What is outbreeding?
Outbringing is a method of breeding animals, which is the crossing of unrelated individuals.
Outbringing differs from other breeding methods in its simplicity and reliability, due to the fact that it does not cause failures in productivity, but rather leads to the birth of offspring with stable productivity.
Thanks to outbreeding, numerous modern breeds of farm animals have emerged since the late 19th century, derived from a variety of local breeds.
The production of more productive offspring using the outbringing method occurs due to the effect of heterosis.
The heterosis method itself has been known for a long time. It was widely used in both crop production and animal husbandry. Using this effect, for example, mules were bred by crossing horses and donkeys.
The first person to define heterosis was Charles Darwin. In the course of his research on plants, he discovered that when cross-pollinated, the plant receives more beneficial effects than when self-pollinated. Darwin explained the cause of heterosis by biological differences and characteristics of male and female gametes.
Later, scientists put forward several more theories about the development of heterosis. The author of one of them is our compatriot M.V. Turbin, who proposed the theory of genetic balance. According to this theory, heterosis occurs under the influence of numerous combinations of genes that have been balanced by evolution in the body. That is, under the influence of both natural and artificial selection when crossing animals, the optimal genomes of the parents create new combinations of genes in the offspring, which cause heterosis.
How does the heterosis effect manifest itself?
Heterosis manifests itself to a greater extent in such traits as the rate of weight gain and egg production, that is, those that have a low heritability. These traits develop under the influence of additive genes. The environment also plays a large role in the development of these traits.
IN modern science Various technologies for obtaining the effect of heterosis have been tested: these are intrabreed matings based on heterogeneous selection, and interline crosses of inbred lines, as well as interspecific and interbreed crossings.
So for example, to get good quality and high yield of beef, crossbreeding of dairy and dairy meat breeds cows with bulls producing meat breeds.
And in poultry farming for both meat and egg production, when crossing different breeds and lines, the offspring acquire heterosis based on signs of viability, feed efficiency and, of course, productivity.
Moreover, heterosis can manifest itself in various forms. Thus, during outbreeding, the offspring may not differ in terms of body weight from their parents, but may significantly exceed them in fertility and vitality.
In order to identify heterosis it is necessary:
- Choose the right line to cross
- Determine which line will be paternal and which maternal
Only when good choice the best combination of these two factors will be achieved. But, of course, this is trial and error.
In this case, the combinative abilities of the selected breeds and lines should be taken into account. This combining ability has two categories:
General combining ability- implies the ability of individuals of a particular species (breed or line) to generally obtain the effect of heterosis when crossed with other species (breeds or lines).
Specific combining ability- implies the ability of individuals of a given breed (line) to produce heterosis when crossed with a certain breed or line.
The phenomenon of heterosis - where was it used?
Since heterosis can achieve impressive growth rates in the productivity and vitality of animals, this phenomenon has found application in animal husbandry and crop production.
In crop production, with the help of heterosis, simply impressive results were achieved - the yield of individual varieties was increased by almost 40 percent.
The leader among heterotic crops is corn. New hybrid varieties of corn have high drought resistance, seed preservation, and increased plant immunity.
The heterosis effect has been successfully used in the hybridization of wheat, sugar beets, sorghum and other crops, as well as in fruit, berry and vegetable growing.
But in poultry farming, the main direction of breeding work where the heterosis method is used is the production of broiler and egg breeds of poultry.
But the use of this effect, of course, is not limited to poultry farming alone. The phenomenon of heterosis is successfully used in beef cattle breeding, sheep farming, camel farming and even fish farming.
In addition, thanks to the research of scientists, the appearance of heterosis during interspecific crossing was established. In addition to the mules mentioned above, other famous breeds were developed in this way.
The famous Santa Gertrude cattle breed, bred in the USA at the beginning of the 20th century, is distinguished by its simply amazing weight gain - the weight of adult animals reaches 700 kg. This breed was developed by crossing the wild Zebu bull with the Shorthorn beef breed of cows.
The achievement of Kazakh livestock breeders was the Arharomerinos sheep breed, bred in the 50s of the last century. This hybrid, obtained by crossing the Argali mountain ram with fine-wool sheep, exceeded all the breeders' expectations in many respects, such as: fertility, strong immunity, meat yield, quality and quantity of wool, and, as a bonus, the ability to live in the mountains.
Disadvantages of heterosis
Despite all its obvious advantages, this effect also has its disadvantages. And, of course, the main one is the short duration of the action of heterosis.
The fact is that hederosis manifests itself most noticeably when crossing individuals that are distant in degree of relationship. Under this condition, the effect is strongly consolidated in the first generation.
Scientists are looking for ways to solve this problem - how to prolong the effect of heterosis. One of the solutions was variable crossing, that is, when hybrids are bred from two different breeds, and then the resulting individuals are crossed with other breeds.
Outbringing, despite the disadvantages of the heterosis effect, remains effective method increasing the productive performance of animals and plants.
Heterosis using heterogeneous selection during intrabreed mating. The use of cross lines, lines of sires and families in purebred breeding, as well as mating of animals belonging to the same breed, but raised in different conditions, are also variants of heterogeneous selection. The heterotic effect in such matings will be discussed below. In this case, we are talking about such a heterogeneous selection in which the mated animals are in the same household, do not have an obvious linear affiliation, or belong to the same related group and therefore are related to each other to one degree or another. Such heterogeneity is most often expressed in the differences between the mated individuals only in certain characteristics, in particular in exterior-constitutional features. [...]
The practice of breeding knows many examples when, with successful crosses of lines, offspring are obtained that are not only distinguished by strong constitution, fertility, vitality, but also significantly superior in basic productive qualities to both mothers and the average indicators of those lines to which the parents belong.[ . ..]
Heterosis also manifests itself in hybrids of salmon fish and can occur when crossing whitefish.[...]
Heterosis does not occur automatically; the degree of its manifestation is largely determined by the genetic (hereditary) characteristics of the crossed pairs.[...]
