Erosion of genetic diversity of crop plants has for several decades been making it necessary to develop initiatives for protecting these plant resources. One strategy is in-situ conservation of crop plants. The model currently advanced involves maintaining the varieties to be conserved isolated in reserves, protected from entry of other varieties from elsewhere and cultivated according to ancestral farming practices. Researchers from the IRD and the CIMMYT of Mexico (1) used work previously conducted in Mexico on maize varieties, or landraces, to devise a different, dynamic, model judged more compatible with agricultural development and closer to the real conditions in which these plants diversified under the constant action of farmers (2). Insofar as local landraces are still grown as crops on sufficiently large areas of land, the introduction of others from outside, favouring a certain rate of gene flow, would in fact be a source of diversity rather than a factor of genetic erosion.
Mexico, the cradle of maize cultivation, is where this member of the Graminae family, a descendent of a local wild grass, teosinte, was domesticated and phenotypically diversified by human action, at least 6 000 years B.P. An in-situ conservation programme jointly run by CIMMYT, INIFAP (Mexican National Institute of Research in Forestry and Agriculture and Livestock Breeding) and the IRD, conducted in the central valleys of Oaxaca, enabled the research team to characterize the genetic structure of the different populations of local maize landraces and measured the impact of farming practices on this diversity. They focused on two types of diversity: phenotypic (concerning the morphological characters of the plants) and the genetic diversity (observed using genetic markers).
Study of the populations of maize landraces cultivated in six villages of this central region of Mexico has revealed that the morphological and agronomic characters in the field, such as ear size, kernel colour, or flowering period, vary depending on the farmer. At genome scale, genetic markers have shown strong homogeneity between the maize populations within the same village and, more surprisingly, between distant villages. This means that the local varieties possess a common genetic base. The diversity observed in characters of direct pertinence to farmers would consequently be the result of the latter’s personal decisions on seed selection, which they make before each crop cycle.
Marie Guillaume | alfa
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