Apomixis

Apomixis (Gk = without mixing) means that seeds are produced asexually from maternal tissue, and this topic merits a minor digression. Apomictic seed is produced naturally in certain grasses, and in the nucellar seeds of some fruit trees, such as mangoes and citrus. The genetic significance of apomixis is that apomictic seedlings are genetically identical to their maternal parent and to each other. They are thus the equivalent of vegetative propagation, with the added advantage that they do not transmit the various diseases that are so commonly carried by conventional vegetative propagating material, such as tubers, cuttings, grafts, corms and bulbs. A genetically engineered apomixis in seed-propagated crops would introduce the many advantages of vegetative propagation, while retaining the many advantages of propagation by seed.

These advantages are worth noting. An artificially induced apomixis would produce an instant genetic stabilisation of any individual plant that is heterozygous. This would preserve the hybrid vigour and other valuable traits of heterozygous plants. These traits are normally lost during the process of seed propagation. Most crops that are propagated by seed must first be made into pure lines, if they are autogamous, or hybrid varieties, if they are allogamous. At the very least, an induced apomixis would save a lot of work for the breeders.

Hybrid vigour (heterosis) is lost in pure lines which, obviously, are homozygous. It is also lost in the second generation of hybrid varieties, in which the seed begins segregating. Apomixis would eliminate these problems. It would also permit true seed propagation of vegetatively propagated crops such as potatoes. (This would save the large quantities of potatoes currently used for vegetative propagation, although we should note that potato crops produced from true seedlings are likely to have a considerably reduced yield). Apomixis would also facilitate the bulk production of seed of hybrid varieties of open-pollinated crops such as maize, by the stabilisation of the inbred parent lines.

There is a very real possibility that molecular biology will develop transgenic lines of crops carrying an apomictic gene. This is apparently the most promising aspect of genetic engineering seen so far in crop improvement. However, the widespread reservations about the use of GMOs will have to be answered before this use of apomixis can be implemented.

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