Heterosis
Heterosis concept.
Related mating is accompanied by inbred depression, increased homozygosity of the inbred offspring, and an increase in the genetic similarity of the offspring to the ancestor. Heterosis has opposite biological and genetic properties.
Under heterosis understand the superiority of the offspring of the first generation over the parental forms in vitality, endurance, productivity, arising from the crossing of different races, animal breeds, zonal types.
The phenomenon of heterosis, or "hybrid strength", was noticed in the practice of animal husbandry in ancient times, in particular when obtaining mules by crossing a donkey with a mare, Charles Darwin was the first to provide a scientific explanation for the "hybrid strength" that occurs in offspring when unrelated organisms are crossed. He attributed this effect to 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), to explain the presence of" hybrid force "by the state of heterozygosity in the genotype of an organism, formed as a result of crossing. The hypothesis of 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 action of the heterozygote Aa to the manifestation of the phenotype is stronger than the homozygous dominant genotype AA(that is, the effect of the action Aa 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 Kible and Pelliu (1910), is based on the fact that when organisms are crossed, carrying different homozygous genes in the genotype, for example AAbb) and aaBB, y crossbred offspring, recessive alleles go into a heterozygous form of the genotype Aabb, in which the harmful effect of recessive genes is eliminated. The influence of dominant genes on the manifestation of heterosis can be explained by a simple total action 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 causing heterosis.
An ecological type of heterosis was revealed (Merkur'eva 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 the increased milkiness of the 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 imported 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 also leads to a heterotic effect. This explanation is called balance heterosis (Dobzhansky, 1952). Later, 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 adaptation 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, the heterozygosity accompanying crossing is under pressure from various factors and thereby a balanced interaction of genes in the genome is created. ,
In the practice of animal husbandry, the so-called negative heterosis is sometimes observed, when the level of the trait in the offspring is below the average of the parents, but slightly higher than the level of the trait of the parent in which it is less developed. The higher the differences in the level of the trait of the parental forms, the more the average level of the trait of offspring approaches the level of the trait of the worst parent. This feature in inheritance is described by Ya L, Glembotskiy in relation to shearing wool in hybrids obtained from crossing Angora goats with coarse-haired goats. The cut of wool in the first generation hybrids was somewhat greater than in coarse-haired goats, but significantly less than in angora goats, in which it was 4-5 times larger than in coarse-haired and crossbred goats.
Research to elucidate the biological foundations of heterosis was 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. Sareenov (1982), which can serve as additional material for understanding heterosis and the effect of crossing. During the work, the amount of DNA, RNA, proteins and the activity of a number of enzymes in tissues and in subcellular structures of cells (nucleus, chromosomes) of purebred and crossbred sheep were determined. Peculiarities of metabolic processes and heterosis were revealed in animals of different origin. 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. Crossbreeding does not cause activation of previously inactive genes obtained through the chromosomes of the parents in hybrids, and does not lead to a radical restructuring of metabolic processes. Instead, there is only a stimulation of the level of intensity of metabolic processes. In the process of ontogenesis, this tension decreases and the effect of heterosis in hybrids decreases.
The biochemical effect of heterosis in hybrids manifested itself in the stimulation of the activity of tissue enzymes (DNase, RNase, etc.), which affect the synthesis of nucleic acids. The activity of enzymes in hybrids occurs in a wider pH range of the environment, which increases the ecological plasticity of hybrid organisms and adaptability to environmental conditions. Consequently, crossing affects the mechanism of regulation of enzyme activity.
Synthesis of RNA in the cell nucleus and translation of directed RNA synthesis of protein molecules in the cytoplasm proceed at a higher level in hybrids. This is facilitated by the enrichment of cell nuclei with non-histone proteins of chromatin, which is a specific stimulator of genome activity. Consequently, crossing stimulated the synthesis of ribosomal RNA, that is, enhanced the transcription process. It is hypothesized that with the help of biologically active substances (hormones, metabolites) that can affect the activity of the genetic apparatus, it is possible to prolong the effect of heterosis during 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 of sensitive copies of structural genes on chromosomes, which form an excess of information in cells and determine the high compatibility of metabolic processes (Severin, 1967).
