Heterosis. Genetic ideas about heterosis

Heterosis is the property of crossbreeds and hybrids of the first generation (Fi) to surpass the original parental forms in terms of biological and economically useful features.

For the first time, the phenomenon of heterosis was described by I. Kölreuter, who worked at the St. Petersburg Academy of Sciences, on the example of an interspecific hybrid, which he received in 1760, crossing two different types of shag. This plant hybrid turned out to be sterile like a mule, and the author called it "the first plant mule."

The scientific term "heterosis" appeared much later. It was proposed by the American researcher J. Shell in 1914 to designate the power of hybrids (crossing effect), and since then it has firmly entered the scientific literature as a synonym for the old name "hybrid power".

At present, it is firmly established that heterosis manifests itself not only in the breeding of animals and birds, but also in the selection of plants, as well as microorganisms. Therefore, heterosis is a general biological phenomenon.

What are the causes of heterosis? On this account, there are several points of view 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 force").

Having studied the experience of English breeders in creating new breeds of farm animals, Charles Darwin noted that inbreeding (inbreeding), which he used to consolidate the desirable traits of outstanding producers in the offspring, leads, like self-pollination in plants, to negative consequences - to depression, while crossing increases, as a rule, the viability of 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 the same reason the degree of difference between the sexual elements that combine in the process of fertilization.

The more parental forms, and hence their germ cells, differ in their biological characteristics, the stronger heterosis is manifested in the offspring, and vice versa, the absence of such differences during closely related long-term mating leads to nibred 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 a favorable effect of dominant genes - heterosis manifests itself as a result of interaction when crossing favorable dominant factors that are present in the original parental forms.

It is assumed that when crossing, a combination of favorable non-allelic dominant genes occurs and their simultaneous suppression of the action of various harmful recessive alleles, which are located in different loci in different lines, and even more so breeds. When crossed, the dominant alleles introduced by one parent (line) may override the recessive alleles received by the hybrid from the other parental form (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" recessive, but also "foreign" dominant genes (hypostatic gene).

The dominance hypothesis is generally accepted, however, it does not fully explain all the questions that arise in connection with the manifestation of heterosis. So, if we proceed from the above hypothesis, then theoretically it should be expected that during polyhybrid crossing, the Aa heterozygote will, to one degree or another, approach the AA homozygote in productivity - approach, but not exceed it.

However, in practice it has long been established that a heterozygote can surpass in power not only the recessive parental form, but also the dominant one, that is, both of its parents. This phenomenon has even received a special name in genetics - overdominance, or monohybrid heterosis.

Hypothesis of heterozygosity (overdominance). From the point of view of the hypothesis of heterozygosity, the manifestation of heterosis is explained, in modern terms, by the heterogeneity of members of the same pair of alleles in hybrid organisms as a result of crossing different parental forms. The combination during hybridization of different-quality gametes of the parents in itself stimulates the faster growth of heterozygous hybrids, their better development, etc. As a result, the hybrid surpasses the initial homozygous parental forms, both recessive and dominant, in terms of power, which causes the effect of overdominance. At the same time, the homozygosity of the parents 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 the 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 act simultaneously, but they, apparently, are not sufficient for a comprehensive explanation of the mechanism for the occurrence of heterosis as a general biological phenomenon. On this occasion, prof. M. V. Lobashov (1969) wrote: “It is difficult to assume that such a complex phenomenon as heterosis is based on a single genetic mechanism.” As for the understanding of the very mechanism of gene interaction in heterosis, according to modern views, the difference between both hypotheses is insignificant or it does not exist at all.

Hypothesis of genetic balance (academician of VASKhNIL N. V. Turbin, 1961. From the point of view of this hypothesis, the phenomenon of heterosis cannot be explained by the action of any one genetic cause - this is a cumulative effect.

Genetic balance hypothesis, accepting in both respects the separate 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 the external environment in general, the conditions for growing hybrids in particular. Particular attention is paid to the cytoplasmic influence. It is assumed that plasma differences between gametes should stimulate vital 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, the importance of biochemical theory of heterosis , according to which crossing leads to an increase in heterozygosity for mutations that regulate 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 well-known geneticist F. Hutt is right in his statement: "Heterosis is still one of the biggest mysteries of genetics."

What is heterosis as a genetic phenomenon? What are the manifestations of heterosis?

Heterosis is a set of phenomena associated with increased viability of hybrids (hybrids), which, as established, manifests itself already in the early stages of development (ontogenesis). In embryos of hybrid chickens, for example, even in embryonic development, metabolic processes are intensified, their development is accelerated, as a result of which the output 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 manifests itself most clearly (clearly) 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 are sold for meat. Consequently, heterosis cannot be fixed hereditarily, it must be received 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 the extinction of heterosis in subsequent generations of hybrids is the result of recombination losses.

Forms of manifestation of heterosis are different. Usually, when two breeds (A and B) are crossed, the level of productivity of the crossbred (AB) offspring is equal to the average productivity of the original breeds. In such cases, one speaks of hypothetical (probable) heterosis.