HETEROSIS [from gr. heteroiosis - change, transformation] - increased vitality and fertility of first-generation hybrids compared to parental forms. First described by Charles Darwin (1859). The phenomenon of G., as a rule, is not observed in the second and subsequent generations. G. is widely used in agriculture. practice.[...]
Heterosis during interspecific crossing. The manifestation of heterosis during interspecific crossing was known in ancient times, when mating donkeys with mares of various breeds produced mules that were superior to both horses and donkeys in longevity, performance (traction force per unit mass) and resistance to various diseases. But in terms of live weight, they occupy an intermediate position and are practically sterile. When crossing a two-humped camel (Bactrian) with a one-humped camel (dromedary), hybrids (bunks) are obtained that significantly exceed the parent forms in live weight and traction capacity without loss of fertility.[...]
Heterosis in plants is the phenomenon of superiority of a hybrid plant over the best of its parents, but in terms of power and degree of development of certain characteristics and properties. The heterotic power of plants manifests itself to the greatest extent in the first generation of hybrids.[...]
Heterosis is understood as the property of animals to surpass the best of the parental forms in viability, growth energy, fertility, constitutional strength, and resistance to diseases.[...]
The phenomenon of heterosis is also observed during purebred breeding of animals (with various forms of heterogeneous selection and crosses of well-combined lines), but it is not expressed as clearly as when crossing representatives of different breeds, as well as during hybridization.[...]
To obtain heterosis during interbreeding, the correct selection of paternal and maternal breeds, as well as the choice of breed representatives, is of great importance. In poultry farming, as I. F. Rostovtsev points out, where there is a rapid change of generations and there is great opportunity selection, methods have been developed for the directed formation of heredity of the original crossed forms, ensuring the manifestation of heterosis in their crossbred offspring.[...]
The use of heterosis in animal husbandry.[...]
Use of heterosis. Obtaining E1 hybrids? from which cross-pollinating plants propagated by seeds will be grown depends on a number of factors. The first condition is the possibility of obtaining and maintaining inbred lines. Some inbred lines become so weakened that they are difficult to maintain. In some plants, inbreeding itself is difficult.[...]
Nikolyukin N.I. Heterosis and its use in fish farming.[...]
Rostovtsev N.F. Heterosis in animal husbandry. M.: Publishing House of the USSR Academy of Sciences.[...]
Thus, with heterosis, it becomes possible to increase the vitality, fertility, precocity and productivity of offspring obtained from crossing organisms that differ in their hereditary qualities or were grown in different conditions. Heterosis in pond fish farming is expressed in accelerated growth of hybrids, increased endurance to unfavorable environmental conditions, winter hardiness, earlier puberty, and higher productivity. At the same time, not only intraspecific, but also more distant hybrids - interspecific and intergeneric - turn out to be fertile, which makes it possible to study heterosis not only in the first, but also in subsequent generations. At the same time, fully manifested in the first hybrid generation, it fades out in subsequent ones. Therefore, heterotic individuals should not be left for the tribe.[...]
Thanks to the heterosis effect, industrial crossing makes it possible to increase the fish productivity of ponds to a greater extent than with purebred breeding. For example, crossing Moldovan females with scaly males of the local outbred stock of the Stavropol fish farm made it possible to increase the fertility of eggs by 12.9-15.1%, and the yield of juveniles per female by 82%. The yield of fry per female increased from 60 (with single-breed selection) to 160 thousand. .[...]
Heterosis acquired the greatest importance in carp breeding. In hybrids of carp and carp, especially Amur carp, heterosis is clearly expressed in terms of growth rate and viability. In the Ukrainian SSR, northern (Ropshinsky) hybrids serve as material for crossing with Ukrainian carp, and crosses from such crossing show strong heterosis in growth. Hybrids used for crossing with Ukrainian carp produce crosses that also exhibit strong heterosis in growth. A cross between a female Ukrainian scaly carp and a male Ropshinsky carp exceeds the parental forms in height by 25%, and their nutritional value is equivalent to that of the Ropshinsky carp, but exceeds that of the Ukrainian scaly carp. Crosses obtained from a female Ropsha-Ukrainian cross with male Ukrainian scaly carp turned out to be heavier in weight than hybrids from crossing a female Ukrainian scaly breed with male Ropsha-Ukrainian carp. Thus, the producers from the first generation of crossing are unequal in the strength of transmission of hereditary characteristics to the offspring and the dominance of the maternal organism, as in a number of experiments conducted by other researchers, is clearly expressed.[...]
Selection for heterosis is directly related to the theory and practice of breeding selection and selection and serves as one of the ways to increase animal productivity. Heterosis in its genetic nature is the opposite of inbreeding depression. One of its features is the greatest degree of expression only in the first generation of hybrids (or crosses). Then heterosis imperceptibly fades and disappears in subsequent generations when hybrids are crossed with each other, unless special measures are taken to preserve the effect of heterosis.[...]
And leagues in F.V. Inbreeding and heterosis of farm animals. [...]
The effect of reproductive heterosis turned out to be highest when crossing Konakovo females with Shostka males and Moscow females with Konakovo males (Table 4).[...]
In dairy cattle breeding, heterosis in terms of milk yield and fat content in milk during interbreeding is rarely observed. Data on heterosis based on milk yield are given by N.F. Rostovtsev from the experience of crossing East Friesian cows with bulls of the Red Gorbatov breed (Table 44). In dairy cattle, the effect of heterosis is observed more often in the total amount of milk fat per lactation, especially when crossing cows of different breeds with Jersey bulls. [...]
Genetic explanation of heterosis. Although the heterosis effect is the most significant indicator of plant improvement, its genetic basis is not fully understood.[...]
