Heterosis- increasing the viability of hybrids due to the inheritance of a certain set of alleles of various genes from their dissimilar parents. This phenomenon is the opposite of inbreeding depression, which often occurs as a result of inbreeding (inbreeding), leading to increased homozygosity. The increase in the viability of first-generation hybrids as a result of heterosis is associated with the transition of genes to a heterozygous state, while recessive semi-lethal alleles that reduce the viability of hybrids do not appear.
In plants (according to A. Gustafson), three forms of heterosis are distinguished:
-----T. n. reproductive heterosis, which results in increased hybrid fertility and productivity,
-----somatic heterosis, increasing the linear dimensions of the hybrid plant and its mass,
-----adaptive heterosis (also called adaptive), which increases the adaptability of hybrids to the action of unfavorable environmental factors.
Geneticists have come up with a way to practically use heterosis. His idea is that, for example, two suitable parental plant lines are bred in seed farms, hybrid seeds are obtained from them, and these seeds are sold to agricultural producers. (The implementation of this idea requires some additional tricks. For example, to obtain seeds of hybrid corn, a variety whose pollen is sterile is used as one of the parents, which avoids self-pollination. The gene for pollen sterility in corn was discovered in our country by geneticist and breeder M.I. Khadzhinov in 1932. Subsequently, with the help of crossings, the sterility gene was introduced into the desired varieties of corn). This method turned out to be very successful. For example, in the USA, since 1968, only hybrids have been grown on all areas occupied by corn. In many countries around the world, commercial seeds of onions, tomatoes, beets, rice, cucumbers, carrots and other crops are obtained using the same technique.
Heterosis is also used in animal husbandry. Broiler poultry farming is based on it. A breed of chickens with high egg production and another breed with rapid growth of chickens were bred. When hens of the first breed are crossed with roosters from the second, a hybrid generation is obtained, characterized by particularly rapid growth.
Hypothesis heterosis, formulated by G. Schell, E. East and H. Hayes, explains the phenomenon of heterosis by the presence of heterozygosity of various loci and the resulting overdominance, that is, when the effect of heterozygote Aa on the manifestation of the phenotype is stronger than the homozygous dominant genotype AA (that is, the effect of the action Aa is more than the action of AA).
Another explanation of heterosis, formulated by Kiebl and Pellew (1910), is based on the fact that when crossing organisms carrying different homozygous genes in the genotype, for example AAbb and aaBB, in the crossbred offspring, recessive alleles pass into the heterozygous form of the AaBb genotype, in which the harmful effects of recessive ones are eliminated genes. The influence of dominant genes on the manifestation of heterosis can be explained by the simple cumulative effect of a large number of dominant genes, that is, there is an additive effect.
45. Genetic structure of populations. Hardy-Weinberg law.
A population is a collection of individuals of the same species that occupy a certain area for a long time and freely interbreed with each other. To one degree or another, isolated from another population.
Each genetic population has a specific genetic structure and gene pool. Gene pool is the set of all genes that members of a population have. Genetic structure determined by the concentration of each gene (or its alleles) in the population, the nature of the genotypes and the frequency of their distribution,
A haploid set of chromosomes contains one complete set of genes, or one genome. Normally, two such sets of genes serve as the main prerequisite for the development of the diploid phase. If there are A individuals in the population, then in the normal diploid state of chromosomes the number of genomes in the population will be 2N.
The genetic structure of a population is usually expressed by the frequency of alleles of each locus and the frequency of homozygous and heterozygous genotypes. The ratio of allele and genotype frequencies in a population exhibits a certain pattern in each specific period of time and across generations of organisms.
Panmixia is free crossing.
An important property of populations is their ability to exhibit high genetic variability, the main source of which lies in the process of reproduction,
The source of increased hereditary variability is the mutation process, during which the appearance of new alleles contributes to the formation of new phenotypes (and genotypes) in the population that were previously absent from it.
The interaction of genes at different loci with each other also affects the genetic variability of the population. It is called co-adaptation of genes. The effect of gene coadaptation in different generations of individuals in a population may be different due to changing conditions in different generations.
Under the influence of selection, the individuals that make up a population develop such an important property as adaptability to environmental conditions. The level of fitness serves as a measure of the progress of a population and is expressed by the intensity of reproduction of individuals and the increase in population size.
The genetic structure of each panmictic population is preserved over a number of generations until some factor takes it out of equilibrium. The preservation of the original genetic structure, that is, the frequency of alleles and genotypes over a number of generations, is called genetic balance and typical of panmictic populations. A population may have equilibrium at some loci and disequilibrium at others.
When a population transitions to a non-equilibrium state, the levels of allele and genotype frequencies change, and a new relationship develops between homozygous and heterozygous genotypes.