Explanations for the heterotic effect can be found in judgments that hybrids have polymorphic types of proteins (isozymes), which differ in some properties.
The parental forms do not have enzyme polymorphism, and when they are crossed, polymorphism and the number of polymorphic loci are formed in hybrids. therefore, there are more of them than the parents. This, according to some scholars (Finchem, 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 combined 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 traits and has a polygenic type inheritance.
VG Shakhbazov (1968) offers a new approach to explaining the heterotic effect. He believes that heterosis has a biophysical basis, since during fertilization there is an exchange of electrical charges of homologous chromosomes, 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 given explanations of the reasons for 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 the practice of animal husbandry, methods of animal breeding are carried out for fixing and enhancing 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 value of the superiority of the trait in hybrid animals over both parental forms. Another type of heterosis is hypothetical, when the traits of crossbreed offspring exceed the arithmetic mean level of the trait of both parents.
If there is no data for one of the breeds from which the crosses were obtained, then their indicators are compared with the parent breed, and the improved indicators of the crosses are called not heterosis, but the effect of crossing.
Summarizing the modern understanding of the phenomena of inbred depression and heterosis, one 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 of inbred lines of poultry using different types of inbreeding and crossing (crossing) of lines to obtain the so-called hybrid bird, which manifests itself as heterosis. For example, in the Netherlands, the Eurybrid company is working with two crosses of hens of the egg direction: Hisex white (white shell, based on Leghorns) and Hisex brown (with the participation of Rhode Island and New Hampshire with brown shell). These two crosses occupy a leading position in the world poultry egg industry.
Work on the creation of a hybrid of egg and meat poultry is being carried out in our country as well. To carry out selection for obtaining 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 unwanted individuals. Of the large number of established lines, about 10-15% of lines remain by the final, with an inbreeding coefficient on average at the level of 37.5% (mating of full siblings for three generations). Next, the remaining lines are crossed with each other to check them for compatibility, then the most successful combinations are left for production crossing and 2-, 3-, 4-line hybrids are obtained,
The use of the effect of heterosis is also used in working with other species of animals, especially in beef cattle breeding, sheep breeding, camel breeding, and fish breeding. 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 heterosexual breeds by obtaining hybrids from crossing cattle with zebu (Santa Gertrude, Beefmaster, Charbray, Bridford - in the USA; San Paulo - in Brazil; Haup Holstein - in Jamaica). In our country, distant hybridization was carried out between fine-fleeced sheep and argali, and a new breed was developed - archaromerinos. In Kyrgyzstan and Altai, yak hybrids with 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 insurmountable obstacle 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 hybrids “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 (commercial) animal husbandry. At the same time, the best part of the queens is isolated from the crosses of the first generation, obtained from crossing the queens of breed A with the producers of breed B, and they are crossed with the manufacturer of breed C, they get hybrids of the second generation, with the manifestation of heterosis when three breeds are combined (A, B, C). Further, hybrids of the second generation can be crossed with the producer of breed D and get more complex hybrids in which the heredity of the original parent breed A and the heredity of the paternal breeds B, C and B. There are no other methods that allow preserving the effect of heterosis in animal husbandry.