To use the phenomenon of heterosis in animal husbandry within individual breeds, a similar method began to create highly inbred lines, in order to subsequently cross them within the same breed, as well as cross lines taken from different breeds. Especially a lot of such work has been carried out in poultry and pig farming. As one would expect, the lines bred in this way either quickly die out or turn out to be significantly less viable and less productive compared to the original material.[...]
A higher severity of heterosis is observed in four-line hybrids, when first, by crossing two paternal lines, cockerels are obtained, and hens are obtained from two maternal lines, and then such two-line hybrids are crossed with each other. The egg production of interline hybrids is 25-30% higher than with conventional purebred breeding or simple industrial crossing.[...]
Subsequently, to explain heterosis, in addition to the hypothesis of additional dominants (additive interaction of genes), other hypotheses were proposed (non-allelic interaction, overdominance, physiological balance, genetic balance, etc.).[...]
Taking into account the specific forms of manifestation of heterosis, the following types are distinguished: reproductive (increased fertility), somatic (development of organs and tissues), adaptive (increased vitality).[...]
Plants are powerful with a clear manifestation of heterosis, 125-135 cm tall, perennial, with a large number (25 - 37 spike-bearing shoots. New shoots of renewal, in the same way as perennial wheat and perennial hybrid rye, develop until late autumn. According to the structure of the spike, G4 hybrids are intermediate between perennial wheat and perennial hybrid rye (Fig. 89). Floral membranes with ostevndous pointed tips. The spike membranes are narrow, long, with well-defined denticles along the keel. The straw under the ear of most plants has pubescence. characteristic of perennial hybrid rye.[...]
To explain the genetic nature of heterosis, a number of other hypotheses have been put forward, for example the dominance hypothesis (Johnson, Pelyu). They believed that heterosis is the effect of a combination of favorably acting dominant alleles of different loci. In other words, in heterotic organisms there is a superiority of the entire set of dominant genes over the set of recessive genes. Close to this is the opinion of L. S. Zhebrovsky, who explains the manifestation of heterosis mainly by the additive effect of positively influencing dominant genes, present in a different set in the parents and combined in the offspring. In this case, the harmful effects of recessive genes are suppressed.[...]
Hybridization in pond fish farming. Hybridization is understood as the crossing of fish of different species to obtain marketable fish products, as well as to develop new, more valuable breeds and hybrid forms that combine valuable properties of the original species.[...]
Various compatibility, as well as the phenomenon of heterosis, observed at various degrees of heterogeneous selection and during crossing, are also due to other more complex, but little studied forms of interaction. We should not forget the very complex nature of the cause-and-effect relationships between hereditary factors and traits: some factors enhance (in ontogenesis) the development of a trait, others weaken it; the trait develops according to the resultant between opposite tendencies.[...]
The issue of obtaining heterosis for the milk productivity of cows has been least studied. Materials from many years of research into the patterns of variability in the productivity of cows of various breeds give reason to have no doubt that the heterogeneous effect on the size of yield is a common fact and that, as a rule, its occurrence is the result of a particularly successful selection of parental pairs during interbreed crosses and other variants of intrabreed mating of animals , having different chain heredity. But the manifestation of heterosis in terms of milk yield has its own specifics. After all, the abundant milk production of cows is one of the most physiologically complex traits. Unlike adaptive traits, which are fixed by natural and artificial selection, abundant milk production develops not as an advantage for the animal, but as a what a person needs quality in the process of long, complex and skillful work on the creation and improvement of dairy and dairy-meat cattle breeds.[...]
Kryzhanovsky O. A., Maslova I. I. Dependence of the heterosis effect on the combining ability of lines. - In the book: Fish selection. - M., 1989. - P. 86.[...]
Summarizing the results of numerous works on inbreeding in various branches of livestock farming and the use of heterosis by crossing inbred lines, I. Johansson and D. Lasch come to the conclusion that close inbreeding, repeated over several generations, leads to significant depression. It primarily affects traits related to the viability of animals (fertility, embryonic and postembryonic mortality, resistance to environmental influences, etc.). When crossing representatives of inbred lines, especially different breeds, increased viability (heterosis) is usually observed in the offspring. The question of whether it is economically justifiable to breed highly inbred lines for subsequent crossing of their representatives with each other, Johansson and Lasch consider, has not yet been clarified. It can be justified if the viability and productivity of animals obtained as a result of a cross of inbred lines will be so higher compared to the non-inbred original forms that it fully compensates for the losses and extra costs, inevitable when breeding and maintaining inbred lines. The use of moderate inbreeding in lines before crossing them with each other can somewhat reduce these losses, but this also reduces the chances of obtaining a more significant effect of heterosis. The far-reaching homozygosity of breeds and lines of our domestic animals is considered impossible by Johansson and Lasch.[...]
The achievements of modern biology, primarily genetics and selection, make it possible to use heterosis in the field of forestry - to create highly productive hybrids capable of producing an annual increase of 20-25 m3 per 1 hectare or more. This especially applies to poplars. For their successful breeding, not only the breeding of varieties is required, but also subsequent extensive testing.[...]
The system of selecting sires based on the principle of line rotation assumed, in addition to avoiding close inbreeding, also obtaining heterosis when crossing lines. But long-term and often stereotyped use of such a system not only leads to great genealogical diversity in each herd, but also to a decrease in the possibility of heterosis.[...]
With variable crossbreeding, genetic differences between animals of subsequent generations become less noticeable and the effect of heterosis decreases. Involving a large number of breeds in crossing does not produce a significant effect. It may not appear at all if less valuable and poorly compatible breeds are selected. Nevertheless, the method of alternating crossing, in which each generation is obtained as a result of fairly diverse matings, makes it possible to use all the advantages of crossing (increased vitality and productivity of crossbreeds) and, with a successful selection of breeds, maintain the phenomenon of heterosis over a number of generations. A significant advantage of variable crossbreeding is that only purebred sires are needed to maintain it to any generation; Crossbred animals are used as queens. In addition, in pig breeding, for example, crossbred queens, especially the first generation, turn out to be better mothers than purebred ones. This method gives good results if well-matched breeds are selected for crossbreeding, and purebred sires used in mating with crossbred queens to produce subsequent generations are selected after their progeny evaluation.[...]
