One of the most striking examples of cytoplasmic inheritance is the phenomenon cytoplasmic male sterility(CMS) found in many plants - corn, onions, beets, flax, etc.
Cytoplasmic male sterility in maize was discovered in the 1930s simultaneously in the USSR by MI Khadzhinov and in the USA by M. Rhodes. Corn is a monoecious plant, its female flowers are collected on the cob, the male flowers are gathered in a panicle. In some varieties of corn, plants were found that had underdeveloped anthers in their panicles, often completely empty, and sometimes with underdeveloped sterile pollen. It turned out that this feature is determined by the characteristics of the cytoplasm. Pollination of male-sterile plants with normal pollen from other plants in most cases produces plants with sterile pollen in the offspring. When this crossing is repeated over a number of generations, the sign of male sterility does not disappear, being transmitted through the maternal line. Even when all 10 pairs of chromosomes from plants with sterile pollen are replaced by chromosomes from plants with fertile pollen, male sterility persists. This served as convincing evidence that the inheritance of this trait is carried out through the cytoplasm. The cytoplasm responsible for the sterility of the pollen was indicated by the symbol cyt S(sterile cytoplasm), and the cytoplasm of plants with fertile pollen is the symbol cyt N(normal cytoplasm).
It has been established that the genotype of a plant can have a definite effect on the action of sterile cytoplasm. Cytoplasm cyt S can cause pollen sterility only if there is a recessive gene in the plant genotype rf homozygous rfrf. If this gene is represented by a dominant allele Rf, that plant cyt S RfRf or cyt S Rfrf has normal pollen. Allele Rf is thus a pollen fertility restorer. Therefore, plants and plants can have fertile pollen. cyt N rfrf, and cyt N Rf–, and cyt S Rf–, and completely sterile - only plants cyt S rfrf. Multiple repetition of crossing ♀ cyt S rfrf × ♂ cyt N rfrf always produces offspring with completely sterile pollen. And only in the case of crossing cyt S rfrf× cyt S RfRf(or cyt N RfRf) offspring can be obtained where all plants will have normal pollen despite the presence of cytoplasm cyt S... It should be emphasized once again that the gene Rf does not change the structure and specificity of the cytoplasm cyt S, but only inhibits the manifestation of its action.
The interaction of nuclear and cytoplasmic hereditary factors that determine CMS can be written as follows:
Notes (edit).
1. CMS factors are localized on mtDNA. Allele of cytoplasmic male fertility cyt N - This is a specific section of mtDNA with a length of 10 kb. When this region is deleted, an allele of cytoplasmic male sterility arises. cyt S
2. The phenomenon of CMS is widely used to obtain hybrid seeds in nightshade, pumpkin cultivated plants, since in this case, to obtain F 1, laborious operations of castration and isolation of mother flowers are not required.
3. CMS is used to ensure the genetic safety of genetically modified crops: the pollen of such GMO lines is sterile.
148. What is cytoplasmic male sterility?
One of the most striking examples of cytoplasmic heredity can be considered cytoplasmic male sterility (CMS), found in many plants - corn, onions, beets, flax.
Let's consider the CMS using corn as an example. Corn is a monoecious plant, the female flowers of which are collected on the cob and the male flowers in a panicle. Sometimes the panicle may contain underdeveloped anthers with sterile pollen. Pollen sterility is determined by some features of the cytoplasm. Pollination of plants with CMS with pollen taken from other plants produces forms with sterile pollen in the offspring. Consequently, the sign of male sterility is transmitted through the maternal line. Even when all 10 pairs of chromosomes of a pollen-sterile plant are replaced by chromosomes of plants with normal pollen, male sterility remains. The cytoplasm responsible for male sterility can be designated as cyt S, and the normal cytoplasm as cyt N. Genetic analysis showed that the genotype of the plant also affects the sterility of the pollen. Cytoplasm cyt S determines sterility only if the genotype contains recessive rfrf genes in a homozygous state - cyt S rfrf. When cyt S RfRf or cyt S Rfrf, the plants have normal fertile pollen. This means that the Rf gene is able to restore pollen fertility. A similar relationship between cytoplasm and genotype made it possible to develop a methodology and draw up a scheme for obtaining interline maize hybrids using CMS (Fig. 37):
Used lines of maize belong to the so-called "fixers" of sterility and "restorers" of fertility. Fertile lines and varieties that retain sterility in the offspring when crossed with a sterile form are called "fixers" of sterility, and lines and varieties that restore the fertility of the offspring of plants with CMS are "restorers" of fertility. There are two types of male sterility known in corn. The study of pollen formation in sterile forms of maize showed that visible disturbances in sporogenesis occur at different stages: in some, degeneration ends at the stage of mononuclear pollen, in others - binuclear. External conditions have a great influence on the manifestation of sterility: temperature, soil and air humidity, as well as the length of the day. Corn varieties with one or another type of CMS have different sensitivity to environmental factors. In plants with the Moldavian type of CMS, the panicles form anthers that do not open, the pollen in them is not viable, although under certain conditions viable pollen can also form. In plants with the Texas type of CMS, the manifestation of sterility is less susceptible to the influence of external influences, and the trait is significantly pronounced: the anthers are strongly degenerated and never open. Both types of CMS are used in breeding.