The structure of the gene pool in a panmictic stationary population is described by the basic law of population genetics - Hardy-Weinberg law, which states that in an ideal population there is a constant ratio of the relative frequencies of alleles and genotypes, which is described by the equation:
(p A + q a) 2 =R 2 AA+ 2∙р∙q Aa + q 2 aa = 1
If relative allele frequencies are known p And q and total population size N generally, then we can calculate the expected, or calculated absolute frequency (that is, the number of individuals) of each genotype. To do this, each term of the equation must be multiplied by N general:
p 2 AA·N generally + 2·p·q Aa ·N generally +q 2 aa N generally = N generally
In this equation:
p 2 AA·N generally – expected absolute frequency (number) of dominant homozygotes AA
2·p·q Aa ·N total – expected absolute frequency (number) of heterozygotes Ahh
q 2 aa N generally – expected absolute frequency (number) of recessive homozygotes ahh
Heterosis (from Greek heteroiosis - change, transformation)
“hybrid vigor”, acceleration of growth and increase in size, increase in vitality and fertility of first generation hybrids in various crossings of both animals and plants. In the second and subsequent generations, G. usually fades out. A distinction is made between true gigantism, the ability of hybrids to leave a large number of fertile offspring, and gigantism, the enlargement of the entire hybrid organism or its individual parts. G. is found in a variety of multicellular animals and plants (including self-pollinators). Phenomena similar to G. are observed during the sexual process in some unicellular organisms. In agriculture Heterosis of animals and cultivated plants often leads to a significant increase in productivity and yield (see below - Heterosis in agriculture). G. and its reverse inbreeding depression (see Inbreeding) were already known to the ancient Greeks, in particular Aristotle. The first scientific studies of hydrogen in plants were carried out by the German botanist J. Kölreuther (1760). Charles Darwin generalized his observations about the benefits of crossings (1876), thereby having a great influence on the work of I.V. Michurin and many other breeders. The term "G." proposed by the American geneticist G. Schell (1914); He was the first to produce “double” interline corn hybrids. The basics of the method of industrial cultivation of these hybrids were developed by D. Jones (1917). The use of hybridization (See Hybridization) in agriculture is expanding from year to year, which also stimulates theoretical research into genetics. Individuals with strongly expressed hybridization have advantages under natural selection (See Natural selection), and therefore the manifestations of genetics intensify, which contributes to increase in genetic variability (See Variation). Stable genetic systems often arise that ensure the preferential survival of heterozygotes for many genes. The study of G., in addition to the usual study of morphological characteristics, requires the use of physiological and biochemical methods that make it possible to detect subtle differences between hybrids and original forms. The study of genetics has also begun at the molecular level: in particular, the structure of specific protein molecules—enzymes, antigens, etc.—is being studied in many hybrids. According to Darwin, genetics is caused by the unification of heterogeneous hereditary inclinations in a fertilized egg. On this basis, two main hypotheses about the mechanism of G. arose. The hypothesis of heterozygosity (“overdominance”, “single-gene” G.) was put forward by American researchers E. East and G. Schell (1908). Two states (two alleles) of the same gene, when combined in a heterozygote (See Heterozygote), complement each other in their effect on the body. Each gene controls the synthesis of a specific polypeptide. In a heterozygote, several different protein chains are synthesized instead of one, and heteropolymers are often formed - “hybrid” molecules (see Complementarity); this could give her an advantage. The hypothesis of dominance (summation of dominant genes) was formulated by American biologists A. V. Bruce (1910), D. Jones (1917) and others. Mutations (changes) of genes in the general mass are harmful. Protection against them is an increase in the dominance (See Dominance) of genes that are “normal” for the population (the evolution of dominance). The combination of favorable dominant genes of two parents in a hybrid leads to genetics. Both hypotheses of genetics can be united by the concept of genetic balance (American scientist J. Lerner, English K. Mather, Russian geneticist N.V. Turbin). G., apparently, is based on the interaction of both allelic and non-allelic genes; however, in all cases, G. is associated with increased heterozygosity of the hybrid and its biochemical enrichment, which causes increased metabolism. Of particular practical and theoretical interest is the problem of fixing G. It can be solved by doubling chromosome sets (see Polyploidy), creating stable heterozygous structures and using all forms of Apomixis a, as well as vegetative propagation of hybrids. G.'s effect can also be strengthened by doubling individual genes or small sections of chromosomes. The role of such duplications in evolution is very great; Therefore, G. should