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 characteristics. The main indicators of heterosis are increased embryonic and postembryonic viability; reduction of feed costs per unit of production; increased early maturity, fertility, productivity; manifestation of wider possibilities of adaptation to changing conditions and new elements of technology. A wide range of heterotic effect, manifested in a variety of reacting characters, 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 G. in production, economically viable methods of obtaining hybrid seed n corn, tomatoes, eggplants, peppers, onions, cucumbers, watermelons, pumpkins, sugar beets, sorghum, rye, alfalfa, and other agricultural crops. plants. A special position is occupied by a group of vegetatively propagated plants in which it is possible to consolidate G. in the offspring, for example, varieties of potatoes and fruit and berry crops derived from hybrid seeds. For practical purposes, G. is used for intervarietal crosses of homozygous varieties of self-pollinating plants, intervarietal (interpopulation) crosses of self-pollinated lines of cross-pollinated plants (paired, three-line, double-four-line, multiple), and variety-line crosses. The advantage of certain types of crossing for each agricultural. 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 properties of incompatibility in some cross-pollinated plants and other hereditary features in the structure of the flower and inflorescence, excluding high costs for castration. When choosing parental forms for obtaining heterotic hybrids, their combinational 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 free-pollinating varieties based on inbreeding in the form of forced self-pollination. Methods for assessing and increasing the combinational ability of lines and other groups of plants used for crosses have been developed.
The greatest effect in the use of G. was achieved in corn. The creation and introduction of corn hybrids into production has made it possible to increase by 20-30% the gross grain harvest in the vast areas occupied by this crop in different countries of the world. Hybrids of corn have been created that combine high yields with good seed quality, drought resistance and immunity to various diseases. Zoned heterotic hybrids of sorghum (Hybrid Early 1, Hybrid Voskhod), heterotic intervarietal hybrids of sugar beet, of which the Yaltushkovsky hybrid is the most widespread. To obtain heterotic forms, sugar beet lines with sterile pollen are increasingly used. G.'s phenomena have also been established in many vegetable and oil-bearing crops. The first results were obtained in the study of G. in hybrids of wheat of the first generation, sterile analogues and restorers of fertility (fertility) were created, and sources of CMS in wheat were identified.
An increase in the power, vitality and productivity of the first generation hybrids in comparison with the 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. Charles Darwin was the first to observe the phenomenon of hybrid strength in corn. In his experiments, this culture had a decrease in productivity and a decrease in plant height as a result of self-pollination, these signs intensified with cross-pollination. Charles Darwin associated the increased vigor 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 manifests itself in increased growth, more intensive metabolism and higher yields. The increased productivity of heterotic hybrids is their main advantage. The increase in yield in hybrids of the first generation of all agricultural 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 culture are occupied by heterotic hybrids. Using heterosis, you can significantly increase agricultural production.
Heterosis does not necessarily enhance all the properties and characteristics of plants. For some of them, it can manifest itself more strongly than for others, and for some it may be absent.
Heterosis is observed when crosses between varieties, as well as between genetically and ecologically distant species and forms. It manifests itself most strongly and lends itself to control when crossing self-pollinated lines. Intsucht 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 a parchment insulator at the very beginning of flowering. On the same plant, the cob is also isolated before the threads appear. The best material for insulating the cob is cellophane. The dimensions of the insulators: for the panicle 20X30 cm, for the ears - 10 × 16 cm. Parchment insulators are glued with wood glue, adding a small amount of chromopic to it, and cellophane insulators - with a saturated solution of zinc chloride.
When the pollen ripens, the panicle is cut off and placed under an insulator along with the ear. Plants obtained from self-pollination are subjected to self-pollination the next year, repeating this procedure for several years. After 4-5 years of induction, a very high degree of evenness is practically achieved in the offspring of the in-line lines, and further self-pollination becomes unnecessary.
The selected lines are further propagated not under insulators, but in special areas where cross-pollination of plants occurs within one line without the danger of disrupting their homogeneity. Due to the low yield and poor growth, the resulting incuht lines cannot be used directly. But among these lines there are some very valuable for certain economically useful characteristics. For example, in corn, lines appear that are resistant to blister smut - a very dangerous disease of this crop, which takes 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 a corn moth, windbreak, etc. Such incuht lines are used in crosses with each other, as well as with varieties.
After the lines achieve uniformity in morphological and physiological characteristics, which usually happens after 4-5 years of self-pollination, they are assessed for combining ability, that is, the ability to produce highly productive hybrids. Distinguish between general and specific combining ability.
The overall combining ability shows the average value of the lines in hybrid combinations. It is determined by the results of crossing the lines with the variety serving as the paternal parent, called in this case the tester.