The phenomenon of cytoplasmic male sterility (CMS)
Cytoplasmic male sterility (CMS, eng. Cytoplasmic male sterility, CMS) - the phenomenon of complete or partial sterility of the androeum of higher plants, the cause of which is the presence of a special mutation in the mitochondrion, that is, in the mitochondrial genome, plant fertility is restored in whole or in part in the presence of a dominant allele of the nuclear fertility restorer gene. First described by Marcus M. Rhodes in corn, also described in petunia, cabbage, sunflower and other plants. The CMS is characterized by the so-called maternal type of inheritance.
General mechanism of CMS
Cytoplasmic male sterility is manifested in the interaction of the nuclear genome with mitochondrion. Mitochondria and plastids as organelles originating from endosymbiontic prokaryotic microorganisms have their own unique genome, and although during the evolution of the eukaryotic cell they lost most of their autonomy and lost most of their genes, some important proteins are still encoded under the control of mitochondrial and plastid genes. Mitochondria and plastids also have a working protein-synthesizing apparatus. Cytoplasmic male sterility arises as a result of a certain mutation in the mitochondrion, as a result of which the plant's androecium degenerates, manifested either in the degeneration of the anthers, or in the non-disclosure of the anthers, or in the formation of non-viable pollen. Genotypes with wild-type mitochondria are designated N or Cyt N (i.e., normal type of cytoplasm), genotypes with a mutant mitochondrion are designated as S or Cyt S (i.e., sterile cytoplasm). In the nuclear genome of plant cells, there are also special genes that restore fertility (eng. restorer of fertility or Rf-genes), the dominant alleles of which fully or partially restore the fertility of the androecium. Only genotypes that have a mutant mitochondrion and are recessive homozygotes for Rf-genes are sterile (Cyt S rfrf), all other genotypes are fertile.
Breeding of F1 hybrids of white cabbage based on CMS
The use of maternal hybrids with cytoplasmic male sterility (CMS) in F1 seed production excludes self-pollination and the presence of siblings in lots of hybrid seeds.
Male sterility occurs in almost 140 species of higher plants and is associated with abnormal development of anthers (non-opening of the anther or abortion of pollen at various stages of microsporogenesis (Bunin, 1994). reproduction of maternal line plants.
After the discovery of specific dominant genes of fertility restorers (Rf genes), it became possible to commercialize CMS lines as maternal parental forms of such important agricultural plants as corn, sorghum, rice, and sunflower.
In plant species Bgasicaceae L. (as well as onions,
beets and carrots), a more complex inheritance of MS is observed, which until recently was reflected in the efficiency of its use in heterotic breeding. The phenotypic manifestation of male sterility in cabbage plants is more often due to the interaction of the cytoplasm with the nuclear genes that are in a homozygous state, that is, essentially refers to nuclear cytoplasmic male sterility. Therefore, unlike, for example, corn, this requires the creation of special lines - sterility fixers, which complicates the selection and seed-growing process. At the same time, in vegetable cruciferous crops such as B. operacea, B. campestris, R sativus, there is no need to restore fertility, since these plants use vegetative organs for food.