be considered as an important stage on the path of evolutionary progress. V. S. Kirpichikov. Heterosis in agriculture. The use of hydrocarbons in plant growing is an important technique for increasing plant productivity. The yield of heterotic hybrids is 10-30% higher than that of conventional varieties. For the use of hydrocarbons in production, cost-effective methods have been developed for producing hybrid seeds (See Hybrid seeds) of corn, tomatoes, eggplants, peppers, onions, cucumbers, watermelons, pumpkins, sugar beets, sorghum, rye, alfalfa, and other agricultural products. . plants. A special position is occupied by a group of vegetatively propagated plants in which it is possible for G. to be established in the offspring, for example, varieties of potatoes and fruit and berry crops bred from hybrid seeds. To use genetics for practical purposes, intervarietal crossings of homozygous varieties of self-pollinating plants, intervarietal (interpopulation) crossings of self-pollinating lines of cross-pollinating plants (paired, trilineal, double-quadrilineal, multiple) and varietal-lineal crossings are used. The advantage of certain types of crossing for each agricultural product. culture is established on the basis of economic assessment. Elimination of difficulties in obtaining hybrid seeds can be facilitated by the use of cytoplasmic male sterility (CMS), the property of incompatibility in some cross-pollinating plants and other hereditary features in the structure of the flower and inflorescence, eliminating the high costs of castration. When choosing parental forms for producing heterotic hybrids, their combinative ability is assessed. Initially, selection in this direction was reduced to the selection of the best genotypes in terms of combinational value from populations of open-pollinated varieties on the basis of inbreeding in the form of forced self-pollination. Methods have been developed for assessing and increasing the combinative ability of lines and other groups of plants used for crossings. The greatest effect in using G. was achieved on corn. The creation and introduction into production of corn hybrids has made it possible to increase the gross grain yield by 20-30% on the vast areas occupied by this crop in different countries of the world. Corn hybrids have been created that combine high yields with good seed quality, drought resistance and immunity to various diseases. Heterotic hybrids of sorghum (Hybrid Early 1, Hybrid Voskhod), heterotic intervarietal hybrids of sugar beet have been zoned, of which the Yaltushkovsky hybrid is the most widespread. To obtain heterotic forms, sugar beet lines with sterile pollen are increasingly used. G. phenomena have also been established in many vegetable and oilseed crops. The first results in the study of G. in first-generation wheat hybrids were obtained, sterile analogues and fertility restorers were created, and sources of CMS in wheat were identified. In animal husbandry, genetic phenomena are observed during hybridization, interbreeding and intrabreeding (interlinear) crossing (See Crossing) and provide a noticeable increase in agricultural productivity. animals. The most widespread use of hybrids is in industrial crossing (see Industrial crossing). In poultry farming, when crossing egg-laying breeds of chickens, for example, Leghorns with Australorps, Rhode Islands, etc., the egg production of first-generation crossbreeds increases by 20-25 eggs per year; crossing meat breeds of chickens with meat and egg breeds leads to an increase in meat qualities (see Broiler); G. based on a complex of characteristics is obtained by crossing closely related lines of chickens of the same breed or by interbreeding. In pig breeding, sheep breeding, and cattle breeding, industrial crossing is used to obtain meat productivity, which is expressed in increasing the early maturity and live weight of animals, increasing the slaughter yield, and improving the quality of the carcass. Pigs of meat-fat (combined) breeds are crossed with boars of meat breeds. Small, unproductive sheep of local breeds are crossed with rams of meat-wool breeds, and fine-wool ewes are crossed with rams of early-ripening meat or semi-fine-fleece breeds. To increase meat productivity, cows of dairy, milk-meat and local meat breeds are crossed with bulls of specialized meat breeds. Lit.: Darwin Ch., The action of cross-pollination and self-pollination in the plant world, trans. from English, M.-L., 1939; Kirpichnikov V.S., Genetic foundations of heterosis, in the collection: Questions of evolution, biogeography, genetics and selection, M., 1960; Hybrid corn. Collection of translations, M., 1964; Joint scientific session on the problems of heterosis. Abstracts of reports, in. 1-6, M., 1966; Use of heterosis in animal husbandry. [Conference materials], Barnaul, 1966; Heterosis in animal husbandry. Bibliographic list, M., 1966; Guzhov Yu. L., Heterosis and harvest M., 1969; Brewbaker J.L., Agricultural Genetics, trans. from English, M., 1966; Turbin N.V., Khotyleva L.V., Use of heterosis in crop production. (Review), M., 1966; Kirpichnikov V.S., General theory of heterosis, l. Genetic mechanisms, “Genetics”, 1967 No. 10; Fincham J. R. S., Genetic complementation, N. Y. - Amst., 1966.
Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .
Synonyms:See what “Heterosis” is in other dictionaries:
Acceleration of growth, increase in size, increase in vitality and fertility of first generation hybrids compared to the parental forms of plants or animals. Typically, heterosis subsides in the second and subsequent generations. Heterosis is widespread... ... Financial Dictionary
Heterosis (translated from Greek as change, transformation) is an increase in the viability of hybrids due to the inheritance of a certain set of alleles of various genes from their dissimilar parents. This phenomenon is the opposite of inbred... Wikipedia
- (from the Greek heteroiosis change, transformation), the property of first-generation hybrids to surpass the best of the parent forms in vitality, fertility and other characteristics. In the second and subsequent generations, heterosis usually fades out.... ... Modern encyclopedia
- (from the Greek heteroiosis change transformation), the property of first-generation hybrids to surpass the best of the parent forms in vitality, fertility and other characteristics. In the second and subsequent generations, heterosis usually fades. Heterosis... Big Encyclopedic Dictionary
- (from the Greek heteroiosis change, transformation), “hybrid power”, the superiority of hybrids in a number of characteristics and properties over the parent forms. The term "G." proposed by J. Schell in 1914. As a rule, G. is characteristic of first-generation hybrids ... Biological encyclopedic dictionary
Excellence Dictionary of Russian synonyms. heterosis noun, number of synonyms: 1 excellence (14) ASIS Dictionary of Synonyms. V.N. Tr... Synonym dictionary
- (from the Greek heteroiosis change, transformation), hybrid power, increased vitality and fertility of first-generation hybrids compared to parental forms. It was first described by Charles Darwin (1859). Theoretical foundations of heterosis... ... Ecological dictionary
HETEROSIS, increased vitality, demonstrated in some cases by hybrid offspring (see HYBRID), not characteristic of the parents... Scientific and technical encyclopedic dictionary
Heterosis. See hybrid vigor. (
Heterosis
The concept of heterosis.
Inbreeding is accompanied by inbreeding depression, increased homozygosity of inbred offspring and increased genetic similarity of the descendant to the ancestor. Heterosis has opposite biological and genetic properties.
Under heterosis understand the superiority of the first generation offspring over the parental forms in viability, endurance, productivity, which arises when crossing different races, animal breeds, and zonal types.
The phenomenon of heterosis, or “hybrid vigor,” was noticed in the practice of animal husbandry in ancient times, in particular when producing mules by crossing a donkey with a mare. Charles Darwin first gave a scientific explanation of the “hybrid vigor” that arises in the offspring when unrelated organisms are crossed. He explained this effect by the biological dissimilarity of male and female gametes, which is caused by the influence of differences in the environment in which the parents live.
Genetic theories of heterosis
The term " heterosis"was introduced by G. Schell (1914), and explained the presence of “hybrid vigor” by the state of heterozygosity in the genotype of an organism, formed as a result of crossing. The heterosis hypothesis, formulated by G. Schell, E. East and H. Hayes, explains the phenomenon of heterozygosity by the presence of heterozygosity of various loci and the resulting overdominance, that is, when the action of a heterozygote Ahh the manifestation of the phenotype is stronger than that of the homozygous dominant genotype AA(that is, the effect of the action Ahh more action AA) The significance of heterozygosity was confirmed by the works of N. P. Dubinin, M. Lerner and other scientists,
Another explanation of heterosis, formulated by Keibl and Pellew (1910), is based on the fact that when crossing organisms carrying different homozygous genes in the genotype, for example AAbb) And aaBB, y crossbred offspring, recessive alleles transform into a heterozygous form of the genotype AaB, in which the harmful effects of recessive genes are eliminated. The influence of dominant genes on the manifestation of heterosis can be explained by the simple total effect of a large number of dominant genes, that is, there is an additive effect.
K. Davenport (1908) and D. Jones (1917) proposed to explain heterosis based on the hypothesis of the interaction of non-allelic dominant genes of both parents, which gives a total effect that causes heterosis.
An ecological type of heterosis has been identified (Merkuryeva and 1980), which is caused by the process of acclimatization and manifests itself in animals of the first ecological generation. This type of heterosis manifested itself in increased milk production of offspring born in the Ryazan region from Ayrshire cows imported from Finland. In subsequent generations, milk yield decreased to a level corresponding to the genetic potential of the introduced group of cows.
Modern ideas about the causes of heterosis are based on the fact that heterosis is the result of the interaction of many genes. Their multiple action leads to a heterotic effect. This explanation is called balance heterosis (Dobzhansky, 1952). Subsequently, Lerner (1954), N.V. Turbin (1961-1968) continued to develop this position. According to their statements, hegerosis is caused by the action of many genes, mutually balanced in the genome in the process of evolution, which determines the optimal development and adaptability of the organism to environmental conditions.
If, during crossing, the optimal genomes of both parents are combined, then the descendants of the first generation have the most favorable situation in the combination of genomes, which leads to the manifestation of heterosis. Consequently, heterozygosity accompanying crossing undergoes pressure from various factors and thereby creates a balanced interaction of genes in the genome ,
In the practice of animal husbandry, so-called negative heterosis is sometimes observed, when the offspring have a level of a trait below the average of the parents, but is slightly higher than the level of the trait of the parent in whom it is less developed. The higher the differences in the trait level of the parental forms, the more the average trait level of the descendants approaches the trait level of the worst parent. This feature of inheritance was described by Ya L, Glembotsky in relation to the cutting of wool in crosses obtained from crossing Angora goats with coarse-haired goats. The wool clipping of the first generation crossbreeds was slightly greater than that of coarse-haired goats, but significantly less than that of Angora goats, in which it was 4-5 times greater compared to coarse-haired and crossbred goats.
