What are the characteristics of hybrid breeding?

Two or more genetically different varieties or types of artificial hybrids, resulting in hybrid offspring due to genetic recombination, hybrids will appear in a variety of variant types, according to the breeding goals, to select comparative identification, breeding into new varieties called cross-breeding.

Hybrid breeding is the most widely used, easy to receive results. It is characterized by significant variation, the progeny separation is large, and can select and breed new varieties of super parentage. For example, the new mint variety Haixiang 1, the new groundnut variety Beijing 1, Beijing 2, Beijing 4, etc. is to use cross-breeding method to breed out.

Crossbreeding can be divided into inter-species hybridization and distant hybridization and the use of hybrid advantage according to the parentage of the proximity of the parentage. Usually referred to as cross-breeding, refers to the sexual hybridization between varieties.

I. Genetic principles of crossbreeding

The reason why crossbreeding is able to create new varieties has the following aspects:

(a) genetic recombination

The genetic material of organisms is mainly in the chromosomes of the nucleus of the cell, and the process of sexual crossbreeding is a combination of chromosomes in the nucleus of the parental sex and the exchange of the process of combining and exchanging chromosomes, and at the same time, the chromosomal genes were recombination, which results in variation in the offspring. The more relative traits involved in hybridization, the richer the genetic variation and the greater the chance of acquiring new types.

(ii) Gene Accumulation

Some favorable traits can be enhanced through crossbreeding. For example, if two parents that are not susceptible to disease are crossed, the offspring are likely to have disease-resistant individuals.

Second, the methods and procedures of crossbreeding

(a) selection of parents

Selecting good parents is the key to improving the effect of crossbreeding, the main principles of selecting parents are:

1. Both parents should have more advantages, and one of the parents must have a better quality and productivity. The more advantages and fewer disadvantages the parents have, the better.

2. When choosing parents, it is best to have a parent that can be adapted to the local natural environment, so that the cultivated varieties are generally able to ensure a good and stable yield.

3. Selection of geographically distant or ecological type of parent hybridization, is conducive to breeding a wide range of adaptability, yield potential of excellent varieties.

(ii) the way of hybridization

The way of hybridization includes the selection of several parents, which varieties as the parent or parent and their sequence in the hybridization program. Only a reasonable configuration of the hybrid combination, in order to show the desired traits, unwanted shortcomings to overcome, in order to more effectively breed new varieties, commonly used hybridization methods are the following:

1. Single cross

Only two parents for hybridization is called a single cross. If A x B is an orthologous cross, then B x A is an inverse cross. Single cross is the simplest type of hybridization in crossbreeding. Among the two parents, the parent that is better adapted to local conditions is generally used as the mother, and the other parent is used as the father.

2. Repeat crosses

More than two crosses between two or more parents are called repeat crosses. There are many types of double crosses depending on the number of parents used and the method of crossbreeding, and in each type of hybrid progeny, the proportion of the genetic composition of each parent is different. For example, double-crossing:

But triple-crossing: A × B → hybrid progeny × C → hybrid generation, the composition of the genetic material of A and B each accounted for 1/4, while C accounted for 1/2.

Recrossing should master the principle of better overall traits, adaptability, and a certain degree of fertility of the parents on the last cross, so that the last parent's genetic material in the hybrid progeny to occupy a larger proportion, so as to breed a better overall traits, adaptability, and some of the yielding parent in the last cross. In order to facilitate the breeding of varieties with better overall traits.

3. Backcrossing

After crossing two varieties, the hybrid offspring and one of the parents to repeat the cross is called backcrossing, backcrossing can be carried out once, can also be carried out several times. The variety used for repeated crosses is called the "rotating parent" and the other variety is called the "non-rotating parent". Both parents can be used as rotating parents. The proportion of genetic material in the offspring of the rotating parent grows larger and the proportion of the non-rotating parent grows smaller, so that a large number of the traits of the rotating parent are retained, and some of the good traits of the non-rotating parent are retained in the offspring of the hybrid through selection.

If the transferred trait is controlled by a recessive gene and cannot be selected in the backcrossed hybrid, it can be allowed to self-cross once after each backcross, and from the segregating progeny, select the plants that need the transferred trait and backcross again. The backcrossing method can also be used for variety improvement.

4. Multiple-parent hybridization

That is, selecting multiple varieties or types of pollen mixed pollination on the stigma of the mother, according to the principle of fertilization selectivity and multiplicity, the hybrid progeny has a high viability and strong adaptability; due to the fertilization of multiple parents, the hybrid plasticity is very large, so the multiple-parent hybrid also has the effect of composite hybridization.