In Norway, under the leadership of the famous geneticist H. Skjervold, a method was developed for creating a synthetic population of dairy cattle, designed to ensure the maintenance of heterosis for a long time. Using this method, a synthetic population of Norwegian red cattle (AF/g) was created, which absorbed approximately 16 breeds and offspring bred in the country, with optimal proportions of blood from each of the original breeds and offspring.[...]
Based on the results of these experiments, the authors come to the conclusion that when offspring-tested sires are used for crossbreeding, the productivity of the first crossbred generation increases significantly (the average effect of heterosis is about 20%). Further crossing maintains heterosis for several generations. At the same time, the first crossbred generation shows the least variability, which then increases. Crossbred cows, unlike purebred cows, are characterized by greater constancy (persistence) of milk yield throughout lactation, and as a result, higher milk productivity.[...]
Sometimes, within each of the two breeds selected for industrial crossing, individuals taken from different, well-combined inbred lines are pre-mated; then the resulting offspring of one breed with obvious signs of heterosis are mated with the same offspring obtained as a result of crossing lines of another breed. Double crosses make good utility animals. In this way (incrossbreeding), poultry farmers in the USA usually obtain custom birds.[...]
Often, with heterogeneous selection, due to a combination of hereditary characteristics of the parents, new valuable qualities appear in the offspring, which each of the parents did not have individually. Successful genealogical combinations can also lead to heterosis in the development of certain characteristics. This is especially observed when carrying out crosses of lines that differ on average from each other, that is, when the mating of representatives of lines is heterogeneous. O, A. Ivanova, analyzing the results of crosses of lines of the Russian trotting breed of horses, found that as the diversity of the line, calculated by the sum of squared deviations of the Indicators of each line from each other, increases, more effective crosses are obtained. Thus, when combining the Bob-Douglas line with the Pass-Rosa line, an offspring is obtained that is distinguished by the highest agility. The degree of heterogeneity of these lines, expressed by the sum of squared deviations of their indicators, is 83.5, and the average agility of the offspring at a distance of 1600 m is 2 min 19 3/a s. The least agility was observed in the offspring obtained from the selection of the same Bob-Douglas line to the Gay-Bingea line. At the same time, the degree of heterogeneity was 9.7, and speed was 2 min 37 4D s.[...]
This method is widely used in breeding farms to improve breeding and productive qualities existing breeds and for breeding new ones in commercial farms to increase the productivity of industrial herds using heterosis. The biological prerequisite for crossing is the expansion of combinative variability and biological enrichment, the appearance of the effect of heterosis - an increased level of development of a number of traits in crosses obtained through crossing.[...]
Test questions. 1. What are the forms and principles of selection? 2. What is the essence of homogeneous and heterogeneous selection? 3. How is selection carried out taking into account genealogical compatibility? 4. Ways to obtain it.[...]
Each of these methods has its own characteristics and can be used to obtain heterosis not for all, but only for certain characteristics. Whatever methods are used to obtain heterosis, the individual characteristics of the manufacturer are of great importance. The more valuable its origin and the higher its ability to transmit its qualities to its offspring, the higher the degree of manifestation of heterosis, other things being equal, will be. The huge role of heterosis in increasing productivity and improving other economically useful traits of animals prompted many scientists to find ways to consolidate it for a long time or at least preserve it for several generations. D. A. Kislovsky was one of the first to theoretically substantiate the possibility of using heterosis in subsequent generations of interbreed variable crossing. He argued that with such crossing the characteristics and positive aspects absorption and industrial crossing.[...]
In this context, biopoliticians, along with anthropologists, of course, contribute to resolving the question of what were driving forces origin of man. Many answers to this question have been given in the literature. There are views about the leading role of changes in environmental conditions (apparently, repeatedly in the process of anthropogenesis), increased radioactive background, which caused abrupt mutational changes with the eventual emergence of Homo sapiens, the effect of heterosis (crossing of local populations that had previously been isolated for a long time ), as well as the role of other factors. Many researchers consider anthropogenesis to be a complex process that occurred under the influence of many factors at once, including: 1) a complex and diverse habitat (savanna); 2) the complex nature of searching and obtaining food with competition for it; 3) complicated social organization and a system of social communication, which is associated with cooperation in obtaining food and especially its protection from competitors; 4) the need for collective care for dependent and slowly developing children in conditions when a woman’s life expectancy was on average 26 years, and the first child was born at 15-17 years [Dolnik, 1994, 1996; Butovskaya, 2000]. Thus, human evolution is viewed largely in the context of biosocial systems and Corning's “teleonomic selection.” The stages of anthropogenesis reflected the stages of change in the social structure; changed biosocial (simply social since the appearance of the genus Homo) systems were not only the correlates of these changes, but also, probably, at least in some cases, their causal factors - an idea consistent with the views of Corning, Jantsch, as well as our compatriot Krasilov. [...]
Excessive inbreeding within a line limits the possibility of obtaining anything new and can lead to stagnation. Proof of this situation can be seen in the cases I often observe in which, in progressive breeds, animals obtained as a result of inbreeding are far from being the best, but only average or good. Line crosses in such cases, as well as the selection of unrelated queens to sires, expand hereditary possibilities, are accompanied by the phenomena of heterosis and ensure further progress; Breeding, with the occasional use of moderately related mating, precedes crosses and supplies material for them. In this case, there is some alternation of either more homogeneous or more heterogeneous selection, and each of them, being associated with the other, finds its place in breeding work. As an example of such a cross with preliminary inbreeding, one can cite the famous trotter Magnanimous, born in 1950 at the Khrenovsky stud farm from Velbot and Castellanshi (see his pedigree). Whalebot and Castellan belong to different lines and were obtained as a result of inbreeding: Whalebot - on Warmik in degree III-III, Castellan - on Impatiens in degree III-III, Kasha in degree III-III and Nezhdanny in degree IV-IV. It should be noted that the Magnanimous is an animal, in a sense, “hybrid”; it is obtained as a result of the mating of two inbred parents taken from different, although not genetic, but zootechnical (breeding) lines.[...]