The specific combining ability is assessed by the results of crossing lines with any one line or a simple hybrid. At the same time, cases are revealed 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 general combinational ability.
To determine the specific combining ability of self-pollinated lines, diallele 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 further attenuation of the hybrid vigor 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 AAbv 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 generations in the work, since only in seedlings of the first hybrid generation, which, due to heterozygosity of parental varieties, had a wide variety of traits and properties, heterosis is fixed with further vegetative reproduction ...
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 obtained 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 from crossing a variety with a self-pollinated line or from crossing a simple interline hybrid with a variety. An example of a variety-linear hybrids of the first type is Bukovinsky HAZ. It was obtained from crossing the German variety Gloria Yanetsky T with a 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 bladder smut, has a high combinational ability, and its plants are usually two-cob.
Hybrid Bukovinsky 3TV has a very high cold resistance, is relatively early ripening and high-yielding, resistant to fly fly, is able to maintain green leaves and stems at full grain maturity.
Dneprovsky 56TV belongs to the cultivar hybrids of the second type. It was obtained from crossing a simple interline hybrid Iskra T with the variety Severodakotskaya TV: Iskra (VIR 26THVIR 27T) x Severodakotskaya TV. Linear hybrids exceed the usual grain yield by an average of 4-5 centners / ha, or 15-20%. New cultivar hybrids have been zoned: Bukovinsky PT, hybrid Collective 244, Dniprovsky 260M.
Simple interline hybrids are obtained by crossing two self-pollinated lines. For example, a simple interline hybrid Ideal was obtained from crossing the self-pollinated lines VIR 28 and VIR 29, and a simple interline hybrid Slava was obtained from crossing VIR 44 and VIR 38.
Simple interline hybrids give a large heterosis, but due to the low yield of the self-pollinated lines forming them, they did not receive widespread use in production for a long time. In recent years, it has been possible, through periodic selection, 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, Nagrada TV, Zakarpatsky 2TV, etc. Simple interline hybrids in production conditions by 10-12 centners / ha and more exceed the best double interlinear and variety-linear hybrids in grain yield. So, the simple hybrid Krasnodar 303TV, widely cultivated in the steppe regions of Ukraine, in the North Caucasus and in the Moldavian SSR, gives 80-90 centners of grain per hectare, and with irrigation - over 150 centners. New simple interlinear hybrids Dneprovskiy 70TV, Krasnodarskiy 301TV, Moldavskiy 385AMV, high-lysine hybrids Krasnodarskiy 303L and Hercules L.
On the basis of simple interlinear hybrids, high-yielding double interlinear and variety-linear hybrids, as well as complex hybrid populations, are created. For example, as a result of crossing a simple interline hybrid Astra (line 346 X Chlinia 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 in comparison with ordinary varieties of 8-12 c / ha, or 25-40%. New double interlinear hybrids have been 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 simple replanting for 3-4 years. Due to the constantly occurring 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 inter-line 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 parent form, and the paternal form was A 619 MB. Zoned three-line hybrids Dneprovsky 460 MB, Collective 101 TV, Kharkov 178 TV.
To obtain hybrid seeds, the parental forms of the hybrids are sown at the hybridization sites.
The laboriousness and high costs of work on removing panicles from plants of maternal forms of hybrids largely hindered the widespread use of the phenomenon of heterosis. The best solution to this problem is to find or create maternal forms of plants that are male sterile, 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 already known to Charles Darwin. He viewed them as the tendency of the species to move from monoeciousness to dioeciousness, which he considered more perfect in evolutionary terms. Thus, the appearance in monoecious plants of individuals with male sterility is a natural phenomenon of the evolutionary process.
Cytoplasmic male sterility (CMS) was first observed by the German geneticist K. Correns in 1904 in a summer savory garden plant. In 1921 the English geneticist W. Betson discovered it in flax, and in 1924 the American geneticist D. Jones discovered it in onions. The CMS in corn was first discovered by the academician of VASKhNIL 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 50s, it was appreciated and found wide practical application, first in the cultivation of corn, and then in many other crops.
