Articles
CONVENTIONAL AND MOLECULAR PLANT BREEDING
Climate changes in the direction of an increase in mean temperature and a decrease in precipitation seem to have been particularly relevant (Blumler and Byrne, 1991). Regions of the world where wild plant species were independently domesticated include China, the Fertile Crescent, Central Mexico, the Central Andes, Sub-Saharan Africa and Eastern North America.
These events took place in the period extending from approximately 10,000 to 4,000 years before present (Smith, 1995).
The first step in the elaboration of agricultural systems involved the development of methods for harvesting, cleaning and using wild seeds, followed by the intentional domestication of crop plants (Kislev, 1984). In the Near East, the first proof of intentional cultivation is the appearance at several sites of seeds of the cultured strains (ex. Triticum monococcum subsp. monococcum) in association with seeds of the wildtype progenitors (ex. T. monococcum subsp. boeoticum) (Zohary and Hopf, 1993). Plant domestication was rapid, and the acquisition by the crop of the domestication syndrome (Hammer, 1984) was a relatively simple process.
The domestication syndrome largely concerned traits that are common to several and different crop species like loss of seed dispersal mechanisms; loss of seed dormancy; rapid seed germination; acquisition of vegetative reproduction or apomixis; autogamy; annualism; increase in seed or organ size; loss of traits that would tend to hinder efficient sowing; and loss of toxic or bitter compounds from the organs to be used as food.
The simple genetic basis of several of these traits has made possible the rapid fixation in the cultivated genotypes of favorable alleles.
Table 1, for example, shows that almost all traits that distinguish cultivated from wild bean genotypes have a simple Mendelian basis (Gepts, 1990). In some cases, the appearance of a new form may have been so sudden in its morphological consequences, as to suggest to the Neolithic breeder the direct domestication of the new plant.
This was the case with the hexaploid wheat T. aestivum subsp. spelta for which no wild progenitor exists.