Among the plant diversity of Brassica L. species, the form with nuclear-cytoplasmic MS was first discovered by Ogura (1968) in an unidentified variety of the Japanese radish subspecies, daikon. Flowers in plants of this form have a number of morphological features: small buds, a loop-shaped pistil or a pistil protruding from the bud, the death of the first buds on the inflorescence, etc. Pollen degeneration in such daikon MS plants occurs at the stage of microsporogenesis and is associated with early collapse of tapetum tissue. This type of sterility is induced by the interaction of sterile cytoplasm S (ogy) with the homozygous recessive nuclear gene msms (rfog rfog) (Bunin, 1994).
In Russia, attempts to use male sterility were made at the TSKHA and at VNIISSOK. In TSKHA Z.G. In the early seventies, Averchenkova discovered several plants with nuclear cytoplasmic male sterility in the Amager611 variety (Averchenkova, 1968). This type of male sterility could not be used in practical breeding, since sterility fixers were not obtained. THEM. Kolesnikov (VNIISSOK) introduced androsteril lines of broccoli from the Czech Republic.
According to the phenotypic manifestation of male sterility, these lines most likely possessed ogy-CMS. Attempts to use these lines directly in the breeding process were not crowned with success due to their shortcomings: the absence of petals in flowers, the death of buds under unfavorable conditions, etc.
Thus, for a long time, the production of cabbage hybrids based on male sterility was very problematic.
The type of YCMS, according to A.V. Kryuchkov (1974) has a number of other disadvantages:
1) it is difficult to maintain the YCMS line in a homozygous state for most genes, since the sterility fixer may be with a different genotype and, as a result, F1 hybrids will be unaligned;
2) in seed production of lines and hybrids, seeds are collected only from a sterile line and their yield is significantly reduced.
No CMS donors were found in cabbage. In 1988 M.S. Bunin introduced the male sterile daikon form from Japan to Russia. Hybridological analysis showed that androsterility in him is controlled by the interaction between the recessive nuclear gene in the homozygous state msms and sterile cytoplasm (S), that is, it corresponds to ogy-CMS.
Currently, there are several CMS systems in cruciferous oilseeds. Rolima-CMS, which was identified as a spontaneous mutation in a Polish rape variety, was widely used in breeding programs for the creation of oilseed rape hybrids.
Sterility fixers were found in rape, a fertility restorer was isolated in the rapeseed variety Italu and in Br. jincea. F1 hybrids of spring rape were created and registered on the basis of polima-CMS: in China - Qui - Yong, in Canada - Hyola 401, in Europe - Hybridol. The jun-CMS system (B. jincea) is more stable than the polima-system, and on its basis, F1 hybrids with partially restored male fertility were obtained in Indian mustard and oilseed rape.
The nap-CMS system, identified by Tompson (1976) and Shiga (1976) in B. parus, is rarely used in heterotic breeding of oilseed rape because of the MS instability in the temperature range from 20 to 26 ° C.
The sam-CMS system (B. samestris) was identified as being quite close to the nap-system with corresponding disadvantages when used in breeding heterotic hybrids.
The nigra-CMS system. Obtained by transferring the genome of B. napus and B. oleracea into the cytoplasm of B. nigra. It is highly stable in B. oleracea and low in B. nigra. With this system, the anthers are modified into petals (petaloid sterility - pp):
male sterility (b) pp (b - cytoplasm of B. nigra);
sterility fixer (c) pp (c - cytoplasm
V. are found in fodder and cabbage;
fertility restorer (c) PP, found in forage and cabbage.
Tоug-CMS system. Created on the basis of V. tourneforii. Used to create the first hybrids of B. juncea.
Mug-CMS system. Found after transfer of the B. napus nucleus into the cytoplasm of Diplotaxis muralis. Transferred to turnips and rapeseed. The manifestation is stable, but there is no sterility fixer.
In recent years, in a number of countries (Holland, Denmark, Belgium, France, Japan) breeding work has been successfully carried out on the use of cabbage forms with CMS.
In Russia in the RSAU - Moscow Agricultural Academy named after K.A. Timiryazev at the N. N. Timofeeva continued the previously begun work on finding and using forms of cabbage with CMS. As a result of the work carried out, a hybrid of medium late cabbage Favorit with the use of CMS was created. The high efficiency of the method of distant hybridization in the transfer of CMS from rape (B. napus) to cabbage (B. pekinensis) was established.
genetic code inbreeding sterility
In many species of plants with bisexual flowers and monoecious, single individuals with sterile male generative organs are occasionally found. 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 formation of individuals with male sterility is a natural phenomenon of the evolutionary process.