Research to elucidate the biological basis of heterosis has been carried out at the Institute of Experimental Biology of the Academy of Sciences of the Kazakh SSR since 1962 under the leadership of Academician F. M. Mukhametgaliev. The research results are summarized in the monograph by A. S. Sareenova (1982), which can serve as additional material for understanding heterosis and the effect of crossing. In the process of work, the amount of DNA, RNA, proteins and the activity of a number of enzymes in the tissues and subcellular structures of cells (nuclei, chromosomes) of purebred and crossbred sheep was determined. Features of metabolic processes and heterosis in animals differing in origin were identified. It turned out that the heterotic effect is not associated with a change in the amount of hereditary substance in a single cell, nucleus or chromosomes. Crossing does not cause the activation of previously inactive genes obtained through the chromosomes of the parents in crossbreeds, and does not lead to a radical restructuring of metabolic processes. Instead, there is only stimulation of the level of intensity of metabolic processes. In the process of ontogenesis, this tension decreases and the effect of heterosis in crossbreeds decreases.
The biochemical effect of heterosis in crossbreds manifested itself in the stimulation of the activity of tissue enzymes (DNAase, RNase, etc.), which affect the synthesis of nucleic acids. The activity of enzymes in crossbreeds occurs in a wider pH range of the environment, which increases the ecological plasticity of crossbred organisms and adaptability to environmental conditions. Consequently, crossing affects the mechanism of regulation of enzyme activity.
RNA synthesis in the cell nucleus and translation of RNA-guided synthesis of protein molecules in the cytoplasm occur at a higher level in crossbreeds. This is facilitated by the enrichment of cell nuclei with non-histone chromatin proteins, which are a specific stimulator of genome activity. Consequently, the crossing stimulated the synthesis of ribosomal RNA, that is, it enhanced the transcription process. It is hypothesized that with the help of biologically active substances (hormones, metabolites), which can influence the activity of the genetic apparatus, it is possible to prolong the effect of heterosis over a longer period of ontogenesis.
There are other biochemical explanations for heterosis. It is believed that the main reason for hybrid power is the formation on chromosomes of sensitive copies of structural genes, which form an excess of information in cells and determine the high compatibility of metabolic processes (Severin, 1967).
Explanations for the heterosis effect can be found in the assumption that crossbreeds have polymorphic types of proteins (isoenzymes), which differ in some properties.
The parental forms do not have polymorphism of enzymes, and when they are crossed, polymorphism and the number of polymorphic loci y are formed in crossbreeds. therefore there are more of them than their parents. This, according to some scientists (Fincham, 1968; Kirpichnikov, 1974), explains the effect of overdominance. F. M. Mukhametgaliev (1975) believes that the mutual stimulation of genomes during fertilization is equivalent to the additive effect of united genetic systems and is the basis for the appearance of heterosis, but is not the cause of the emergence of new qualities in the genetic material, therefore heterosis manifests itself in quantitative changes in characteristics and has a polygenic type inheritance.
A new approach to explaining the heterosis effect is proposed by V. G. Shakhbazov (1968). He believes that heterosis has a biophysical basis, since during fertilization an exchange of electrical charges of homologous chromosomes occurs, which increases the activity of chromosomes in hybrid zygotes. This leads to the accumulation of acidic proteins and RNA, increases the nucleolus-nuclear ratio and increases the rate of mitotic division.
The above explanations of the causes of the heterosis effect indicate a lack of unity in the scientific explanation of the phenomenon of heterosis, and therefore the problem remains for further study and consideration. Despite this, in animal husbandry practice, animal selection techniques are used to consolidate and enhance the effect of heterosis. There are several techniques for calculating the magnitude of the heterosis effect. The so-called true type of heterosis is distinguished, which is determined by the magnitude of the superiority of the trait in crossbred animals over both parental forms. Another type of heterosis is hypothetical, when the characteristics of the crossbred offspring exceed the arithmetic average level of the trait of both parents.
If there is no data on one of the breeds from which the crossbreeds are obtained, then their performance is compared with the parent breed, and the improved performance of the crossbreed is called not heterosis, but the effect of crossing.
Summarizing the modern understanding of the phenomena of inbreeding depression and heterosis, we can draw conclusions about the need to use both phenomena in practical breeding work.