In terms of their biological essence, interline crosses during purebred breeding are not fundamentally different from interbreeding. They are based on the same patterns discovered by Charles Darwin. This is a genetic difference in sexual elements in mated animals, enriching the heredity of the offspring, stimulating its development and increasing the vitality of the animals. Professor Yurasov defines a line as a micronode. From this point of view, a successful cross of lines can be considered as a result of the manifestation of heterosis. Just as heterosis during interbreeding crosses is the result of the dissimilarity of the germ cells of the parents, so heterosis during line crosses can be explained by the same phenomenon.[...]
The experiments of American researchers Schell, East, Geis, Jones and others established that during inbreeding, i.e. during artificial self-pollination of corn (a typical cross-pollinating plant with same-sex flowers), the plants experience a sharp depression (their growth decreases, the cobs become smaller and grains in them, the yield sharply decreases). A similar depression is observed in the first (several) inbred generations; the low-yielding dwarf forms obtained as a result of further inbreeding remain more or less unchanged. When such depressed, but unequal forms are crossed with each other, already in the first generation all the signs of pronounced heterosis appear: powerful plants with a large number enlarged cobs, strewn with a mass of large, full-weight grains. When these vegetatively powerful plants (the product of crossing) then begin to reproduce by self-pollination, then in the next 3-5 generations a sharp decrease in vegetative power and productivity is again observed: a return to dwarf low-yielding forms is observed. Thus, heterosis is a property of only the first generation obtained as a result of crossing. It disappears with further breeding of crossbreeding products “in itself”.[...]
V. A. Altshuler, E, Ya. Borisenko and A. I. Polyakov, complementing the hypothesis of obligate heterozygosity, gave it an evolutionary interpretation. Each new gene arises in a heterozygous state and is exposed to natural selection. Many of the newly emerged gene changes have a pleiotropic (multiple) effect. In one direction this action is beneficial, in another it is neutral or even harmful to the body. In the process of evolution, those organisms survive in which the positive effect of genes was revealed in a heterozygous state, and the harmful effect was found in a recessive state. The emergence of genes with double action, that is, with obligate-heterozygous action, is a consequence of the evolutionary process. Heterosis is primarily beneficial to the animal’s organism itself, which results from crossbreeding. It follows that a high degree of heterozygosity is the cause of heterosis.[...]
In Knox's experiments (1946-1954), the egg production of red Rhode Island and white Leghorn inbred lines (inbreeding coefficient from 60 to 80%) was 163 and 185 eggs, respectively, against 217 and 212 eggs in non-inbred birds of the same breeds. At the same time, the egg production of crosses obtained as a result of crossing inbred Rhode Island roosters with inbred White Leghorn hens increased to 233 eggs, and with a reciprocal combination of inbred hens and roosters of these breeds - up to 258 eggs. The results of tests of inbred lines of chickens of four breeds (New Hampshire, Rhode Island Red, Striped Plymouth Rock and Austrolorp) and the results of their crossings carried out by Nordskog in Iowa (1954) are also known. It was shown that the effect of heterosis on the growth rate of young animals up to 8 weeks of age during interline crosses within the same breed was expressed by 4%, and when crossing different breeds - 7%; hatchability was the highest, and chicken mortality was the lowest also with interbreeding; The egg production of a crossbred bird (with interbreeding) exceeded by 12%, and the average egg production of birds of inbred lines obtained by crossing lines in one breed by 10%.
Arden. Pony. Khaynak. Sheep breeds. Methods of animal selection. Karakul breed. Russian trotting breed. Selection. Santa Ynez. Ayrshire breed. Inbreeding. Brown Swiss breed. The oldest breed. Basic methods of animal selection. Interspecific hybrids. Soviet merino. Unrelated crossing. Features of animal selection. Archaromerinos. Hampshire. Sheep breeding. Hereford breed.
“Achievements in animal selection” - Dragged camel. Selection methods. Interspecific hybridization. Liger. The concept of selection. Wild sheep argali. Camelama. Selection. Hybrid pheasant. Outbreeding. Heterosis. Hybrid animals. Mule. Features of animal selection. Orca dolphin. Zebroids. Levopard. Interspecific animal hybrids. Inbreeding. Animal selection. Selection of breeding material. Dog-wolf.
“Selection of farm animals” - Semi-fine wool sheep. Trotting horses. Meat breeds of pigs. Wild species. Horse pack and draft horses. Fine fleece sheep. Artificial insemination. Method of selection work. Fat and meat-greasy breeds of pigs. Bull. Crossbreeding. Modern methods selection. Classification of breeds. Meat breeds. Hybridization. Riding horses. Plates. Animal selection. Breeding methods. Cloning. Success of selection.
“Basics of selection of organisms” - Fundamentals of selection of organisms. N.I. Vavilov. Selection tasks. East Asian Center. The law of homological series of hereditary variability. What does selection do? Mediterranean center. Selection methods. Central American Center. South-West Asian Center. Basic methods of plant breeding. South Asian Center. South American Center. Plant selection. Textbook assignment. Modern selection.
“Breeding work in horse breeding” - State Book of Breeding Animals. Federal law about livestock breeding. Development of the genetic basis of selection in horse breeding. Detailed characteristics of breeding stock. One of the most important acceleration factors scientific and technical progress. Valuation is an assessment of breeding and productive qualities. Horse production goals. Method of reproductive crossing. The birth of Bars 1 was preceded by trial and error work.