In corn, there are two types of CMS: Texas (T) and Moldovan (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 Moldovan 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 formed in the anthers. The Texas and Moldovan types of CMS differ from each other in that each of them has its own lines that reinforce sterility or restore fertility.
The method of obtaining hybrid corn seeds without removing the tassels on the basis of CMS began to be used in the early 50s. To create hybrids of corn on a sterile basis, it is necessary to have: sterile analogs of self-pollinated lines or varieties; lines - sterility fixers; lines - fertility restorers.
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Heterosis (from the Greek heteroiosis - change, transformation)
"Hybrid vigor", acceleration of growth and increase in size, increased viability and fertility of the first generation hybrids during various crosses of both animals and plants. In the second and subsequent generations, G. usually dies out. Distinguish between true G. - the ability of hybrids to leave a large number of fertile offspring, and gigantism - an increase in 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 and in some unicellular organisms. At S.-kh. animals and cultivated plants G. often leads to a significant increase in productivity and yield (see below - Heterosis in agriculture). G. and the opposite inbred depression (see. Inbreeding) were already known to the ancient Greeks, in particular to Aristotle. The first scientific studies of G. in plants were carried out by the German botanist I. Kölreuter (1760). Ch. Darwin generalized observations about the benefits of crossing (1876), thereby exerting a great influence on the work of IV Michurin and many other breeders. The term "G." suggested by the American geneticist G. Schell (1914); he was the first to obtain "double" interline maize hybrids. The foundations 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 stimulates theoretical studies of G. an increase in genetic variability (see Variation). Often, stable genetic systems arise that ensure the predominant survival of heterozygotes for many genes. The study of G., in addition to the usual study of morphological characters, requires the use of physiological and biochemical techniques that make it possible to detect subtle differences between hybrids and the original forms. The study of G. at the molecular level has also begun: in particular, the structure of specific protein molecules - enzymes, antigens, and others - is being investigated in many hybrids. According to Darwin, G. is caused by the union of heterogeneous hereditary inclinations in a fertilized egg. On this basis, two main hypotheses arose about the mechanism of H. The hypothesis of heterozygosity ("overdominance", "one-gene" G.) was put forward by the 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 governs 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); it can give her an edge. 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. An increase in dominance (see Dominance) of genes "normal" for a population (evolution of dominance) serves as protection against them. The combination of favorable dominant genes of two parents in a hybrid leads to G. Both hypotheses of G. can be combined by the concept of genetic balance (American scientist J. Lerner, English K. Mater, 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, as well as vegetative propagation of hybrids. G.'s effect can be fixed and with doubling of individual genes or small sections of chromosomes. The role of such duplications in evolution is very great; therefore, G. should be regarded as an important stage on the path of evolutionary progress. V. S. Kirpichikov. Heterosis in agriculture. The use of G. in plant growing is an important method for increasing the productivity of plants. The yield of heterotic hybrids is 10-30% higher than that of conventional varieties. To use G. in production, economically viable methods have been developed for producing hybrid seeds (see Hybrid seeds) of corn, tomatoes, eggplants, peppers, onions, cucumbers, watermelons, pumpkin, sugar beets, sorghum, rye, alfalfa, and other agricultural crops. ... plants. A special position is occupied by a group of vegetatively propagated plants in which it is possible to consolidate G. in the offspring, for example, varieties of potatoes and fruit and berry crops derived from hybrid seeds. For practical purposes, G. is used for intervarietal crosses of homozygous varieties of self-pollinating plants, intervarietal (interpopulation) crosses of self-pollinated lines of cross-pollinated plants (paired, three-line, double-four-line, multiple), and variety-line crosses. The advantage of certain types of crossing for each agricultural. 