Practical application of heterosis
Modern livestock farming is characterized by the use of crossbreeding, accompanied by a heterotic effect, especially for egg and broiler poultry farming . This system includes two main stages; breeding inbred lines of birds using different types of inbreeding and crossing (crossing) lines to obtain a so-called hybrid bird that exhibits heterosis. For example, in the Netherlands, the Eurybrid company works with two crosses of egg-laying chickens: “Hisex White” (white shell, based on Leghorns) and “Hisex Brown” (with the participation of Rhode Island and New Hampshire with a brown shell). These two crosses occupy a leading position in the world egg production.
Work on creating hybrid egg and meat poultry is also being carried out in our country. To carry out selection for heterosis, inbred lines are bred by mating according to the “brother x sister” type for 3-4 generations or more, combining this with strict culling of undesirable individuals. Of the large number of established lines, about 10-15% of lines remain at the end, with an inbreeding coefficient on average of 37.5% (mating of full sibs for three generations). Next, the remaining lines are crossed with each other to check their compatibility, then the most successful combinations are left for production crossing and 2-, 3-, 4-line hybrids are obtained,
The use of the heterosis effect is also used in working with other types of animals, especially in beef cattle breeding, sheep farming, camel farming, and fish farming. Methods for obtaining the effect of heterosis are varied. Heterosis manifests itself during interspecific crossing of animals: obtaining mules from crossing a donkey with a mare, breeding new heterotic breeds by obtaining hybrids from crossing cattle with zebu (Santa Gertrude, Beefmaster, Charbray, Bridford - in the USA; Sao Paulo - in Brazil; Haup Holstein - in Jamaica). In our country, distant hybridization was carried out between fine-wool sheep and argali and a new breed was developed - arharomerinos. In Kyrgyzstan and Altai, hybrids of yak and Simmental cattle were obtained.
Distant hybridization is accompanied by the manifestation of heterosis for a number of economically valuable traits.
The problem of obtaining and enhancing the effect of heterosis has not been fully resolved. The main obstacle that cannot be overcome is the loss of the heterotic effect in the second generation, that is, the heterosis obtained in the first generation is not consolidated, but is lost in subsequent generations when breeding crosses “in themselves.” Some methods allow heterosis to be maintained over several generations. One of the most accessible and effective methods is variable crossing, used in commercial livestock farming. At the same time, from the first-generation crossbreeds obtained from crossing queens of breed A with sires of breed B, the best part of the queens is isolated and crossed with the sire of breed C, and second-generation crosses are obtained, with the manifestation of heterosis when three breeds are combined (A, B, C). Next, the second generation crossbreeds can be crossed with the sire of breed D and obtain more complex crossbreeds, which represent the heredity of the original maternal breed A and the heredity of the paternal breeds B, C and B. No other methods have been developed in animal husbandry to preserve the effect of heterosis.
In the practice of modern animal husbandry, it has been proven that the effect of heterosis is diverse and is expressed in the improvement of valuable economic traits. The main indicators of heterosis are an increase in embryonic and postembryonic viability; reduction of feed costs per unit of production; increasing early maturity, fertility, productivity; manifestation of greater opportunities for adaptation to changing conditions and new elements of technology. The wide range of heterotic effect, manifested in a variety of reacting characteristics, is a reflection of physiological and biochemical processes caused by the peculiarities of the genetic apparatus of heterotic animals.
The use of Heterosis in crop production is an important technique for increasing plant productivity. The yield of heterotic hybrids is 10-30% higher than that of conventional varieties. For the use of hydrocarbons in production, cost-effective methods have been developed for obtaining hybrid seed n corn, tomatoes, eggplants, peppers, onions, cucumbers, watermelons, pumpkins, sugar beets, sorghum, rye, alfalfa and other agricultural products. plants. A special position is occupied by a group of vegetatively propagated plants in which it is possible for G. to be established in the offspring, for example, varieties of potatoes and fruit and berry crops bred from hybrid seeds. To use genetics for practical purposes, intervarietal crossings of homozygous varieties of self-pollinating plants, intervarietal (interpopulation) crossings of self-pollinating lines of cross-pollinating plants (paired, trilineal, double-quadrilineal, multiple) and varietal-lineal crossings are used. The advantage of certain types of crossing for each agricultural product. culture is established on the basis of economic assessment. Elimination of difficulties in obtaining hybrid seeds can be facilitated by the use of cytoplasmic male sterility (CMS), the property of incompatibility in some cross-pollinating plants and other hereditary features in the structure of the flower and inflorescence, eliminating the high costs of castration. When choosing parental forms for producing heterotic hybrids, their combinative ability is assessed. Initially, selection in this direction was reduced to selecting the best genotypes in terms of combinational value from populations of open-pollinated varieties based on inbreeding in the form of forced self-pollination. Methods have been developed for assessing and increasing the combinative ability of lines and other groups of plants used for crossings.