“Trotting horse breeds” - Characteristics of the breed. Breeding procedures. Peter Tzi Great. Speedy Crown. Perm type. Oryol trotting breed. Diversity of conformation of the standardbred breed. A modern racing trotter. Standardbred breed. Standardbred trotter. Hambletonian. Exterior disadvantages. The founder of the breed. A.G. Orlov on Barsa I. The type developed under the influence of harsh climatic conditions. The influence of ancestors on the breed.
Heterosis is the property of crosses and hybrids of the first generation (Fi) to surpass the original parental forms in biological and economically useful characteristics.
The phenomenon of heterosis was first described by I. Kölreuter, who worked at the St. Petersburg Academy of Sciences, using the example of an interspecific hybrid that he obtained in 1760, crossing two different types shag. This plant hybrid turned out to be sterile like a mule and the author called it “the first plant mule.”
Scientific term"heterosis" appeared much later. It was proposed by the American researcher J. Schell in 1914 to denote the power of hybrids (the effect of crossing), and since then it has firmly entered the scientific literature as a synonym for the old name “hybrid vigor.”
It is now firmly established that heterosis manifests itself not only in the breeding of animals and birds, but also in the selection of plants and microorganisms. Consequently, heterosis is a general biological phenomenon.
What are the causes of heterosis? There are several points of view on this matter in the form of separate independent hypotheses.
One of the first is set out in the fundamental work of Charles Darwin, “The Action of Cross-Pollination and Self-Pollination in the Plant World,” in which he outlined the issues of heterosis (“hybrid vigor”).
Having studied the experience of English breeders in creating new breeds of farm animals, Charles Darwin noted that related mating (inbreeding), which he used to consolidate the desired characteristics of outstanding producers in the offspring, leads, like self-pollination in plants, to negative consequences- to depression, while crossing, as a rule, increases the viability of the offspring due to the manifestation of the effect of crossing (heterosis).
Darwin suggested that at the core these two phenomena - inbreeding depression and heterosis - lies same reason - the degree of difference between the sexual elements that come together during the process of fertilization.
The more the parental forms, and therefore their germ cells, differ in their biological characteristics, the more heterosis is manifested in the offspring, and vice versa, the absence of such differences during closely related long-term mating leads to nibular depression.
Based on these considerations, Charles Darwin proclaimed the “great law of nature”, according to which, from the point of view of the evolution of a species, crossing is always beneficial, and inbreeding (inbreeding in animals, self-pollination in plants) is harmful.
Dominance hypothesis (Jones, 1972). This hypothesis is based on the idea of the beneficial action of dominant genes - heterosis manifests itself as a result of interaction when crossing favorable dominant factors present in the original parental forms.
It is assumed that during crossing there is a combination of beneficially acting non-allelic dominant genes and their simultaneous suppression of the action of various harmful recessive alleles, which are located in different loci in different lines, and especially breeds. When crossing, dominant alleles contributed by one parent (line) may overlap recessive alleles received by the hybrid from the other parent (line).
The manifestation of heterosis is also possible due to the phenomenon of epistasis, when individual non-allelic genes (epistatic gene) suppress not only “their own” recessive genes, but also “foreign” dominant genes (hypostatic gene).
The dominance hypothesis is generally accepted, however, it does not fully explain all the issues that arise in connection with the manifestation of heterosis. So, if we proceed from the above hypothesis, then theoretically we should expect that during a polyhybrid crossing, the heterozygote Aa will, to one degree or another, approach the productivity of the homozygote AA - approach, but not exceed it.
However, in practice it has long been established that a heterozygote can be superior in power not only to the recessive parental form, but also to the dominant one, that is, both of its parents. This phenomenon even received a special name in genetics - overdominance, or monohybrid heterosis.
Heterozygosity hypothesis (overdominance). From the point of view of the heterozygosity hypothesis, the manifestation of heterosis is explained, in modern terms, by the different quality of members of the same pair of alleles in hybrid organisms as a result of crossing different initial parental forms. The combination during hybridization of different quality gametes of parents in itself stimulates faster growth of heterozygous hybrids, their better development, etc. As a result, the hybrid is superior in power to the original homozygous parental forms, both recessive and dominant, which determines the effect of overdominance. At the same time, parental homozygosity has a depressing effect on the viability of the offspring, which is expressed by the formula Aa>AA>aa.
It is assumed that in a heterozygote, both alleles of one locus perform different functions, mutually complementing each other in the biochemical process. In this case, the effect of heterosis will be higher, the more the alleles of each locus differ functionally from each other, the more they complement each other.
The causes of heterosis indicated in these hypotheses may operate simultaneously, but they are apparently not sufficient for a comprehensive explanation of the mechanism of heterosis as a general biological phenomenon. On this occasion, Prof. M.V. Lobashov (1969) wrote: “It is difficult to imagine that such a complex phenomenon as heterosis is based on a single genetic mechanism.” As for understanding the very mechanism of gene interaction during heterosis, according to modern views, the difference between both hypotheses is insignificant or non-existent.
The genetic balance hypothesis (Academician N.V. Turbin of the Russian Academy of Agricultural Sciences, 1961. From the point of view of this hypothesis, the phenomenon of heterosis cannot be explained by the action of any one genetic cause - it is a total effect.
Genetic balance hypothesis, accepting in both respects the individual provisions of the previously stated hypotheses, more attention, however, pays attention to the mutual influence of non-allelic genes, physical and biochemical factors, as well as external environment in general, the conditions for growing hybrids in particular. Special attention is given cytoplasmic influence. It is assumed that plasmatic differences between gametes should stimulate life processes in a hybrid organism. The balance of gene systems makes populations the most adaptive and productive in specific environmental conditions.
It should be noted that in recent years it has also become increasingly important biochemical theory of heterosis , according to which crossing leads to an increase in heterozygosity for mutations regulating protein synthesis - hence the manifestation of heterosis occurs due to the enrichment of biochemical processes in the cells and tissues of the hybrid organism.