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 properties of incompatibility in some cross-pollinated plants and other hereditary features in the structure of the flower and inflorescence, excluding high costs for castration. When choosing parental forms for obtaining heterotic hybrids, their combinational 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 free-pollinating varieties on the basis of Inbreeding and in the form of forced self-pollination. Methods for assessing and increasing the combinational ability of lines and other groups of plants used for crosses have been developed. The greatest effect in the use of G. was achieved in corn. The creation and introduction of corn hybrids into production has made it possible to increase by 20-30% the gross grain harvest in the vast areas occupied by this crop in different countries of the world. Hybrids of corn have been created that combine high yields with good seed quality, drought resistance and immunity to various diseases. Zoned heterotic hybrids of sorghum (Hybrid Early 1, Hybrid Voskhod), heterotic intervarietal hybrids of sugar beet, of which the Yaltushkovsky hybrid is the most widespread. To obtain heterotic forms, sugar beet lines with sterile pollen are increasingly used. G.'s phenomena have also been established in many vegetable and oil-bearing crops. The first results were obtained in the study of G. in hybrids of wheat of the first generation, sterile analogues and restorers of fertility (fertility) were created, and sources of CMS in wheat were identified. In animal husbandry, G.'s phenomena are observed during hybridization, interbreed and intrabreed (interline) crossing (see Crossing) and provide a noticeable increase in agricultural productivity. animals. The most widespread use of G. in industrial crossing (see Industrial crossing). In poultry farming, when egg-bearing breeds of chickens are crossed, for example, Leghorns with Australorp, Rhodeland, etc., the egg production of hybrids of the first generation 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. according to a complex of characters is obtained by crossing closely related lines of chickens of the same breed or by interbreeding crosses. In pig breeding, sheep breeding, and cattle breeding, industrial crossing is used to obtain G. for meat productivity, which is expressed in an increase in the early maturation and live weight of animals, an increase in the slaughter yield, and an improvement in the quality of carcasses. Pigs of meat-greasy (combined) breeds are crossed with boars of meat breeds. Small, unproductive sheep of local breeds are crossed with rams of meat-wool breeds, fine-wool mothers - with rams of early maturing meat or semi-fine-wool breeds. To increase the meat productivity, dairy cows, dairy-meat and local meat breeds are crossed with bulls of specialized meat breeds. Lit .: Darwin Ch., The effect of cross-pollination and self-pollination in the plant world, trans. from English, M.-L., 1939; Kirpichnikov VS, Genetic bases 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, v. 1-6, M., 1966; The use of heterosis in animal husbandry. [Conference proceedings], Barnaul, 1966; Heterosis in animal husbandry. Bibliographic list, M., 1966; Guzhov Yu. L., Heterosis and M.'s harvest, 1969; Bryubaker J.L., Agricultural genetics, trans. from English, M., 1966; Turbin N.V., Khotyleva L.V., The 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, increased vitality and fertility of the first generation hybrids in comparison with the parental forms of plants or animals. Usually in the second and subsequent generations, heterosis dies out. Heterosis is widespread ... ... Financial vocabulary
Heterosis (translated from Greek, 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 Greek heteroiosis change, transformation), the property of the first generation hybrids to surpass the best of the parental forms in vitality, fertility and other characteristics. In the second and subsequent generations, heterosis usually fades. ... ... Modern encyclopedia
- (from the Greek. heteroiosis change, transformation), the property of the first generation hybrids to surpass the best of the parental forms in vitality, fertility and other characteristics. In the second and subsequent generations, heterosis usually fades away. Heterosis ... Big Encyclopedic Dictionary
- (from the Greek. heteroiosis change, transformation), "hybrid power", the superiority of hybrids in a number of traits and properties over parental forms. The term "G." proposed by J. Schell in 1914. As a rule, G. is characteristic of hybrids of the first generation ... Biological encyclopedic dictionary
Excellence Dictionary of Russian synonyms. heterosis noun, number of synonyms: 1 superiority (14) ASIS Synonym Dictionary. V.N. Tr ... Synonym dictionary
- (from the Greek heteroiosis change, transformation), hybrid power, increased vitality and fertility of the first generation hybrids in comparison with the parental forms. It was first described by Charles Darwin (1859). The theoretical foundations of heterosis ... ... Ecological Dictionary
