Hybridization is the process of crossing two genetically different individuals to create new genotypes. For example, a cross between a parent 1, with the genetic makeup (genotype) BB, and parent 2, with bb, produces progeny with the genetic makeup Bb, which is a hybrid (the first filial generation or F1). Hybridization was the basis of Gregor Mendel’s historic experiments with garden peas. Inheritance studies require crossing plants with contrasting or complementary traits.
Hybridization of plants occurs in nature through various mechanisms. Some plants (such as the oil palm) are insect-pollinated, and others (such as maize, or corn) are wind-pollinated.
Such plants are referred to as cross-pollinated plants. Natural hybridization has played a significant role in producing new genetic combinations and is the norm in cross-pollinated plants. It is a common way of generating genetic variability.
In plants with perfect flowers (autogamous, having flowers with both stamens and pistils), cross pollination rarely occurs. Such plants (such as wheat and rice) are called self-pollinated plants.
Flowers bearing only pistils or stamens are said to be imperfect flowers. Plants that have separate pistillate and staminate flowers on the same plant (such as maize) are called monoecious. Plants that have male and female flowers on separate plants (such as asparagus) are called dioecious.
Through artificial means (controlled pollination), hybridization of both cross-pollinated and self-pollinated plants can be accomplished. Artificial hybridization is an important aspect of improving both cross-pollinated and self-pollinated plants. The breeder must know the time of development of reproductive structures of the species, treatments to promote and synchronize flowering, and pollinating techniques.
Applications to Agriculture
|Applications to Agriculture|
Around 1761 Joseph Gottlieb Kölreuter was the first to report on hybrid vigor in interspecific crosses of various species of Nicotiana. He concluded that cross-fertilization was generally beneficial and self-fertilization was not. In 1799 T. A. Knight conjectured that because of widespread existence of cross-pollination in nature, it must be the norm.
Charles Darwin reported the results of his experiments with maize. He indicated that in twenty-four crosses, there was an increase in plant height, which was attributed to hybridization, and that decrease in plant height was associated with self-pollination (or selfing).
He also noted that crossing of inbred plants could reverse the deleterious effects of selfing or inbreeding. In 1862 Darwin wrote, “Nature tells us, in the most emphatic manner, that she abhors perpetual self-fertilization.” In the late 1800’s William J. Beal evaluated hybrids between maize varieties.
He observed that some hybrids yielded 50 percent more than the mean of their parents. S.W. Johnson provided an explanation for hybrid vigor in 1891.G.W.McClure reported in 1892 that hybrids between maize varieties were superior to the mean of the two parents.
Exploitation of Heterosis
The phenomenon of heterosis has been exploited in crop plants, such as maize, sorghum, sunflower, onion, and tomato. Maize (corn) was the first crop in the United States in which hybrids were produced from inbred lines. It was George Shull who, following the rediscovery of Mendel’s laws of inheritance in 1900, conducted the first experiments on inbreeding and crossing, or hybridizing, of inbred lines.
Shull suggested that inbreeding within a maize variety resulted in pure (homozygous) lines and that hybrid vigor resulted from crossing of pure lines because heterozygosity was created at many allelic sites. Hybrid maize was introduced in the United States in the late 1920’s and early 1930’s, after which U.S. maize production increased dramatically from the use of hybrids.
Heterosis now drives a multi billion-dollar business in agriculture. Yield improvement made in various crops in which heterosis was detected has been tremendous. In 1932 in the United States, 44.8 million hectares (111 million acres) were required to produce 51 million metric tons of maize grain, with a mean yield of 1.66 metric tons per hectare.
In 1994 it took only 32 million hectares (79 million acres) to produce 280 million metric tons of grain, with a mean yield of 8.69 metric tons per hectare. In the United States in 1996, twenty-one vegetable crops occupied 1,576,494 hectares (3.9million acres), with a mean of 63 percent of the crop in hybrids.
Heterosis saved an estimated 220,337 hectares (544,459 acres) of agricultural land per year, feeding 18 percent more people without an increase in land use. From 1986 to 1995, the best rice hybrids showed a 17 percent yield advantage over the best inbred-rice varieties at the International Rice Research Institute.
Despite the impact that heterosis has had on crop production, its molecular genetic basis is still not clear. It is hoped that with the progress being made in the genetic sequencing of various plant species, a better understanding of heterosis will emerge.
|Exploitation of Heterosis|
There are barriers, however, for accomplishing interspecific and intergeneric crosses. Plants of the same species cross easily and produce fertile progeny. Wide crosses are difficult to make and generally produce sterile progeny because of chromosome-pairing difficulties during meiosis.
Triticale is the only human-made cereal crop, which is a cross between the genus Triticum (wheat) and the genus Secale (rye). The first fertile triticale was produced in 1891.
Some of the interspecific and intergeneric barriers should be overcome via the newer techniques of gene transfer. It is expected that genes from wild relatives of cultivated plants will continue to be sought to correct defects in other wise high-yielding varieties.