Male sterility was first discovered by K. Correns in 1904 in a summer savory garden plant. In 1921 V. Batson found it in flax, in 1924, American geneticist D. Jones - in onions, in 1929 A.I. Kuptsov - in sunflower.
In 1932 M.I. Hajishimo from him the American geneticist M. Rhodes discovered male sterile plants in corn. Later it was found that male sterility is widespread among flowering plants. Mutations causing male sterility are now described in most cultivated plants.
Male sterility occurs in the absence of pollen or its inability to fertilize and manifests itself in three main forms:
- 1) Male generative organs - stamens - do not develop at all; a similar phenomenon is observed in plants of some types of tobacco;
- 2) Anthers in flowers are formed, but their pollen is not viable; this form of sterility is most common in maize;
- 3) Normal pollen is formed in the anthers, but they do not crack and the pollen does not fall on the stigmas; this is a very rare phenomenon sometimes observed in some varieties of tomato.
Male sterility can be genetically determined by genes for nuclear sterility and the interaction of nuclear genes and plasmogens. In accordance with this, two types of male sterility are distinguished: nuclear, or gene, and cytoplasmic. Nuclear sterility is caused by mutations in the ms chromosomal genes. Due to the fact that the genes for sterility are recessive, and the genes for fertility are dominant, with this type of inheritance of sterility from crossing sterility of plants with fertile all F1 plants are fertile (msms x MsMs Msms), and in F2 there is a splitting into fertile and sterile forms in relation to 3: 1 in subsequent generations, the number of sterile plants from such a crossing is continuously decreasing. Currently, methods of using gene sterility are being developed to obtain heterotic hybrids of cotton, sunflower and some other crops.
Complete male sterility due to one recessive gene ms was found in a sample of wild one-year-old beet. By the method of saturating crosses, this gene was transferred to sugar beets. In this culture, it acts independently of genes X and Z, which restore pollen fertility in forms with S-cytoplasm.
Three hypotheses have been put forward to explain the causes of cytoplasmic sterility. One of them, known as viral, associates the occurrence of male sterility with a viral infection, which can be transmitted during sexual reproduction through the cytoplasm of the egg.
The second hypothesis considers the emergence of CMS as a result of a mismatch between the cytoplasm and nucleus of different species during distant hybridization. Indeed, in some cases, for example, when common wheat Triticum aestivum is crossed with Tr Aestivum, forms with CMS appear. However, in many cultures, CMS was found that was not associated with distant hybridization. Therefore, the greatest recognition is currently received by the hypothesis considering the emergence of CMS as a result of specific mutations of plasmogenes.
It can be argued that cytoplasmic male sterility is due to hereditary changes (mutations) in the cytoplasm. It is usually fully preserved in F1 and subsequent generations in all plants. With this type of inheritance, a sterile plant, for example corn, pollinated with pollen of another variety or line, gives birth to offspring in which the panicle remains sterile, and the other traits change, as is usually the case with hybridization. The sign of male sterility persists even when all 10 pairs of chromosomes in maize of such pollen-sterile plants are replaced in repeated crosses by chromosomes from plants with normal, fertile pollen. It follows from this that male sterility is steadily transmitted from generation to generation through the maternal line, and the hereditary factors that determine it are not located in the chromosomes of the nucleus.
The character of CMS inheritance has been well studied in reciprocal crosses of male-sterile plants, sometimes producing small amounts of fertile pollen, with normal fertile plants. When plants are pollinated with a sterile line with fertile pollen, the sign of sterility is transmitted to hybrids of F1 and subsequent generations. If this crossing continues, then there is a gradual replacement of the genes of the sterile line with the genes of the line with fertile pollen. The cytoplasm of the sterile maternal line is gradually saturated with the nuclear hereditary material of the paternal fertile line.
