genes from wild Brazilian cassava, conferring disease resistance, were incorporated into local varieties (Prescott-Allen and Prescott-Allen, 1982). In the U.S., disease-resistant, wild Asian species of sugarcane (Saccharum sp.) helped to save the U.S. sugar cane industry from collapse (Prescott-Allen and Prescott-Allen, 1982). Many other cases that have benefited agriculture in all parts of the world can be cited, as well.
In addition to these sources of genetic diversity, new DNA sequences can be created or introduced into crop species. For example, mutations are a source of new diversity and can be induced by chemical mutagens or ionizing radiation. And with modern genetic engineering techniques, all organisms, at least in theory, can contain potentially useful genes which could be transferred between crops and induced to express themselves. These new genes then become integrated into the plant genome and are passed from generation to generation.
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