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heterosis(hĕt'ərō`sĭs): see hybridhybrid
, term applied by plant and animal breeders to the offspring of a cross between two different subspecies or species, and by geneticists to the offspring of parents differing in any genetic characteristic (see genetics).
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Hybrid vigor or increase in size, yield, and performance found in hybrids, especially if the parents have previously been inbred. The application of heterosis has been one of the most important contributions of genetics to scientific agriculture in providing hybrid corn, and vigorous, high-yielding hybrids in other plants and in livestock. See Breeding (animal), Breeding (plant) See Genetics, Mendelism
There are two principal hypotheses to account for the association of size and vigor with heterozygosity, dominance and overdominance. The dominance hypothesis notes that any noninbred population carries a number of recessive genes that are harmful to a greater or lesser extent, but which are rendered ineffective by their dominant alleles. As they become homozygous through inbreeding, they exert their harmful effect. With hybridization, some of the detrimental recessives contributed to the hybrid by one parent are masked by dominant alleles from the other, and an increase in vigor is the result. The alternative hypothesis is that there are loci at which the heterozygote is superior in vigor to either homozygote. This, the overdominance hypothesis, also has the consequence that vigor is proportional to heterozygosity. The dominance hypothesis has been more widely accepted, but the two are very difficult to distinguish experimentally, and it is likely that overdominant loci are playing an appreciable role in heterosis, particularly in determining why one hybrid is better than another. See Dominance
(hybrid vigor), accelerated growth and increased dimensions, endurance, and fertility of various first-generation animal and plant hybrids. Heterosis is usually attenuated in the second and later generations. True heterosis, which is the ability of hybrids to leave a large number of fertile descendants, is distinguished from gigantism, which is the enlargement of the entire hybrid organism or of its individual parts. Heterosis is found in various multicellular animals and plants, including self-pollinating ones. Phenomena resembling heterosis are also observed in the sexual processes of some unicellular organisms. Heterosis often results in a considerable increase in the productivity and yield of agricultural animals and crops.
Heterosis and its converse, inbred depression, were already known to the ancient Greeks, particularly Aristotle. The first scientific studies of heterosis in plants were carried out by the German botanist J. Kölreuter (1760). Darwin generalized observations on the use of hybridizations (1876), thus greatly influencing the work of I. V. Michurin and many other breeders. The term “heterosis” was proposed in 1914 by the American geneticist G. Shull, who was the first to obtain double interlinear corn hybrids. In 1917, D. Jones developed the principles of a method of industrial cultivation of these hybrids. The application of hybridization in agriculture has increased over the years, stimulating theoretical investigation of heterosis. Species with marked heterosis have advantages in natural selection; thus, the phenomenon of heterosis increases and contributes to increased genetic variability. Often, stable genetic systems arise, ensuring predominant survival of heterozygotes by numerous genes.
Aside from the usual study of morphological traits, investigation of heterosis requires the application of physiological and biochemical methods, making it possible to detect fine differences between hybrids and the original forms. Heterosis has also begun to be studied at the molecular level. In particular, the structures of specific protein molecules of many hybrids (for example, enzymes and antigens) are being studied.
According to Darwin, heterosis depends on the conjunction of diverse hereditary tendencies in the fertilized egg. On the basis of this, two main hypotheses regarding the mechanism of heterosis were established. The hypothesis of heterozygosis (overdominance or single-gene heterosis) was proposed by the American researchers E. East and G. Shull. When they combine in the heterozygote, two states (two alleles) of one and the same gene reinforce each other in their action on the organism. Each gene controls the synthesis of a particular polypeptide. In heterozygotes a few different protein chains are synthesized instead of one, and often heteropolymers—that is, hybrid molecules—are formed. This process may be advantageous for the heterozygotes. The hypothesis of dominance (summation of dominant genes) was formulated by a number of American biologists, including A. V. Bruce (1910) and D. Jones (1917). Mutations (changes) of genes are, on the whole, harmful. A defense against them is increased dominance of normal genes for a population of genes (evolution of dominance). Combination in a hybrid of favorable dominant genes of two parents results in heterosis.
The two hypotheses on heterosis can be combined in the concept of genetic balance, which was developed by the American scientist D. Jones, the English scientist K. Mather, and the Russian geneticist N. V. Turbin. Obviously, heterosis is based on the interaction of both allelic and nonallelic genes; however, in all cases heterosis is associated with enhanced heterozygosity of the hybrid and with its biochemical enrichment, which also depends on an increase in the rate of metabolism.
Of particular practical and theoretical interest is the problem of the stabilization of heterosis. It can be solved by doubling sets of chromosomes, creating stable heterozygous structures, and using every possible form of apomixis, as well as by vegetative reproduction of hybrids. The effect of heterosis may also be fixed by doubling individual genes or small parts of chromosomes. The role of such duplications in evolution is very great; therefore, heterosis is considered an important stage in evolutionary progress.