Often the productivity of hybrid (F 1) animals is significantly higher than the average productivity of the parents, and sometimes it exceeds the performance of the best of the parental forms - absolute (true) heterosis.

In other cases, however, the productivity of hybrids exceeds the performance of only one of the parents, the worst is relative heterosis:

where: Pg - a sign of a hybrid; Pl - a sign of the best breed; Pm - a sign of the parent breed; By - a sign of 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 crossbreeding make economic sense. Therefore, practicing breeders under heterosis understand the property of hybrids (F 1) to surpass the best of the parental forms in certain ways.

Some scientists, taking into account the specifics of the forms of manifestation of heterosis, distinguish its independent types:

reproductive heterosis - a higher overall productivity of animals associated with an increase in fecundity (fertility) and a more powerful development of their reproductive organs;

somatic heterosis - a stronger development of vegetative parts (in plants), organs and body parts (in animals);

adaptive heterosis - increased viability of animals, their better adaptability.

In mules, for example, somatic heterosis is strongly pronounced, that is, a large live weight; higher traction force; increased longevity; special endurance; but at the same time, the reproductive system is underdeveloped. They are usually infertile. The above is an example of private heterosis (powerful development does not concern the entire body of the animal, but only its individual features), in contrast to general heterosis, when the overall body weight of the animal develops, the metabolic processes in the body as a whole increase, which ensures an increase in its productivity. (Beauty, 1979).

It should be noted that heterosis is manifested in crossbreeds and hybrids - interspecific, interbreed, interline - according to a limited number of characters. It never manifests itself by the sum of all parental traits. 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.

Mestizo hens of the first generation, obtained from crossing beef roosters with hens of egg-laying (light) breeds, may exceed the original parental forms in egg production, but in live weight they occupy an intermediate position.

From here heterosis should be understood as the superiority of offspring - crossbreeds or hybrids - over parental forms, not in all, but only in certain, specific ways.

Literature

1. Vavilov N. I. Centers of origin of cultivated plants.- Tr. on Applied Botany and Breeding, 1926, v. 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 in the development of animal genetics in the USSR and its connection with selection. - In: Genetic Foundations of Animal Breeding. M., 1969, p. 9-25.

Heterosis is the property of crossbreeds and hybrids of the first generation (Fi) to surpass the original parental forms in terms of biological and economically useful features.

What are the causes of heterosis? On this account, there are several points of view in the form of separate independent hypotheses.

What is heterosis as a genetic phenomenon? What are the manifestations of heterosis?

In other cases, however, the productivity of hybrids exceeds the performance of only one of the parents, the worst is relative heterosis:

where: Pg - a sign of a hybrid; Pl - a sign of the best breed; Pm - a sign of the parent breed; By - a sign of paternal breed.

Some scientists, taking into account the specifics of the forms of manifestation of heterosis, distinguish its independent types:

reproductive heterosis - a higher overall productivity of animals associated with an increase in fecundity (fertility) and a more powerful development of their reproductive organs;

somatic heterosis - a stronger development of vegetative parts (in plants), organs and body parts (in animals);

adaptive heterosis - increased viability of animals, their better adaptability.

From here heterosis should be understood as the superiority of offspring - crossbreeds or hybrids - over parental forms, not in all, but only in certain, specific ways.

Literature

1. Vavilov N. I. Centers of origin of cultivated plants.- Tr. on Applied Botany and Breeding, 1926, v. 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 in the development of animal genetics in the USSR and its connection with selection. - In: Genetic Foundations of Animal Breeding. M., 1969, p. 9-25.

Heterosis using heterogeneous selection in intrabreed mating. The use of lines, lines of producers and families in purebred breeding of crosses, as well as the mating of animals belonging to the same breed, but grown under different conditions, are also options for heterogeneous selection. The heterosis effect during such mating will be discussed below. In this case, we are talking about such a heterogeneous selection, in which the mated animals are in the same farm, do not have a clear 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 difference between mated individuals only in some respects, in particular, in conformation-constitutional features.[ ...]

The practice of breeding knows many examples when, with successful crosses of lines, offspring are obtained that are distinguished not only by the strength of the constitution, fertility, vitality, but also significantly exceed the main productive qualities of mothers, and the average indicators of those lines to which the parents belong. ..]

Heterosis also manifests itself in hybrids of salmonids and can occur when crossing whitefishes.[ ...]

Heterosis does not occur automatically, the degree of its manifestation is largely determined by the genetic (hereditary) characteristics of the interbreeding pairs.[ ...]

HETEROSIS [from gr. heteroiosis - change, transformation] - increased viability and fertility of first-generation hybrids compared to parental forms. First described by C. Darwin (1859). G.'s phenomenon, as a rule, is not observed in the second and subsequent generations. G. is widely used in page - x. practice.[ ...]