HETEROSIS, increased vitality, demonstrated in some cases by hybrid offspring (see HYBRID), not characteristic of parents ... Scientific and technical encyclopedic dictionary
Heterosis. See hybrid power. (
HETEROSIS HETEROSIS
(from the Greek. heteroiosis - change, transformation), "hybrid power", the superiority of hybrids in a number of traits and properties over parental forms. The term "G." proposed by J. Schell in 1914. As a rule, G. is characteristic of hybrids of the first generation obtained by crossing unrelated forms: dec. lines, breeds (varieties) and even species. In subsequent generations (crossing hybrids among themselves), its effect is weakened and disappears. The hypothesis of "overdominance", or monogenic G., assumes that heterozygotes by definition. the gene is superior in characteristics to the corresponding homozygotes. The phenomenon illustrating this hypothesis is inter-allelic complementation. A number of other hypotheses are based on the assumption that the hybrid has a greater number of dominant alleles of different genes in comparison with the parental forms and the interaction between these alleles. Synthetic hypotheses are based on both intragenic and intergenic interactions. The importance of heterozygosity as the basis of G. is also evidenced by the fact that in natural populations, individuals are heterozygous for a large number of genes. Moreover, in the heterozygous state, many are preserved. alleles showing, in a homozygous state, adverse effects on vital signs. G. is of great importance in agricultural. practice (in agricultural animals and plants, G. often leads to a significant increase in productivity and productivity: the production of simple and double interline corn hybrids made it possible to increase the gross grain harvest by 20-30%), but its use is often insufficiently effective, i.e. because the problem of fixing G. in a number of generations has not yet been resolved. As approaches to solving this problem, vegetative reproduction of heterotic forms, polyploidy and decomp. irregular forms of sexual reproduction (apomixis, parthenogenesis, etc.).
.(Source: "Biological Encyclopedic Dictionary." - M .: Sov.Encyclopedia, 1986.)
heterosis(hybrid vigor, hybrid vigor), the superiority of the first generation hybrids over the parental forms in viability, productivity, fertility and a number of other traits. To obtain the effect of hybrid power, it is important as parents to choose unrelated forms, representing different lines, breeds, even species. In practice, the best parental pairs yielding the most valuable hybrids are selected through numerous crosses to reveal the most successful combination of different lines. When the next generations are crossed with each other, heterosis weakens and fades.
Heterosis is based on a sharp increase in heterozygosity in first-generation hybrids and superiority heterozygote for certain genes over the corresponding homozygotes... Thus, the phenomenon of hybrid power is the opposite of the result of closely related crossbreeding - inbreeding, which has adverse consequences for the offspring. The genetic mechanism of heterosis (it has not been fully elucidated) is also associated with the presence of a greater number of dominant genes in the hybrid compared to the parents, interacting with each other in a favorable direction.
Heterosis is widely used in agricultural practice to increase agricultural yields. crops and productivity of agricultural animals. In the 1930s. US breeders have dramatically increased corn yields by using hybrid seeds. One of the important tasks breeding- the search for ways to "fix" heterosis, ie. preserving it in a number of generations.
.(Source: "Biology. Modern illustrated encyclopedia." Ed. A. P. Gorkin; Moscow: Rosmen, 2006.)
Synonyms:
See what "HETEROSIS" is in other dictionaries:
Acceleration of growth, increase in size, increased vitality and fertility of the first generation hybrids in comparison with the parental forms of plants or animals. Usually in the second and subsequent generations, heterosis dies out. Heterosis is widespread ... ... Financial vocabulary
Heterosis (translated from Greek, 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 Greek heteroiosis change, transformation), the property of the first generation hybrids to surpass the best of the parental forms in vitality, fertility and other characteristics. In the second and subsequent generations, heterosis usually fades. ... ... Modern encyclopedia
Heterosis- an increase in the viability of hybrids due to the inheritance of a certain set of alleles of various genes from their heterogeneous parents. This phenomenon is the opposite of inbred depression, which often occurs as a result of inbreeding (closely related crossing), leading to an increase in homozygosity. An 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 are not manifested.