The greatest effect in using G. was achieved on corn. The creation and introduction into production of corn hybrids has made it possible to increase the gross grain yield by 20-30% on the vast areas occupied by this crop in different countries of the world. Corn hybrids have been created that combine high yields with good seed quality, drought resistance and immunity to various diseases. Heterotic hybrids of sorghum (Hybrid Early 1, Hybrid Voskhod), heterotic intervarietal hybrids of sugar beet have been zoned, of which the Yaltushkovsky hybrid is the most widespread. To obtain heterotic forms, sugar beet lines with sterile pollen are increasingly used. G. phenomena have also been established in many vegetable and oilseed crops. The first results in the study of G. in first-generation wheat hybrids were obtained, sterile analogues and fertility restorers were created, and sources of CMS in wheat were identified.
Every summer resident or gardener knows what heterosis is. Another thing is that often we simply do not correlate this biological concept and the effect it causes with this complex word. And although in the fundamental scientific community there are still debates about the mechanisms and effect of heterosis, the average person has long been using it in everyday life. In the article we will try to clearly and clearly explain what heterosis is, what effect it causes and how it is used in human economic activity.
Academic knowledge
Heterosis is a biological phenomenon of “hybrid strength”, namely an increase in vitality, rapid growth and larger sizes of hybrids in the first generation compared to parental organisms. Filial 1 or F1, as the descendants of the first generation are designated, if the parents were not of the same species, may or may not have the effect of heterosis. Changes do not appear in subsequent generations. The effect of heterosis is not always beneficial for a particular organism. As is known, hybrids are often low in fertility (the ability to reproduce fertile offspring) up to and including infertility.
History of discovery
What heterosis is, namely the strengthening of hybrids, was known even before Mendel’s laws of inheritance. The term was introduced in 1914 by the American biologist W. Schell. But back in 1765, the Russian naturalist and biologist I. Kelreuter was the first to describe the hybrid vigor of the descendants of shag and panicle tobacco, which he obtained through artificial cross-pollination of plants. And C. Darwin even wrote a separate work on the power of hybrid forms - “The Effect of Cross-Pollination and Self-Pollination in the Plant World” (1876). Hybrid corn seeds were produced in large quantities by D. Bill at the Michigan Agricultural College as early as 1878.
Mechanism: complementation or overdominance
What is heterosis from the point of view of its mechanisms is still a question today. Among competent scientists, there are two theories about the mechanism of this phenomenon:
- The theory of complementarity explains the phenomenon of strengthening of hybrids by the “turning on” of genes that the parental forms had in the homozygous (aa or AA) state. In the first generation of hybrids, the heterozygous gene allele (Aa) causes heterosis.
- The theory of overdominance. According to this version, heterosis is caused by increased dominance of dominant alleles in hybrids and shutdown of repressors (oppressors) of genes. A key role in this is occupied by the theory of gene hierarchy, according to which the genome contains key regulators - genes that play an important role in the inclusion of certain genes.
Recent studies of molecular biology, however, prove the presence in heterotic hybrids of mechanisms of both complementation and overdominance, and the presence of key “switches” of genes, which order the music in the orchestra of genome implementation.
Types of heterosis
There are the following types of this phenomenon:
- A reproductive form in which there is an increase in fertility and better development of reproductive organs, seeds or fruits.
- Somatic heterosis is caused by the coincidence of alleles and greater development of vegetative and somatic organs.
- The adaptive form helps to increase the viability and stability of hybrid organisms.
Heterosis in nature and human economic activity
In nature, interspecific inbreeding arose along with sexual reproduction. And in evolutionary teaching there is even one of the methods of speciation - hybridization. Man has been using the effect of heterosis since time immemorial. For example, horse-donkey (or dromedary camel) hybrids are known for their strength and increased endurance. The hybrid form “bister”, obtained from crossing beluga and sterlet, is actively used in fish farming, as it grows faster than the parent forms. In crop production, we have long switched to hybrid crop production. The productivity of hybrids of corn, onions, tomatoes, cucumbers and bell peppers is 25% higher than pure lines (indicated by the “F1” icon on the seeds).
Heterosis in humans: jers and mongs
The heterosis effect occurs in interracial marriages. This topic is quite sensitive, especially in the context of pseudo-scientific speculation about the degeneration of the nation and other populist statements. But geneticists and molecular biologists unequivocally state that heterosis is caused by the same mechanisms in interethnic marriages, and the general biological parameters of the offspring may be higher than those of the parents. When the effect of heterosis manifests itself (which is more likely if the parents have pure national characteristics), people are called the word “jer” from GeteRozis, GR. If the effect of heterosis does not manifest itself, the general parameters of the children will be worse than those of the parents. Such people are called "mong" from MONGrel.
And finally
For humanity, the last 100-150 years have become the years of development of transport communications and population migration, the destruction of class, religious and ethnic barriers. This led to the mixing of nations and nationalities, interracial and interethnic marriages. And maybe it is the effect of heterosis in biology that is an increase in population growth in most countries by 5-15 cm and the onset of puberty by 13-14 years instead of 16-18 just 100 years ago?