Heterosis in interspecific crossing. The manifestation of heterosis during interspecific crossing was known in antiquity, when, when donkeys were mated with mares of various breeds, mules were obtained that were superior to both horses and donkeys in longevity, working capacity (traction force per unit mass) and resistance to various diseases. But in terms of live weight, they occupy an intermediate position and are practically barren. When a two-humped camel (Bactrian) is crossed with a single-humped camel (dromedary), hybrids (bunk beds) are obtained that significantly exceed the parental forms in terms of live weight, traction ability without loss of fertility.[ ...]

Heterosis in plants is the phenomenon of the superiority of a hybrid plant over the best of its parents, but the power and degree of development of certain traits and properties. The heterotic power of plants is manifested 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 terms of viability, growth energy, fertility, constitutional strength, resistance to diseases.[ ...]

The phenomenon of heterosis is also observed in purebred breeding of animals (with various forms of heterogeneous selection and crosses of well-matched lines), but it is not expressed as clearly as when crossing representatives of different breeds, as well as during hybridization.[ ...]

To obtain heterosis in 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 a great opportunity for selection, methods have been developed for the directed formation of the heredity of the original crossed forms, which ensure the manifestation of heterosis in their crossbred offspring.[ ...]

The use of heterosis in animal husbandry.[ ...]

The use of heterosis. Obtaining E1 hybrids? from which seed-propagated cross-pollinating plants 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 is difficult on its own.[ ...]

Nikolyukin N. I. Heterosis and its use in fish farming.[ ...]

Rostovtsev N. F. Heterosis in animal husbandry. M.: Publishing House of the Academy of Sciences of the USSR.[ ...]

Thus, with heterosis, it becomes possible to increase the viability, fertility, early maturity and productivity of offspring obtained from crossing organisms that differ in their hereditary qualities or grown under different conditions. Heterosis in pond fish farming is expressed in the accelerated growth of hybrids, increased endurance to adverse 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 manifesting itself in the first hybrid generation, it fades in subsequent ones. Therefore, heterotic individuals should not be left to the tribe.[ ...]

Due to the effect of heterosis, industrial crossing makes it possible to increase the fish productivity of ponds to a greater extent than with purebred breeding. So, for example, crossing Moldovan females with scaly males of the local outbred herd of the Stavropolsky 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 output 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 with 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 carps, and crosses from such crossing have a strong heterosis in growth. The hybrids used for crossing with the Ukrainian carp give crossbreeds, which also show strong growth heterosis. A cross between female Ukrainian scaly carp and male Ropshinsky carp exceeds parental forms in growth by 25%, and their nutritional value is equivalent to the nutritional value of Ropshinsky carps, but exceeds that of Ukrainian scaly carps. Crossbreeds obtained from a female of the Ropshinsky-Ukrainian crossbreed with males of the Ukrainian scaly carp turned out to be more by weight than hybrids from crossing a female of the Ukrainian scaly breed with males of the Ropshinsky-Ukrainian carp. Thus, the producers from the first generation of crossing are unequal in terms of the strength of the transmission of hereditary characteristics to offspring and the dominance of the maternal organism, as in a number of experiments conducted by other researchers, is clearly expressed.[ ...]

Selection for obtaining heterosis is directly related to the theory and practice of breeding selection and selection and serves as one of the ways to increase the productivity of animals. Heterosis is genetically the opposite of ipbreeding depression. One of its features is the highest degree of expression only in the first generation of hybrids (or crosses). Then heterosis imperceptibly fades and disappears in the following generations when hybrids are crossed with each other, unless special measures are taken to preserve the effect of heterosis.[ ...]

And lie in F. V. Inbreeding and heterosis of farm animals.[ ...]

The effect of reproductive heterosis was the 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 in terms of milk yield are given by N. F. Rostovtsev from the experience of crossing East Frisian cows with bulls of the Red Gorbatov breed (Table 44). In dairy cattle, the effect of heterosis is observed more often in terms of the total amount of milk fat per lactation, especially when crossing cows of various breeds with Jersey bulls.[ ...]

Genetic explanation of heterosis. Although the effect of heterosis 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 strongly 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 breeding. As expected, the lines derived in this way either quickly die out, or turn out to be much less viable and less productive compared to the source material.[ ...]

A higher severity of heterosis is observed in four-line hybrids, when, first, males are obtained by crossing two paternal lines, and chickens 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, in order 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 specifics of the forms of manifestation of heterosis, the following types are distinguished: reproductive (increased fertility), somatic (development of organs and tissues), adaptive (increased viability).[ ...]

Powerful plants with a clear manifestation of heterosis 125-135 cm tall, perennial, with a large number (25-37 spike-bearing shoots. New shoots of renewal, as well as perennial wheat and perennial hybrid rye, develop! Until late autumn. According to the structure of the ear, G4 hybrids are intermediate between perennial wheat and perennial hybrid rye (Fig. 89).Flower films with stalks pointed.Spike films are narrow, long, with well-defined denticles along their keel.The culm under the ear in most plants has pubescence.The last two characters are characteristic of perennial hybrid rye.[ ...]

