Plant domestication can be thought of as a two-step process. In the first step, plants acquire traits in what is called the "domestication syndrome" that make the plant worth the labor of cultivation. These include traits that allow a crop to be reliably sown, cultivated and harvested, such as uniform seed germination and fruit ripening.
In the second step, the now domesticated plant is selected for improved qualities. It is in this stage, for example, that farmers might breed many different varieties of a crop that differ in grain taste, fruit color or fruit shape.
In the case of grains two of the most important traits in the domestication syndrome are the loss of shattering, and the loss of seed dormancy.
Shattering, or the tendency of seeds to break off the central grain stalk once mature, is an advantage for wild grains, because it helps to ensure the seeds disperse. But a crop plant must retain its seeds long enough that the seed heads can be gathered at harvest.
The shattering trait provides a good example of the apparent conflict between the genetic and archeological data. Artificial domestication experiments show that it is possible to breed nonshattering cereals quite quickly. But Old World archeological data indicates that nonshattering cereals appeared only gradually, and typically only after the emergence of another domestic trait: larger grain size.
The increase in grain size suggests the plant was already under cultivation, and that the seeds were being sown, or buried, rather than blown about on the surface. Why would the nonshattering trait emerge later than larger grain size?
"The answer, at least ins some cases," Olsen suggests, "is that a mutation that led to a complete loss of shattering might make harvesting easier but it would also make threshing much harder." So the nonshattering trait might have lagged behind other domestic traits because it required an optimal combinatio
|Contact: Diana Lutz|
Washington University in St. Louis