V. S. KIRPICHNIKOV
Heterosis in agriculture. In the cultivation of plants, heterosis is an important way to increase productivity. Crops from heterotic hybrids are 10-30 percent greater than from ordinary varieties. In order to use heterosis in production, economically profitable means have been developed of obtaining hybrid seeds of corn, tomatoes, eggplants, peppers, onions, cucumbers, watermelons, gourds, sugar beets, sorghum, rye, lucerne, and other agricultural plants. A special position is occupied by a group of vegetatively reproducing plants—for example, a variety of potatoes and fruit and berry crops obtained from hybrid seeds—in which it is possible to fix heterosis in the descendants. To use heterosis for a practical purpose, intervarietal crossing of homozygous varieties of self-pollinating plants, intervarietal (interpopulation) crossing of self-pollinated lines of cross-pollinated plants (conjugate, trilinear, bilinear, quadrilinear, and multilinear ones), and strain-line crosses are done. The advantage of certain types of crossing for each agricultural crop is established on a basis of economic evaluation. Elimination of difficulties in obtaining hybrid seeds can be facilitated by the use of cytoplasmic male sterility, the property of incompatibility in some cross-pollinated plants, and other hereditary peculiarities in the structure of the flower and raceme, excluding large expenditures on castration. In choosing parental forms to obtain heterotic hybrids, their combination capacity is assessed. Originally, selection in this direction led to separation of the genotypes with better combination value from the population of free-pollinating varieties on the basis of inbreeding by forced self-pollination. Methods have been developed to evaluate and increase the combination capacity of lines and other groups of plants that are used for crossing.
The maximum effect in the application of heterosis is obtained with corn. The creation and introduction into production of corn hybrids made it possible to increase by 20-30 percent the total harvests of grain on the enormous areas occupied by that crop in various countries of the world. Corn hybrids have been created that combine high yield and good seed quality, drought resistance, and immunity to various diseases. Heterotic sorghum hybrids (Early 1 hybrid, Rise hybrid) and heterotic intervarietal sugar beet hybrids, of which the Ialtushkovskii hybrid has become most widespread, have been zoned. A line of sugar beet with a sterile pollen is being used increasingly to obtain heterotic forms. The phenomena of heterosis are also established with many vegetable and oil-yielding crops. First results have been obtained in studying heterosis in first-generation wheat hybrids, and sterile analogues and fertility reducers produced from sources of cytoplasmic male sterility in wheat have been discovered.
In livestock breeding the phenomena of heterosis are observed in hybridization—intervarietal and intravarietal (interlinear) crossing. Heterosis causes a notable increase in the productivity of agricultural animals, and it has become most widely used in industrial crossing. In poultry farming, when egg-producing varieties of chickens are crossed—for example, leghorns with Australops or Rhode Islands—the egg production of first-generation crosses increases by 20-25 eggs per year. The crossing of meat breeds with meat-egg breeds of chickens improves meat quality. According to a complex of traits, heterosis is obtained when crossing closely related lines of fowls of one breed or by intervarietal crossing. In pig, sheep, and cattle raising, industrial crossing is used to obtain heterosis for meat productivity, which is manifested in earlier maturation, increased liveweights, increased dressing percentage, and improved quality of the carcass. Meat-lard (combined) breeds of pigs are crossed with meat varieties of boars. Local breeds of small, low-productivity sheep are crossed with meat-wool sheep, and fine wool parents are crossed with early maturing meat or semifine wool breeds. To raise meat productivity, milk cows, milk-meat, and local meat breeds are crossed with specialized meat breeds of bulls.
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Kirpichnikov, V. S. “Geneticheskie osnovy geterozisa.” In the collection Voprosy evoliutsii, biogeografii, genetiki i selektsii. Moscow, 1960.
Gibridnaia kukuryza: Sbornik perevodov. Moscow, 1964.
Ob’’edinennaia nauchnaia sessiia po problemam geterozisa: Tezisy dokladov, issues 1-6. Moscow, 1966.
Ispol’zovanie geterozisa v zhivotnovodstve (conference materials). Barnaul, 1966.
Geterozis v zhivotnovodstve: Bibliograficheskii spisok.… Moscow, 1966.
Guzhov, Iu. L. Geterozis i urozhai. Moscow, 1969.
Brewbaker, J. L. Sel’skokhoziaistvennaia genetika. Moscow, 1966. (Translated from English.)
Turbin, N. V., and L. V. Khotyleva. I spol’zovanie geterozisa v rastenievodstve (Obzor). Moscow, 1966.
Kirpichnikov, V. S. “Obshchaia teoriia geterozisa, 1: Geneticheskie mekhanizmy.” Genetika, 1967, no. 10.
Fincham, J. R. Genetic Complementation. New York-Amsterdam, 1966.