To explain the genetic nature of heterosis, a number of other hypotheses were 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, the superiority of the entire set of dominant genes over the set of recessive genes is observed. Close to this is the opinion of L. S. Zhebrovsky, who explains the manifestation of heterosis mainly by the additive action of positively influencing dominant genes present in a different set in parents and combined in descendants. In this case, the harmful effect of recessive genes is extinguished.[ ...]

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 the valuable properties of the original species.[ ...]

Different 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 causal relationships between hereditary factors and traits: some factors enhance (in ontogeny) the development of a trait, while others weaken it; the sign develops according to the resultant between opposite tendencies.[ ...]

The issue of obtaining heterosis in terms of milk production of cows has been studied least of all. The materials of long-term studies of the patterns of variability in the productivity of cows of various breeds give reason not to doubt that the heterogeneous effect on the size of the yield is a frequent fact and that, as a rule, its occurrence is the result of a particularly successful selection of parental pairs in interlinear crosses and other variants of intrabreed mating of animals with different chain inheritance. But the manifestation of heterosis in terms of milk yield has its own specifics. After all, the abundance of milk in cows is one of the most physiologically complex traits. In contrast to adaptive traits, which are fixed by natural and artificial selection, abundant milk production develops not as a quality beneficial for an animal, but as a quality necessary for a person in the process of long, complex and skillful work to create and improve dairy and dairy-beef cattle breeds. ..]

Kryzhanovsky OA, Maslova II Dependence of the effect of heterosis on the combination ability of lines. - In the book: Selection of fish. - M., 1989. - S. 86.[ ...]

Summing up the results of numerous works on inbreeding in various branches of animal husbandry 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 the traits associated with the viability of animals (fertility, embryonic and postembryonic mortality, resistance to environmental influences, etc.). When crossing representatives of inbred lines, especially different breeds, in the offspring, as a rule, increased viability (heterosis) is observed. The question of whether the breeding of strongly inbred lines for subsequent crossing, their representatives among themselves, is economically justified, Johansson and Lasch consider it has not yet been clarified. It can be justified if the viability and productivity of animals obtained as a result of cross-breeding of inbred lines will be so much higher in comparison with non-bred initial forms that it fully compensates for the losses and extra costs that are 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. Far-reaching homozygosation of breeds and lines of our domestic animals Johansson and Lasch consider it impossible.[ ...]

Achievements of modern biology, primarily genetics and breeding, allow the use of heterosis in the field of forestry - to create highly productive hybrids capable of producing an annual increase of 20-25 m3 per 1 ha or more. This is especially true for poplars. Their successful breeding requires not only the breeding of varieties, but also the subsequent extensive testing.[ ...]

The system of selection of sires based on the principle of rotation of lines assumed, in addition to avoiding close inbreeding, also obtaining heterosis during line crosses. But the prolonged and often stereotyped application of such a system not only leads to great genealogical diversity in each herd, but also to a decrease in the possibility of heterosis manifestation.[ ...]

With variable crossing, genetic differences between animals of subsequent generations become less noticeable and the effect of heterosis decreases. Involvement in crossing a large number of breeds does not give a significant effect. It may not appear at all if less valuable and poorly combined breeds are selected. Nevertheless, the method of variable crossing, in which each generation is obtained as a result of quite diverse matings, makes it possible to use all the advantages of crossing (increased viability and productivity of hybrids) and, with successful selection of breeds, to maintain the phenomenon of heterosis for several generations. A significant advantage of variable crossbreeding lies in the fact that only purebred sires are needed to carry it out to any generation; hybrid animals are used as queens. In addition, in pig breeding, for example, crossbred queens, especially of 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 obtain next generations are selected after their evaluation by offspring.[ ...]

In Norway, under the guidance of the well-known 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 the optimal proportions of the blood of each of the original breeds and offspring.[ ...]

Based on the results of these experiments, the authors conclude that when sires tested for offspring are used for crosses, the productivity of the first hybrid generation is significantly increased (the average effect of heterosis is about 20%). Further crossing maintains heterosis for several generations. At the same time, the first crossbred generation exhibits 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-matched inbred lines are pre-mated; then the resulting offspring of one breed with clear signs of heterosis are mated with the same offspring obtained as a result of crossing lines of another breed. Double crosses are good use animals. In this way (incrossbreeding) poultry farmers in the USA usually get custom birds.[ ...]

Often, in heterogeneous selection, due to the combination of the hereditary characteristics of the parents, new valuable qualities appear in the offspring, which each of the parents did not have separately. Successful genealogical combinations can also lead to heterosis in the development of certain traits. This is especially observed in the implementation of crosses of lines that differ on average from each other, that is, when the mating of line representatives is heterogeneous. O. A. Ivanova, analyzing the results of crosses of lines of the Russian trotting breed of horses, found that as the variability of the line, calculated by the sum of the squared deviations of the Indicators of each line from each other, increases, more effective crosses are obtained. So, when the Bob-Douglas line is combined with the Pass-Rose line, a litter is obtained that is distinguished by the highest agility. The degree of heterogeneity of these lines, expressed as the sum of the 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 playfulness was in the offspring obtained from the selection of the same Bob-Douglas line to the Gay-Bingeia line. At the same time, the degree of heterogeneity was 9.7, and agility - 2 min 37 4D s.[ ...]