With each crossing, the maternal line has less and less of its hereditary factors, they are replaced by the factors of the line taken for the saturating crossing. As a result of six to seven backcrosses and selection, plants are obtained that are similar in all respects to the paternal line, but possessing male sterility. They are called sterile counterparts of the fertile lines used as the paternal form. When plants of fertile lines are pollinated with pollen, which is occasionally formed in plants of sterile lines, F1 hybrids have fertile pollen and upon further reproduction give plants only with fertile pollen. Consequently, CMS cannot be transmitted through the male plant, but is steadily transmitted from generation to generation through the maternal line.
The results of the considered crossing, it would seem, leave no doubt that the CMS trait is genetically related only to extrachromosomal factors. But further study of the inheritance of CMS showed that not all crosses of sterile plants with fertile ones result in offspring with sterile pollen. In some cases, the sign of sterility is completely suppressed in F1 hybrids and does not manifest itself at all during their further reproduction, or, starting from F2, splitting into fertile and pollen-sterile plants occurs.
As a result of the study and generalization of experimental material on the inheritance of male sterility, the idea arose that this property is due to the interaction of the cytoplasm and genes of chromosomes, which together make up the genetic system. The cytoplasm, which determines the sterility of the pollen, is called CITs (sterile cytoplasm), and the cytoplasm, which gives plants with fertile pollen, is called CITn (normal cytoplasm). There is a dominant gene Rf localized in chromosomes (from the initial letters restoring fertility - restoring fertility), which, without changing the structure and specificity of sterile cytoplasm, at the same time prevents its manifestation. Sterile cytoplasm manifests its effect only in combination with the recessive alleys of this gene. Therefore, only the combination of CIT srfrf can lead to the development of sterile pollen. Fertile pollen is formed on the basis of normal cytoplasm in combinations of CITn RfRf, CITn Rfrf and CITn rfrf and on the basis of sterile cytoplasm in combinations of CITs RfRf and CITs Rfrf. Thus, maternal inheritance of CMS is only possible in plant crosses CITs rfrf x CITn rfrf CITs rfrf (sterility is fixed).
Crossing CITs rfrf x CITN (s) RfRf all plants will be fertile, that is, there is a complete restoration of fertility.
We analyzed the simplest case of sterility inheritance, associated with the interaction of sterile cytoplasm and one allelic gene pair. Currently, more complex genetic systems of CMS associated with the manifestation of sterility of pollen with two and three genes have been studied. Male sterility in sugar beets is due to the interaction of sterile cytoplasm (CITs) with two nuclear genes (X and Y) and is transmitted in offspring only through the maternal line. The plant with two recessive genes and sterile cytoplasm has the Sxxzz genetic structure, which gives complete sterility of the pollen. Semi-sterile plant types are heterozygous for genes X and Z forms: SXxzz, SXXzz, SxxZz, SxxZZ, SXxZz, SXXZz, SXxZZ, SXXZZ. Fertile plant populations (CITn) have individuals with different hereditary structures, and certain types of sterility can be manifested in their offspring. Pollination with pollen from Nxxzz plants produces completely sterile lines. If the pollinator is diheterozygous for genes X and Y, then the offspring will have all three types of sterility: Sxxzz x NXxZz> SXxZz, SXxzz, SxxZz, Sxxzz. The manifestation of sterility, in addition to genetic factors, is somewhat influenced by external conditions. For example, male sterility is better restored in cool weather, sufficient moisture in the soil and air during the flowering period, with a shortened day and a lack of nitrogen in the soil.
CMS is widely used to create heterotic hybrids of maize and some other crops on a sterile basis. CMS causes a number of changes in corn plants: the number of leaves decreases (by 3-4%), plant growth decreases (up to 4-5%), and a slight depression is observed in other characteristics. The degree of manifestation of depression depends on the genotype of the lines: in some it is more pronounced, in others it is weaker. In some lines with sterile cytoplasm, plant growth even slightly increases. Depression in lines with CMS is partially relieved by the action of restorer genes. The sterility of the cytoplasm, on average, does not have a negative effect on the productivity of hybrids. In unfavorable weather conditions, sterile forms, when pollinated with pollen of fertile plants, turn out to be more productive.
Some scientists believe that the direct cause of the formation of forms with CMS is a violation of protein synthesis as a result of a mutation in the nucleus, leading to improper microsporogenesis, while others associate degeneration of pollen grains with a disruption in the supply of anthers to sterile plants.