HETEROSIS (from the Greek?τερο?οσις - change, transformation) (hybrid power), the superiority of hybrids in a number of characteristics and properties over the parent forms; manifests itself in increased viability, fertility and productivity of hybrids. The term “heterosis” was proposed by the American geneticist G. Schell in 1914, although the phenomenon of heterosis itself has been known since ancient times, and its first scientific description was made by the German botanist I. Kölreuther (1760). The peculiarity of heterosis is that it manifests itself in all first-generation hybrids obtained from crossing unrelated forms: breeds, varieties, lines and even species. In a series of subsequent generations (crossing hybrids with each other), the proportion of descendants with heterosis progressively decreases and then disappears. The superiority of hybrids over the best of the parental forms is designated as true heterosis. In the case when hybrids exceed only the average value of both parents, they speak of potential heterosis. The greatest effect of heterosis can manifest itself in double interline hybrids - descendants from crossing two different simple interline hybrids. Heterosis of plants and animals is used as the most important technique for increasing yields and productivity. It is important to decipher the mechanisms of heterosis, as well as develop methods for its consolidation. None of the proposed hypotheses (heterozygosity, favorable dominant genes, genetic balance, formation of compensatory gene complexes, etc.) fully explains the mechanisms of heterosis. It is believed that there are many reasons for its occurrence. As approaches to solving the problem of fixing heterosis, polyploidy, the creation of stable heterozygous structures and the use of all forms of apomixis, as well as vegetative propagation of hybrids, duplication of individual genes or small sections of chromosomes, transplantation of nuclei of somatic cells in animals into denucleated (enucleated) eggs (cellular engineering).
V. S. Mikheev.
Heterosis in animal husbandry is observed: during interspecific hybridization (for example, a mule obtained by crossing a donkey with a mare surpasses its parents in strength, endurance and performance); with crossbreeding (for example, crossbreeds obtained from industrial crossing of Large White and Estonian pigs have an average daily increase in live weight that is 6-10% higher than their purebred parents); with interline crossings (for example, hybrid laying hens produce 30-50 more eggs per year than hens of the original lines); with heterogeneous selection of parental pairs of the same breed. The degree of manifestation of heterosis depends on the compatibility (combinative ability) of the parents (lines, breeds). Hybrids or crosses may be superior in one or more characteristics to the best of the parental forms (for example, the live weight of a Cornish rooster at 6 weeks is 2.0 kg, a Plymouth rock hen is 1.7 kg, and the average live weight of the offspring obtained from them is 2 ,1 kg); may have an indicator of any trait that exceeds the arithmetic average of this trait in the parents (for example, the egg production of hens of line A is 320 eggs in 72 weeks of life, line B is 280 eggs, and hybrid laying hens AB is 309 eggs). Sometimes first-generation hybrids are superior to the parental forms in a trait derived from the other two, each of which has an intermediate type of inheritance. For example, the milk yield of a black-and-white cow during lactation is 6000 kg of milk with 3.0% fat content, a Jersey cow is 3000 kg of milk with 6.0% fat content, and crosses of these breeds are 4500 kg of milk with 4.5% fat content. In terms of milk yield and fat content, heterosis is not observed. Moreover, the total amount of milk fat obtained during lactation from crossbred cows (202.5 kg) is greater than from purebred cows (180 kg).
Lit.: Kochish I. I., Sidorenko L. I., Shcherbatov V. I. Biology of agricultural poultry. M., 2005; Bakai A.V., Kocsis I.I., Skripnichenko G.G. Genetics. M., 2006.
I. I. Kocsis.
Heterosis in plant growing is manifested in the superiority of first generation hybrids (F1) over the best of the parental forms in one or more characteristics, including an improvement in plant habitus, an increase in the mass of vegetative and generative organs, an improvement in biochemical and physiological characteristics; increased adaptability, increased productivity by 15-50%. The more the crossed parental forms differ in morphological, biological, physiological, adaptive and other characteristics, the more heterosis manifests itself. Selection of F1 hybrids consists of several stages. First, they search for and create plant forms with characteristics or mechanisms that prevent self-pollination (dioecy, sterility, etc.). At the next stage, through repeated self-pollination of cross-pollinating plants, inbred lines that are homozygous for the main economically valuable traits are created. Next, through reciprocal crossings with other inbred lines, they are assessed for their combinative ability, and the best hybrids and their parental lines are isolated. The final stage consists of production of seeds of heterotic hybrids using various crosses (intervarietal, varietal, linear, etc.). They cultivate highly productive F1 hybrids of grains, legumes, vegetables, ornamental and other crops. Some branches of crop production (for example, vegetable growing in protected soil) by the beginning of the 20th century had completely switched to the use of heterotic hybrids. The effect of heterosis is preserved in plants of the next generation only during vegetative propagation.