This method is widely used in breeding farms to improve the breeding and productive qualities of existing breeds and to breed 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 hybrids obtained by 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 signs. 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 the ability to transmit its qualities to 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 interbreeding variable crossing. He argued that with such crossing, as it were, the features and positive aspects of absorption and industrial crossing are combined.[ ...]

In this context, biopolitics, along with anthropologists, of course, contribute to the question of what were the driving forces behind the origin of man. Many answers have been given to this question in the literature. There are views on the leading role of changing environmental conditions (apparently, repeatedly in the process of anthropogenesis), an increased radioactive background that caused spasmodic mutational changes with the emergence of, ultimately, Homo sapiens, the effect of heterosis (crossing of local populations that were previously isolated for a long time). ), as well as the role of other factors. Many researchers consider anthropogenesis to be a complex process that proceeded under the influence of many factors at once, including: 1) a complex and diverse habitat (savannah); 2) the complex nature of the search for and obtaining food with competition for it; 3) a complicated social organization and 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 the average lifespan of a woman was 26 years, and the first child appeared 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 the change in the social structure; changed biosocial (simply social since the emergence of the genus Homo) systems were not only correlates of these changes, but probably, at least in some cases, their causal factors - an idea consistent with the views of Corning, Janch, and also our compatriot Krasilov. [...]

The abuse of inbreeding within the line limits the possibility of getting something new and can lead to stagnation. As proof of this position, I often see cases where, in progressive breeds, animals obtained as a result of related mating are far from being the best, but only average or good. Crosses of lines in such cases, as well as the selection of unrelated queens to producers, expand the hereditary possibilities, are accompanied by the phenomena of heterosis and ensure further progress; breeding with the use of occasionally moderately related mating, preceding crosses, supplies material for them. At the same time, there is a certain alternation of now more homogeneous, now more heterogeneous selection, and each of them, being connected with the other, finds its place in breeding work. An example of such a cross line with preliminary inbreeding is the well-known trotter Magnanimous, born in 1950 in the Khrenovsky stud from Velbot and Castellansha (see his pedigree). Velbot and Castellansha belong to different lines and are obtained as a result of inbreeding: Velbot - on Warmik in the degree III-III, Castellansha - on Touchy in the degree III-III, Kasha in the degree III-III and Unexpected in the degree IV-IV. It should be noted that the Magnanimous is an animal, in a sense, a “hybrid”, he was obtained as a result of mating two inbred parents taken from different, though not genetic, but zootechnical (factory) lines.[ ...]

In their biological essence, interline crosses in purebred breeding do not fundamentally differ from interbreeding. They are based on the same regularities discovered by Ch. Darwin. This is a genetic difference in the sex elements in mated animals, enriching the heredity of the offspring, stimulating its development and increasing the viability of animals. Professor Yura-sov defines the lineage as a micronorod. From this point of view, a successful cross of lines can be considered as the result of the manifestation of heterosis. Just as heterosis in interbreeding is the result of the dissimilarity of the germ cells of the parents, so heterosis in crosses of lines can be explained by the same phenomenon.[ ...]

The experiments of American researchers Shell, East, Geis, Jones, and others found that during inbreeding, i.e., during artificial self-pollination of corn (a typical cross-pollinating plant with same-sex flowers), plants experience a sharp depression (their growth decreases, cobs become smaller and grains in them, the yield drops sharply). A similar depression is observed in the first (several) inbred generations; the dwarf low-yielding forms obtained as a result of further inbreeding remain more or less unchanged. When such depressive, but unequal forms are crossed with each other, all the signs of pronounced heterosis appear already in the first generation: powerful plants are obtained with a large number of enlarged cobs dotted with a mass of large full-weight grains. When these vegetatively powerful plants (the product of crossing) then begin to multiply by self-pollination, then in the next 3-5 generations, a sharp decrease in vegetative power and productivity is again noted: a return to dwarf low-yielding forms is observed. Thus, heterosis is a property of only the first generation resulting from crossing. It disappears with further breeding of the cross products "in itself".[ ...]

V. A. Altshuler, E. Ya. Borisenko and A. I. Polyakov, supplementing the hypothesis of obligate heterozygosity, gave it an evolutionary interpretation. Each new gene occurs in a heterozygous state and is subject to natural selection. Many of the newly emerging 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 the genes was revealed in the heterozygous state, and the harmful effect turned out to be in the recessive state. The emergence of genes with double action, that is, with obligate heterozygous, is a consequence of the evolutionary process. Heterosis is primarily useful to the animal organism itself, which comes from crossing. It follows that a high degree of heterozygosity is the cause of heterosis.[ ...]