When crossing specially similar lines of corn, you can get hybrids that are 25-30% higher in yield than the best varieties. Such lines are sown in alternating rows at hybridization sites. But to obtain hybrid seeds, it is necessary to manually remove all panicles on the plants of the mother form before flowering. This work is labor intensive and must be done very carefully. Therefore, the widespread industrial use of corn hybrids was restrained for a long time. The discovery and use of CMS radically solved the problem of hybrid corn production. By means of returnable saturating crosses, sterile analogs of maternal lines were obtained, maize hybrids were transferred to a sterile basis, and they began to be cultivated without the cost of manual labor for breaking brooms.
The widespread use of hybrids in crops such as sorghum, onions, cucumbers, and tomatoes became possible only thanks to the discovery of CMS, since manual castration of flowers is practically impossible in them.
For the same reason, it was impossible to use heterosis in the main grain crop, wheat, although when crossing specially selected varieties, it manifests itself no less strongly than in corn. Now geneticists and breeders are working to create heterotic wheat hybrids on a sterile basis.
There are 2 types of male sterility (non-viability of pollen):
· Gene male sterility, or nuclear (determined by nuclear genes). It is caused by the genes of the nucleus and is inherited in accordance with the laws of Mnedelya.
· Cytoplasmic male sterility (determined by mtDNA mutations). It is caused by the interaction of nuclear genes and plasmagens, it is transmitted only through the maternal line
Male sterility was first discovered in 1904. Correns at the garden plant summer savory. To date, CMS has been established in more than 300 species. In most of them, it was obtained experimentally by interspecies and intergeneric hybridization. Plants with CMS of hybrid origin include flax, tobacco, wheat, rice, cotton, potatoes, tomato, barley, etc. .
Cases of spontaneous CMS were registered in 21 species, of which 19 are cross-grain and only 2 (broad beans and barley) are self-pollinated. In forage legumes and barley, forms with CMS were found in the crops of samples characterized by an open flowering type. Plants with spontaneous CMS include corn, sugar beet, ouk, carrot, sorghum, millet, rye, sunflower, etc.
Corn is one of the first plants in which cytoplasmic male sterility was discovered. This culture has 3 types of sterility: Moldovan, Texas and Brazilian. CMS is determined by the characteristics of the cytoplasm caused by mutations in some mitochondrial genes. CMS is associated with mtDNA mutations.
In maize with Texas-type CMS, the mitochondrial gene T –urf 13 was identified, which determines the formation of a 13 kD protein. The gene coding for this protein is formed from gene fragments of the original ribosome coding RNA.
In rice with CMS type CMS –bo (Chinsurah boro ǀǀ), the presence of an additional modified copy of the atp6 gene: β –atp6 in mtDNA, and in beans (Phaseolus vulgaris and Phaseolus coccineus species), the region of mtDNA causing CMS consists of two genes: orf98 and orf239. The orf239 gene product is found only in generative tissues.
In radishes with CMS at the 5 / –end, the orf105 gene is installed, which disrupts the structure of the reading frame of the atp6 gene and causes CMS.
The cytoplasm in sterile plants is designated cyt S, and in normal plants with fertile pollen, cyt N. Sterile cytoplasm manifests its effect only in combination with recessive chromosome genes in a homozygous state. If the gene is in the dominant state (Rf), then the specificity of the sterile cytoplasm is not manifested, and the plant forms fertile pollen. The genetic structure of plants with the Moldavian type of sterility, where the trait is due to the cytoplasm and a pair of genes, is as follows:
1. Cyt S rfrf sterile pollen
2. Cyt S RfRf fertile pollen
3. Cit S Rfrf fertile pollen
4. Cit N rfrf fertile pollen
5. Cit s Rfrf fertile pollen
6. Cyt S RfRf fertile pollen
The discovery of the CMS made it possible to use the phenomenon of heterosis in the production of crops such as sorghum, onion, tomato, sunflower, rice, etc., in which manual castration of flowers is required to obtain hybrids is almost impossible.
The Texas type of sterility was found to be associated with instability in helminthosporiosis. At present, seed production of corn hybrids is carried out on the Moldovan and Brazilian types of cytoplasm, but not on the Texas one.