The significance of the above hypotheses is undeniable, but none of them can yet be recognized as a generally accepted theory of heterosis. Perhaps the famous geneticist F. Hutt is right in his statement: “Heterosis still represents one of the biggest mysteries of genetics.”
What is heterosis as a genetic phenomenon? What are the forms of manifestation of heterosis?
Heterosis is a set of phenomena associated with increased viability of crosses (hybrids), which, as established, manifests itself already in the early stages of development (ontogenesis). In embryos of hybrid chickens, for example, even during embryonic development, metabolic processes are enhanced, their development is accelerated, as a result of which the hatch and quality of day-old young animals are higher compared to the same indicators of linear chickens (Zlochevskaya, 1968).
Heterosis is an unstable (short-term) phenomenon; it is most clearly (clearly) manifested only in the first generation (F 1) of crossing. Crossbred (hybrid) animals, when further bred, do not produce similar heterotic offspring; they do not remain “constant” in heterosis. Therefore, they are not left for the tribe, but sold for meat. Hence, Heterosis cannot be fixed hereditarily; it must be acquired anew every time.
In some cases, heterosis can be maintained at a relatively high level in subsequent generations, but in such cases special methods are used - variable crossing, etc. It is believed that extinction of heterosis in subsequent generations of hybrids is the result of recombination losses.
The forms of manifestation of heterosis are different. Usually, when crossing two breeds (A and B), the level of productivity of the crossbred (AB) offspring is equal to the average productivity of the original breeds. In such cases, they speak of hypothetical (probable) heterosis. 
Often the productivity of crossbred (F 1) animals turns out to be significantly higher than the average productivity of the parents, and sometimes it exceeds the indicators of the best of the parental forms - absolute (true) heterosis. 
In other cases, however, the productivity of crossbreeds exceeds that of only one of the parents, the worst being relative heterosis: 
where: Pg is a sign of a hybrid; Pl - sign best breed; PM - a sign of the parent breed; Po is a sign of the paternal breed.
Of course, from purely practical considerations, the effect of heterosis is of greatest interest only in the case when the hybrid offspring exceeds the best of the parents in its overall economic value. Only in such cases does crossing have economic sense. Therefore, practicing breeders understand heterosis as the property of hybrids (F 1) to surpass the best of the parent forms in certain characteristics.
Some scientists, taking into account the specific forms of manifestation of heterosis, identify its independent types:
reproductive heterosis - higher overall productivity of animals associated with increased fertility (fertility) and more powerful development of their reproductive organs;
somatic heterosis - stronger development of vegetative parts (in plants), organs and body parts (in animals);
adaptive heterosis - increased vitality of animals, their better adaptability.
In mules, for example, somatic heterosis is strongly expressed, that is, large live weight; higher traction force; increased longevity; special endurance; but at the same time, the reproductive system is underdeveloped. As a rule, they are infertile. The above is an example of partial heterosis (powerful development does not concern the entire body of the animal, but only its individual characteristics), in contrast to general heterosis, when there is a development of the total body weight of the animal, an increase in metabolic processes in the body as a whole, which ensures an increase in its productivity (Beauty, 1979).
It should be noted that heterosis manifests itself in crosses and hybrids - interspecific, interbreed, interlinear - according to a limited number of characteristics. It never manifests itself by the sum of all parental characteristics. Crossbreeds (hybrids) are superior to their parents not in all indicators of productivity, not in all traits, not in their sum, but only partially, in individual traits (or a group of traits) or even in a single trait.
First-generation mixed breed chickens, obtained from crossing roosters of meat breeds with chickens of egg-laying (light) breeds, can exceed the original parental forms in egg production, but in terms of live weight they occupy an intermediate position.
From here heterosis should be understood as the superiority of offspring - crosses or hybrids - over parental forms, not in all, but only in certain, specific characteristics.
Literature
1. Vavilov N.I. Centers of origin of cultivated plants. - Tr. on applied botany and selection, 1926, vol. XVI, p. 5-138.
2. Gershenzon S.M. Fundamentals of modern genetics. - Kyiv, 1979. - 506 p.
3. Lobashev M. E., Vatti K. V., Tikhomirova M. M. Genetics with the basics of selection. - M., 1979. - 304 p.
4. Rokitsky P. F. Some stages of the development of animal genetics in the USSR and its connection with selection. - In the collection: Genetic foundations of animal selection. M., 1969, p. 9-25.
Heterosis (hybrid vigor) is an increase in power, viability and productivity (or hybrid vigor) of first-generation hybrids compared to parental forms.
The concept of heterosis was introduced into science by the American geneticist W. Shell in 1914. This phenomenon was first discovered in 1772 by I. Kelreuter when crossing two types of tobacco. Later, Charles Darwin studied this phenomenon in experiments on corn pollination.
Heterosis in nature is characteristic of all organisms. It arose along with the advent of diploidity and the sexual process and is directly related to heterozygosity.
We can talk about heterosis in the case when the hybrid offspring is superior to both parents in certain characteristics, for example, in the degree of development of vegetative organs, productivity, resistance to diseases, pests, etc.
However, only certain pairs of parental forms produce heterotic offspring. Breeders cannot say for sure which pairs will give the greatest effect from crossing, and this is mainly determined through experiments. It is also known that heterosis is fully manifested only in the first generation; in subsequent generations, the hybrid power sharply decreases (Fig. 4). This phenomenon is associated with a decrease in plant heterozygosity in hybrid populations.
Fig.4. Manifestation of the effect of heterosis in F 1 and its decrease in subsequent generations, % - relative yield of hybrids F 2 and F 3 compared to F 1
Long-term self-pollination leads to the appearance of plants that are homozygous for most or almost all genes. In the case of heterozygotes of the original form, one pair of alleles in each subsequent generation, during self-pollination, the proportion of heterozygous plants will decrease by half and the proportion of homozygous individuals will correspondingly increase: in the first generation there will be 50%, in the second 75%, in the third 87.5%, etc. d. The proportion of heterozygous and homozygous individuals during self-pollination is calculated using the formulas: (1/2) n for heterozygotes and 1-(1/2) n for homozygotes, where n is the number of self-pollinations.