In the experiments of Knox (1946-1954), the egg production of red Rhode Islands and white leghorns of 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 hybrids obtained as a result of crossing inbred roosters of the Rhode Islands with inbred hens of the White Leggorn breed increased to 233 eggs, and with a reciprocal combination of inbred hens and roosters of these breeds - up to 258 eggs. Also known are the results of testing inbred lines of chickens of four breeds (New Hampshire, red Rhode Island, striped Plymouth Rock and Austrolorp) and the results of their crossings conducted by Nordsky in Iowa (1954). It was shown that the effect of heterosis on the growth rate of young animals up to 8 weeks of age with interline crosses within the same breed was 4%, and when crossing different breeds - 7%; hatchability was the highest, and the mortality of chickens was also the lowest in interbreeding; The egg production of a crossbred bird (with interbreed crossings) exceeded by 12%, and that obtained by crossing lines in one breed - by 10%, the average egg production of a bird of inbred lines.

The increase in power, viability and productivity of hybrids of the first generation in comparison with parental forms is called heterosis.

The concept of heterosis as a manifestation of "hybrid strength" was introduced into science by the American geneticist W. Schell in 1914. For the first time, the phenomenon of hybrid strength was observed by Charles Darwin in corn. In his experiments, the productivity of this culture decreased and the height of the plants decreased as a result of self-pollination, and these signs intensified during cross-pollination. Ch. Darwin associated the increased power of plants obtained as a result of crossing with hereditary differences in parental gametes.

Heterosis in nature is a very ancient phenomenon. It is directly related to the emergence and improvement in the process of evolution of the method of cross-pollination. Natural selection over the centuries has created numerous restrictions on homozygosity and equally numerous adaptations for the implementation of heterozygosity.

Heterosis in hybrids is manifested in increased growth, more intensive metabolism and greater yields. The increased yield of heterotic hybrids is their main advantage. The increase in yield in hybrids of the first generation of all crops is on average 15-30%, while their early maturity often increases. For example, in tomatoes, heterogeneous hybrids begin to bear fruit 10-12 days earlier and exceed the yield of the original parental varieties by 45-50%. In Bulgaria, all areas of this crop are occupied by heterotic hybrids. Using heterosis, it is possible to significantly increase agricultural production.

With heterosis, it is not necessary that all the properties and characteristics of plants are strengthened. For some of them, it may manifest itself more strongly than for others, and for some it may be absent.

Heterosis is observed when crossing between varieties, as well as between genetically and ecologically distant species and forms. It manifests itself most strongly and can be controlled when crossing self-pollinated lines. Intsukht makes it possible to decompose a variety-population into its constituent biotypes (lines). The technique of induction is simple. For example, in corn, the panicle is covered with parchment insulator at the very beginning of flowering. On the same plant, the cob is also isolated before it has threads. The best material for cob insulation is cellophane. Dimensions of insulators: for a panicle 20X30 cm, for cobs - 10 × 16 cm. Parchment insulators are glued with carpentry glue, adding a small amount of chromium peak to it, and cellophane insulators with a saturated solution of zinc chloride.

When pollen ripens, the panicle is cut off and placed under the insulator along with the cob. Plants obtained from self-pollination are subjected to self-pollination the next year, repeating this procedure for several years. After 4-5 years of incubation, a very high degree of evenness in the progeny of incubation lines is practically achieved, and further self-pollination becomes redundant.

The selected lines are subsequently propagated not under insulators, but in special areas where cross-pollination of plants occurs within the same line without the danger of violating their uniformity. The resulting inbred lines cannot be used directly due to low yield and poor growth. But among these lines are very valuable for some economically useful traits. For example, lines appear in corn that are resistant to blister smut, a very dangerous disease of this crop that kills up to 10% of the crop. Some lines are distinguished by a high content of fat or protein in seeds, high early maturity, short stature, resistance to damage by corn borer, windbreaker, etc. Such inbred lines are used in crossings with each other, as well as with varieties.

After the lines reach uniformity in morphological and physiological characteristics, which usually happens after 4-5 years of self-pollination, they are evaluated for combination ability, that is, the ability to produce highly productive hybrids. Distinguish between general and specific combinational ability.

The total combination ability shows the average value of lines in hybrid combinations. It is determined by the results of crossing lines with a variety serving as a paternal parent, called in this case a tester.

Specific combining ability is evaluated by the results of crossing lines with any one line or a simple hybrid. In this case, cases are identified when some combinations turn out to be better or worse than one would expect on the basis of the average quality of the studied lines, established by assessing the overall combination ability.

To determine the specific combinational ability of self-pollinated lines, diallelic crosses are used, in which each line is crossed with all the others to obtain and evaluate all possible combinations.

One of the characteristic features of heterosis is its greatest manifestation in hybrids of the first generation, a sharp decrease in the second generation and a further attenuation of the hybrid power of plants in subsequent generations. This is due to a decrease in the number of heterozygous individuals. For example, if when crossing two self-pollinated lines AAbb and aaBB in the first generation there will be 100% heterozygous plants, then in the second generation their number will decrease by 2 times, and in the third - by 4 times, etc.

