By: Graham Scoles, Ph.D., PAg
Hybridization refers to the natural or deliberate pollination between two plants. It occurs all the time in the wild as many plants are cross-pollinated and when the pollen from one plant lands on the stigma of another plant, and if fertilization is successful in producing seed, then hybridization has occurred. The resulting seed will carry the two sets of genes from either parent and will be similar, but not identical, to either parent. When cross-pollination is the norm for a species then hybridization between plants of that species occurs frequently.
In self-pollinated species, which includes many crops, the pollen from a flower will often fertilize that same flower or another flower on that same plant; normally we would not refer to it as hybridization as the resulting seed will be genetically identical to its parent.
Plant breeders use hybridization as the starting point of a breeding program in self-pollinated crops. Pollen from one line is used to pollinate the (emasculated) flower of another line. The plant breeder will select the two parental lines with the understanding that by combining the good traits of both lines something superior will be produced. As it is a self-pollinated crop the breeder selects what appear to be desirable progeny over a number of generations in the hope of eventually identifying a few lines that are superior. These are then subject to further testing and may eventually become a new variety/cultivar. During this process the plant breeder discards large amounts of material each year.
When breeding a cross-pollinated crop a different process has to be used. Natural cross-pollination among a selected group of plants is allowed to occur and then desirable plants are identified from the progeny and again allowed to cross-pollinate. This occurs for a number of cycles before testing to see if the group of plants is superior to the starting material. Cultivars of self- and cross-pollinated crops breed true (their progeny are the same as the parents) and the grain that is harvested can be used as seed to establish next year’s crop.
Not all crops are purely self- or cross-pollinated, some are intermediate such as canola and the breeding techniques adjusted appropriately.
The value of hybrid cultivars was discovered through experiments with corn in the 1920/30s. Corn is a cross-pollinated crop. But if a corn plant is forced to self-pollinate by covering the silks (=stigmas) from external sources of wind-blown pollen and then pollinating the silks with pollen from the tassels of the same plant seed can be produced. When grown the next generation the progeny do not grow as vigorously as their parent and over many generations of enforced self-pollinating they become even weaker. This is referred to as inbreeding depression. This can be so severe that the weakest lines fail to produce seed but some inbred lines have adequate vigor to be maintained. It was discovered that if inbred lines were hybridized, in certain cases the progeny were more vigorous (larger, higher-yielding) than the starting material from which they have been derived. This is known as either hybrid vigor or heterosis. This discovery eventually led to the development of the hybrid corn industry and subsequently other crops.
Whereas other cultivars breed true and their grain can be used as seed for another generation, this is not true of hybrid varieties. Cross-pollination is, of course, necessary among the plants (in e.g. a hybrid cornfield) for grain to be produced. But if that grain is used as seed for next year’s crop it will not breed true, the crop will exhibit tremendous variability in all characteristics (height, maturity etc.). In the case of canola hybrids, this would also extend to quality characteristics such as glucosinolate content, erucic acid content and any other oil quality characters of that hybrid and make the seed from that crop unacceptable. Thus farmers must obtain seed of a hybrid (which is produced under controlled conditions by the seed company) each year. Many vegetables are also sold as hybrids such as tomatoes. Saving some seed for planting next year will reveal the hidden variability of a hybrid. As in other crops, new hybrid cultivars (based on new combinations of inbred lines) are being brought to market all the time.