Inbreeding (obtaining offspring from crossing related individuals) of the original form, heterozygous for one pair of alleles (Aa), leads to the emergence of homozygotes of two different types.
Thus, forms that are heterozygous for any pair of alleles, as a result of long-term inbreeding, turn into lines that are homozygous for these alleles. The more pairs of alleles for which the original plant is heterozygous, the more different homozygous lines can be obtained during prolonged self-pollination.
Heterosis in general form is the result of the creation of heterozygosity in the process of crossing and the emergence of new interactions between genes. The most important difference between heterotic hybrids and conventional hybrid varieties is that they are used in production only in the first generation, and therefore they must be obtained annually from annual crops.
With heterosis, all properties and characteristics of the organism do not necessarily increase. One of them may be more pronounced than the others, and according to some signs, heterosis may be completely absent. That is, the discrete nature of heredity determines the discrete manifestation of heterosis.
There are various theories of heterosis. The first attempt at a theoretical explanation of this phenomenon was made by American researchers G. Schell and E. East in 1908. Scientists believed that the combination of gametes with a different set of genes during hybridization in itself stimulates more rapid development of the embryo and the growth of the hybrid. That is, heterozygosity is the main reason for hybrid vigor. This theory was called theory of overdominance (heterozygosity).
The phenomenon of overdominance is manifested in the fact that in a number of cases heterozygous individuals (Aa) are superior in power not only to individuals with a recessive homozygote (aa), but also to individuals with a dominant homozygote (AA):
Aa>AA>aa
The heterozygosity hypothesis has received experimental confirmation in the works of many scientists on various objects (barley, corn, tomatoes and other crops).
The theory of overdominance indicates the importance for the emergence of hybrid power of hereditary heterogeneity, which is created as a result of the heterozygous state in the same loci of homologous chromosomes. Since, as a rule, maximum heterosis manifests itself as a result of crossing unrelated individuals, then, according to the theory, the effect of the heterozygous state is higher, the more different the alleles of each locus are.
The theory of heterozygosity explains many cases of heterosis, but does not cover the entire diversity of this complex phenomenon.
Dominance theory (interaction of favorable dominant factors) was formulated by G. Davenport in 1908, supplemented by A. B. Brus and most fully outlined by D. Jones in 1917. It is based on the idea that in the process of evolution, some genes that have a beneficial effect on the growth and development of the organism become dominant and semi-dominant, while other genes that act unfavorably become recessive. According to this theory, heterosis is associated with the multilateral action of dominant genes: firstly, they suppress the possible harmful effects of their recessive alleles; secondly, they have an additive (summative) effect in relation to many quantitative traits, in relation to which heterosis manifests itself. For example, if a trait is controlled by two dominant genes A and B, then their combination in a hybrid will cause the most pronounced such trait (Fig. 5).
Fig.5.
According to this theory, when crossing two self-pollinated lines aaBBccDD and AAbbCCdd, the reduced viability of the first is associated with the manifestation of the harmful effects of recessive genes a and c, and the second - with b and d. In the AaBbCcDd hybrid obtained from crossing these lines, dominant genes will protect the heterozygote from the action of recessive alleles.
Some dominant genes can suppress the action not only of their recessive alleles, but also of dominant genes of other pairs of alleles. This phenomenon is called epistasis.
The dominance hypothesis generally explains the main features of heterosis and inbreeding depression. On its basis, corn selection was successfully carried out for many years.
The theory of dominance also does not exclude the theory of heterozygosity; they can also be considered as components of one general theory heterosis.
Genetic balance concept was put forward by Lerner (1954) and supported by N.V. Turbin (1961). It proceeds from the fact that the nature of the manifestation of any trait in each parental variety or line is the result of a certain balance developed during selection in the multidirectional action on the trait of many genes and conditions environment in which the development of the organism occurs. When crossing parental forms that differ in hereditary terms, the resulting hybrids change the genetic balance, which can cause a change in the degree of manifestation of a particular trait.
This concept allows us to combine the theories of heterozygosity and dominance, to take into account various factors(cytoplasmic, physiological, biochemical) and environmental conditions. However, it does not give an idea of specific gravity each of the factors and types of interaction of hereditary factors underlying heterosis.
The concept of compensatory complexes proposed by V. A. Strunnikov in 1983, which combined ideas about allelic and non-allelic interactions of genes that provide heterosis. A compensatory complex is a complex of genes and their alleles, which is selected when obtaining inbred lines. It neutralizes the negative effects of a high level of homozygosity, and during hybridization it gives the effect of heterosis.
In agricultural countries, heterosis is widely used in the cultivation of corn, sugar and fodder beets, sorghum, onions, tomatoes, peppers, cucumbers, carrots, pumpkins, castor beans and other crops. The development of methods for creating heterotic hybrids of various crops is one of the greatest achievements of modern genetics.
First of all, the phenomenon of heterosis in on a large scale began to be used in corn. Conventional varieties of this crop are almost completely replaced by heterotic hybrids of the first generation. The main crops of corn for grain are occupied by double interlinear (Fig. 5) and varietal linear hybrids. Double interline hybrids are obtained from crossing two simple interline hybrids (four self-pollinated lines), and variety-line hybrids are obtained from crossing a variety with a self-pollinated line or a simple interline hybrid with a variety.
Unlike combination breeding, in which crossings are carried out to create genetic variability for selection, in the selection of heterotic hybrids, crossing serves to produce seeds in large quantities and their further practical use.
Heterosis is a general biological phenomenon characteristic of all types of living organisms. In practice, it is also used in livestock and poultry farming. Carrying out interbreed and interline crossings makes it possible to significantly increase the production of meat, milk, eggs and other products with the same feed costs.
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