I. V. Michurin repeatedly pointed out the advantages of seedlings of the first generation and categorically objected to the use of hybrids of the second and third generation in the work, since only in seedlings of the first hybrid generation, which, due to the heterozygosity of parental varieties, had a wide variety of traits and properties, heterosis is fixed during further vegetative propagation .

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 are produced annually.

Among field crops, heterosis is now most widely used in maize. The usual varieties of this culture are almost completely replaced by heterotic hybrids, which are represented by the following main types. Varietal hybrids are obtained by crossing a variety with a self-pollinated line or by crossing a simple interlinear hybrid with a variety. An example of variety-linear hybrids of the first type is the Bukovinian HAZ. It was obtained by crossing the German variety Gloria Yanetsky T with the self-pollinated line VIR 44TV. This line, which is the paternal form of the hybrid, is one of the best self-pollinated lines. It is highly resistant to drought and blister smut, has a high combination ability, its plants are usually two-cob.

The hybrid Bukovina 3TV is characterized by very high cold resistance, relatively early ripening and high yielding, resistant to swedish fly, able to keep green leaves and stems when the grain is fully ripe.

The variety-linear hybrids of the second type include Dneprovsky 56TV. It was obtained by crossing a simple interline hybrid Iskra T with the Severodakotskaya TV variety: Iskra (VIR 26THVIR 27T) x Severodakotskaya TV. Variety-linear hybrids exceed ordinary varieties in terms of grain yield by an average of 4-5 q/ha, or 15-20%. New variety-linear hybrids are zoned: Bukovinsky PT, Collective 244 hybrid, Dneprovsky 260M.

Simple interline hybrids are obtained by crossing two self-pollinated lines. For example, from crossing self-pollinated lines VIR 28 and VIR 29, a simple interline hybrid Ideal was obtained, and from crossing VIR 44 and VIR 38, a simple interline hybrid Slava.

Simple interline hybrids give great heterosis, but due to the low yield of the self-pollinated lines that form them, they have not been widely used in production for a long time. In recent years, through periodic selection, it has been possible to increase the productivity of self-pollinated lines and, on their basis, to create several hybrids of this type, including such highly productive ones as Krasnodar 303TV, Odessa 50MV, Novinka, Naprada TV, Zakarpatsky 2TV, etc. Simple interline hybrids in production conditions 10-12 c/ha and more exceed the grain yield of the best double interlinear and variety-linear hybrids. So, widely cultivated in the steppe regions of Ukraine, in the North Caucasus and in the Moldavian SSR, the simple hybrid Krasnodar 303TV yields 80-90 centners of grain per hectare, and over 150 centners when irrigated. New simple interline hybrids Dneprovsky 70TV, Krasnodarsky 301TV, Moldavsky 385AMV, high-lysine hybrids Krasnodarsky 303L and Hercules L.

On the basis of simple interlinear hybrids, high-yielding double interlinear and varietal hybrids, as well as complex hybrid populations, are created. For example, as a result of crossing a simple interline hybrid Astra (line 346 X Hlinia Wud) with a simple hybrid Atlas (line 502 X line 21), a double interline hybrid Moldavsky 330 was obtained. Double interline hybrids give an increase in grain yield compared to conventional varieties 8-12 q /ha, or 25-40%. New double interline hybrids are zoned: Zherebkovsky 90 MB, Chuisky 60TB, Dneprovsky 505 MB, Moldavsky 330, Povolzhsky IV.

Complex hybrid populations, or synthetic varieties, are obtained by mixing the seeds of several self-pollinated lines or 2-4 double interline hybrids. Unlike other types of hybrids, they can be cultivated without a noticeable decrease in heterosis by simply reseeding for 3-4 years. Due to constantly ongoing cross-pollination, heterosis in such a population can be maintained at a sufficiently high level for several generations.

Three-line hybrids are obtained by crossing simple interline hybrids with self-pollinated lines. For example, when creating a three-line hybrid Dneprovsky 460 MB, a simple hybrid Dneprovsky 20 M (line VIRIUM x line T 1353M) was taken as the maternal form, and line A 619 MB was taken as the paternal form. Zoned three-line hybrids Dneprovsky 460 MB, Collective 101 TV, Kharkov 178 TV.

To obtain hybrid seeds, parental forms of hybrids are sown in hybridization areas.

The labor intensity and high costs of removing panicles from plants of maternal forms of hybrids to a large extent prevented the widespread use of the phenomenon of heterosis. The best solution to this problem is to find or create maternal forms of plants with male sterility, which would eliminate the need for artificial castration.

Attention was drawn to the fact that in many plant species with bisexual flowers occasionally there are single individuals with sterile male generative organs. Such facts were known even to Charles Darwin. He considered them as the tendency of a species to move from monoecious to dioecious, which he considered evolutionarily more perfect. Thus, the appearance of male-sterile individuals in monoecious plants is a natural phenomenon of the evolutionary process.