In plants (according to A. Gustafson), three forms of heterosis are distinguished:
-----T. n. reproductive heterosis, as a result of which the fertility of hybrids and productivity increases,
----- somatic heterosis, which increases 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 make practical use of heterosis. His idea is that, for example, seed farms breed two suitable parental lines of plants, 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 is used as one of the parents whose pollen is sterile, which makes it possible to avoid self-pollination. The gene for sterility of pollen in corn was discovered in our country by geneticist and breeder M.I. Khadzhinov in 1932. Later, with the help of crosses, the sterility gene was introduced into the desired varieties of corn). This method turned out to be very successful. For example, in the United States, since 1968, only hybrids have been grown in all areas occupied by corn. In many countries of the world, using the same method, marketable seeds of onions, tomatoes, beets, rice, cucumbers, carrots and other crops are obtained.
Heterosis is also used in animal husbandry. Broiler poultry farming is based on it. A breed of chickens with a high egg production and another breed with a rapid growth of chickens were bred. When crossing chickens of the first breed with roosters from the second, a hybrid generation is obtained, characterized by especially 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 the heterozygote Aa on the manifestation of the phenotype turns out to be stronger than that of the homozygous dominant genotype AA (that is, the effect of the action Aa is more than AA action).
Another explanation of heterosis, formulated by Keible and Pelliu (1910), is based on the fact that when organisms carrying different homozygous genes in the genotype are crossed, for example, AAbb and aaBB, in the hybrid offspring recessive alleles turn into a heterozygous form of the AaBb genotype, which eliminates the harmful genes. The influence of dominant genes on the manifestation of heterosis can be explained by a simple total action of a large number of dominant genes, that is, there is an additive effect.
45. Genetic structure of populations. Hardy-Weinberg's law.
Population - a set of individuals of one species, occupying a certain area for a long time and freely interbreeding with each other. To one degree or another, they are isolated from another population.
Each genetic population has a specific genetic structure and gene pool. Gene pool call the set of all genes that members of the population have. Genetic structure is determined by the concentration of each gene (or its alleles) in the population, the nature of 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 are the main prerequisite for the development of the diploid phase. If there are A individuals in the population, then with a 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 the frequencies of alleles and genotypes in a population shows a certain pattern in each specific period of time and across generations of organisms.
Panmixia is a free crossing.
An important property of populations is their ability to exhibit high genetic variability, the main source of which is laid in the process of reproduction,
The source of the increase in hereditary variability is the mutational process, during which the appearance of new alleles contributes to the formation of new phenotypes (and genotypes) in the population that were previously absent in it.
The interaction of genes of different loci with each other also affects the genetic variability of the population, It is called coadaptation 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, such an important property as adaptation to environmental conditions is formed in the individuals that make up the population. 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 an increase in the population size.
The genetic structure of each panmictic population is preserved in a number of generations until some time, until some factor brings it out of equilibrium. Preservation of the original genetic structure, that is, the frequency of alleles and genotypes in a number of generations, is called genetic balance and is typical of panmictic populations. A population can have an equilibrium at some loci and a disequilibrium state at others.
With the transition of a population to a non-equilibrium state, the levels of frequencies of alleles and genotypes change, and a new ratio is formed 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 the relative allele frequencies are known p and q and the total population N in general, then you can calculate the expected, or calculated absolute frequency (that is, the number of individuals) of each genotype. To do this, each term in the equation must be multiplied by N total:
p 2 AA N total + 2P q Aa N total + q 2 aa N total = N total
In this equation:
p 2 AA N total - the expected absolute frequency (number) of dominant homozygotes AA
2P q Aa N total - the expected absolute frequency (number) of heterozygotes Aa
q 2 aa N total - the expected absolute frequency (number) of recessive homozygotes aa
