Agriculture Reference
In-Depth Information
12.3
BRIEF HISTORY OF GENETIC MODIFICATION
It is useful to describe three stages of plant and animal modification:
1. Domestication
2. Conventional breeding
3. Transgenic modification
12.3.1 Domestication
Domestication involves the adaptation of a wild plant or animal to serve human needs.
For example, domestication of dogs probably involved humans raising the progeny of
wild wolves or wolflike species. Domestication of wheat meant that humans gathered,
stored, and intentionally replanted seeds of plants that eventually became what we call
wheat today.
We normally think of domestication as a benign process. In reality, it is an event
signaling a major loss of functions in the plant or animal concerned. 3 Most wild plants
have characteristics that make them undesirable for human food. The seed may be too
small, too difficult to collect, too bitter or even poisonous, to hard to preserve, or just not
taste good. For example, domestication of the potato involved selection of plants with
levels of bitter glycoalkaloids below 20 mg/100 g. 4 If the potatoes had higher levels,
they were rejected. This resulted in a selection of only a few types of wild potatoes
for human use and cultivation. All others were considered unfit for human use. Early
farmers selected a potato with less resistance to insects in the wild but which could
be used for food by humans. Further selection was made for color, size of tuber, and
time of harvest.
Similar events certainly occurred for wheat and rice. 5 For plants to survive in
nature, they must have a means of dispersing their seed. Wild relatives of rice accom-
plished this by readily dropping their seeds when mature. This also helped avoid being
eaten and destroyed by birds. However, for humans wanting to use this seed it was not a
desirable trait. Stooping over to pick small seeds on the ground was too hard. It was
easier to pick the heads of rice that still held seeds. This meant only plants that did
not drop their seeds at maturity were collected and preserved for planting. Plants that
had dropped their seeds were not saved. Naturally, the early farmers selected the
largest, most visible seeds to collect. This also meant that plants with small or dark-
colored seeds were not selected. The effect this had on our present-day crops is
easily seen in Figure 12.4. The seeds inside the circle are domesticated crops and
those outside the circle are their wild relatives. The seeds are, clockwise from top,
peanuts, maize, rice, coffee, soybean, hops, pistachio, and sorghum.
In some cases this selection occurred at the same time as a mutation in the species.
Maize is an example of a crop with no clear wild progenitor. We can find relatives of
maize in the wild, but nothing that appears like maize. Scientists theorize that a single
mutation in teosinte or pod corn resulted in the now familiar ear of maize. This made
harvesting and utilization of the grains much easier than that of other species. If early
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