Cytoplasmic male sterility (CMS) was first observed by the German geneticist K. Korrens in 1904 in summer savory garden plants. In 1921, the English geneticist W. Betson discovered it in flax, and in 1924, the American geneticist D. Jones found it in onions. CMS in maize was first discovered by VASKhNIL Academician M. I. Khadzhinov in 1932 and, independently of him, simultaneously by the American geneticist M. Rhodes. Individuals with CMS transmit this property by inheritance only through mother plants.

This remarkable discovery has not been used in breeding for a long time. But since the 1950s, it has been appreciated and found wide practical application, first in the cultivation of corn, and then in many other crops.

There are two types of CMS in corn: Texas (T) and Moldavian (M). The Texas type of CMS, which produces almost completely sterile ears, was discovered by the American geneticist D. Rogers at the Texas Experimental Station in 1944, and the Moldavian type of CMS was discovered by G.S. Galeev at the Kuban VIR station in 1953 in a sample of local corn from Moldova. With this type of sterility, a small amount of viable pollen is produced in the anthers. The Texas and Moldavian types of CMS differ from each other in that each of them has its own lines that fix sterility or restore fertility.

The CMS-based method of obtaining hybrid corn seeds without panicle removal began to be used in the early 1950s. To create maize hybrids on a sterile basis, it is necessary to have: sterile analogues of self-pollinated lines or varieties; lines - fixatives of sterility; lines - restorers of fertility.

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What is outbreeding?

Outbreeding is a method of breeding animals, which is the crossing of unrelated individuals.

Outbreeding 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.

Since the end of the 19th century, outbreeding has given rise to numerous modern breeds of farm animals derived from a variety of local breeds.

Obtaining more productive offspring with the outbreeding method is due to the effect of heterosis.

The heterosis method itself has been known for a long time. It is widely used in both crop and livestock production. With the help of this effect, for example, mules were bred by crossing horses and donkeys.

Charles Darwin was the first to define heterosis. In the course of his research on plants, he discovered that a plant receives a more beneficial effect when cross-pollinated 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 for 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 is the effect of heterosis manifested?

Heterosis manifests itself to a greater extent in such traits as the rate of weight gain, egg production, that is, those that have a low heritability index. These traits develop under the influence of additive genes. The environment also plays an important role in the development of these traits.

In modern science, various technologies for obtaining the effect of heterosis have been tested: these are intrabreed mating by heterogeneous selection, and interline crosses of inbred lines, as well as interspecific and interbreed crossings.

For example, to obtain good quality and high yield of beef, crossbreeding of dairy and dairy-beef breeds of cows with beef bulls is used.

And in the poultry industry, both meat and egg, when crossing different breeds and lines, the offspring acquires heterosis in terms of viability, feed efficiency and, of course, productivity.

Moreover, heterosis can manifest itself in various forms. Thus, during outbreeding, offspring may not be distinguished from their parents in terms of live weight, however, they may significantly exceed them in terms of fertility and viability.

In order to identify heterosis, it is necessary:

Choose the right line for crossing
Determine which line will be paternal and which will be maternal

Only with a good choice of these two factors will the best combination be achieved. But of course it's trial and error.

In this case, the combinational abilities of the selected breeds and lines should be taken into account. This combination ability has two categories:

General combination ability- implies the ability of individuals of a particular species (breed or line) to generally receive the effect of heterosis when crossed with other species (breed or line).

Specific combination ability- implies the ability of individuals of a given breed (line) to give 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 animal productivity and viability, this phenomenon has found application in animal husbandry in 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 heterosis crops is corn. New hybrid varieties of corn have high resistance to drought, seed preservation, and increased plant immunity.

The effect of heterosis is successfully used in the hybridization of wheat, sugar beet, sorghum and other crops, as well as in fruit and berry and vegetable crop production.

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 breeding, camel breeding and even fish farming.

In addition, thanks to the research of scientists, the appearance of heterosis during interspecific crossing was also established. In addition to the mules mentioned above, other well-known breeds were bred in this way.

The famous Santa Gertrude cattle breed, bred in the USA at the beginning of the 20th century, is distinguished by simply amazing weight gain - the weight of adults reaches 700 kg. And this breed was bred by crossing the wild bull Zebu with the meat breed of Shorthorn cows.

The achievement of Kazakh livestock breeders was the breed of sheep Arkharomerinos, bred in the 50s of the last century. This hybrid, obtained by crossing Arkhara mountain sheep with fine-fleeced sheep, exceeded all breeders' expectations in many ways, 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 drawbacks. And, of course, the main one is the short duration of heterosis.

The fact is that hederosis is most noticeable when crossing individuals that are distant in terms of the degree of kinship. Under this condition, the effect is strongly fixed 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.

Outbreeding, despite the shortcomings of the heterosis effect, remains an effective method for increasing the productive performance of animals and plants.