45. Heterosis. Definition, discovery and its basic laws. Types of heterosis according to Gustafson.
Heterosis (from the Greek - change, transformation) - the phenomenon of superiority on various grounds of the first generation hybrids over the parental forms (the term heterosis was proposed by Schell in 1914)
For plants (according to Gustafson), there are three types of heterosis:
Somatic
Reproductive
Responsive
In somatic heterosis, the first hybrid generation surpasses the parental forms in plant mass. Reproductive heterosis determines the superiority of F 1 in seed productivity. And adaptive heterosis is manifested in the increased resistance of hybrids to unfavorable environmental factors.
The phenomenon of heterosis was first described by Kelreiter (1761 –1766), who studied tobacco hybrids. He made three interesting conclusions that were later confirmed:
The amount of heterosis depends on the degree of difference between parental forms
Hybrid vigor (heterosis) is of particular importance in evolution
From a practical point of view, hybrid power is of great value to arborists
Darwin in his work "the effect of cross-pollination and self-pollination in the plant world" showed the advantage of hybrids over non-hybrids, in particular in maize. Until now, a genetic theory of heterosis has not been created, because. many mechanisms in the manifestation of heterosis are unclear.
46. Hypothesis of overdominance, explaining the phenomenon of heterosis (or monohybrid heterosis).
This hypothesis, independently of each other, was formulated by Shell and East in 1908. Its essence lies in the fact that heterosis is the effect of the interaction of a heterozygous pair of genes.
The over-dominance hypothesis is distinguished from the first by two provisions:
· Both alleles are active;
It is impossible to fix (even theoretically) heterosis, i.e. to get a homozygote equal in effect to a heterozygote with overdominance.
A definite confirmation of this hypothesis was the discovery of the so-called monogenic heterosis. The following examples give an idea of this type of heterosis.
Additional action of alleles. In corn, as shown by Stadler (1942), there are multiple R? Determining the pigmentation of various parts of the plant. A plant heterozygous for these alleles is always more pigmented than a homozygous one:
R g R g - aleurone is colored, anthers are not colored
R r r r - aleurone is not stained, anthers are colored
R g r r - aleurone is colored, anthers are colored.
Alternative ways of synthesis. Suppose that the G 1 allele determines the production of red pigment at t = 27, and the G 2 allele at t = 10 0 C. In this case, in the absence of dominance, the G 1 G 2 genotype can synthesize red pigment at both high and low temperatures.
Synthesis of the optimal amount of a certain substance. In experiments with seedlings in barley, it was shown that dominant and recessive homozygous genotypes for the K gene (KK and KK) fix carbon dioxide at approximately the same rate, and in the heterozygous genotype (Kk) photosynthesis increased by ≈50%
In general, both of these hypotheses explain the same end result and therefore cannot be considered mutually exclusive.
47. Hypothesis of dominance, explaining the phenomenon of heterosis. Ways of fixing heterosis.
This hypothesis has a more complete name - the hypothesis of the interaction of favorable dominant factors. She explains heterosis by a combination of favorable dominant genes as a result of crossing and their suppression of harmful recessive alleles. Developed by American geneticist Johnson (1917).
The manifestation of heterosis when crossing inbred lines, according to this hypothesis, is the result of suppression of the action of harmful recessive genes by their favorable dominant alleles, because it is unlikely that in different lines all recessive genes were at the same loci.
"The contribution of dominant and recessive genotypes to the development of a quantitative trait"
P: aaBBccDDee × AabbCCddEE
1+2+1+2+1 = 7 2+1+2+1+2 = 8
It is also believed that the effect of heterosis is also influenced by the degree of interaction between dominant genes (epistasis * - the interaction of genes that affect the severity of the trait)
In general, it is customary to consider, according to this hypothesis, heterosis as any excess of indicators in hybrids over the midpoint between its parents.
Based on the theory of dominance, heterosis as a whole should be considered as the summation of the d values for each of the genes, contributing to the development of the trait in F 1: Heterosis = Σd
This formula must be modified if the parental forms are not completely homozygous for each of the polygenes.
In this case, it takes the form:
Heterosis in F 1 = Σ dy 2, where y is a factor representing the difference in gene frequency between the two crossed forms (y = 1 or 100% when the parent is homozygous for different alleles at each locus).
Since heterozygosity in F 2 decreases by 2 times, and heterosis in the second generation is reduced by 2 times: Heterosis in F 2 = Σ dy 2/2