5. Polymerization hybridization

is a variety of varieties, first two parents of the hybrid (single cross), the next year (or the next generation) and then the single hybrid hybrid two to two hybrid (double cross), and then the next generation, in the double hybrid hybrid hybrids between a variety of ways.

6. Random hybridization

Self-crossing in a mixed group of good varieties or self-crossing lines of selected plants, self-crossing seeds into the line, and then in the line of progeny of all possible crosses, the hybrid seeds of the various groups of mixed planting, the establishment of a new group. Repeated self-crossing and hybridization can lead to reorganization of gene groups, and then new varieties can be bred by group selection or hybrid selection. This method is long and costly, but produces a high level of varieties.

(C) hybridization technology

1. Basic information to be understood and mastered before hybridization

(1) Understanding and regulating the flowering period

Only by understanding the flowering period of the parents, can we try to make them meet at the same time. The method of regulating the flowering period can be used to delay or advance the flowering period of hybrid parents by staging the sowing of seeds or regulating the number of hours of light.

(2) Knowing the pollen life span and storage conditions

If the pollen life span is short, then it is necessary to crossbreed quickly, and the pollen life span varies a lot from species to species. The pollen of Dihuang still has a 28% germination rate after 5 days at room temperature, and the fruiting rate is still high after 1 month in the refrigerator to pollinate the stigma. Ginseng pollen can be kept at room temperature for 3-4 days. Astragalus pollen stored at room temperature for a week loss of germination power, stored in the refrigerator for a month still have 67% germination rate. Dryness, low temperature and less oxygen are conducive to prolonging pollen life. Generally under natural conditions, the pollen life of self-pollinated plants is shorter than that of cross-pollinated plants.

(3) Understanding stigma vitality and duration

Hybridization should be carried out when the stigma vitality is the strongest, the determination of stigma vitality is generally used staged pollination method, the highest rate of fruitfulness of the pollination period for the stigma vitality of the period. Tests have proved that the stigma of medicinal plants is suitable for pollination, ginseng at the time of flowering, astragalus in the corolla ≥ calyx, flowers want to open when not open, the ground in the flower 1-2 days, hairy flower digitalis in the flowering of the stigma bifurcation.

2. Operation techniques of hybridization

(1) Selection of inflorescences to organize flowers

Selection of robust inflorescences, the number of flowers to be retained in each inflorescence varies with the different types of medicinal plants, such as dihuang, yuanhu, beimu, etc., it is appropriate to retain about 5 flowers in each inflorescence, and ginseng, American ginseng, etc., may be retained in the range of 10-20 flowers. In addition, it also depends on how much material, if the material is more, you can choose more inflorescences, less flowers per sequence, so that the hybrid nutrients are sufficient, the grain is large and heavy, so that the hybrid is robust.

(2) de-masculinization bag

The parent plant in the pollen is not dispersed before the de-masculinization, and immediately put on a cellophane bag, as a parent plant to organize the flowers should also be bagged to isolate. De-sexing generally use pointed tweezers, after making a plant should be sterilized tweezers with 70% alcohol to kill the pollen, so as not to bring unwanted pollen to the stigma of the next plant, the formation of pseudohybridization.

(3) pollination

Demasculinized and bagged female stigma maturity, the parent pollen will be taken in a small petri dish, dipped in pollen with a brush on the stigma or tweezers up just pollen pollen capsule directly in the stigma a few times, so that the pollen falls on the stigma, and then bagged, hanging tags, record the name of the parents, the date of the de-masculinization and pollination, the number of hybridized flowers, and so on. Seeds were harvested at maturity, fruits of each combination were counted, and saved and sown by hybrid combination.

(D) Selection methods and procedures for hybrid progeny

Hybrid progeny, due to the complexity of the genetic basis, have traits segregated from the F2 generation, and must be selected through successive directional selection in order to stabilize the heredity gradually. Depending on the pollination and reproduction methods of the plant, different selection methods can be used.

1. Lineage method

This is the most widely used method of crossbreeding, which uses a single-plant selection method. That is, the hybrid seed sown in combination, from F2 (single-cross) or F1 (double-cross) segregation generation to choose a good single plant, planted into the line, its offspring become the strain, and later in the good strain to choose a single plant, until the selection of the strain is good and neat and consistent. These neat and uniform strains become strains. In the selection process, each generation has a systematic numbering, breeding varieties can be traced back to its various generations of kinship, so it is called genealogy. The whole process of cross-breeding work consists of the following different experimental beds:

(1) Parental nursery

Parental material for planting hybridization, the same combination of parents to be planted next to each other, if a variety needs to be combined with a number of varieties with the combination, but also should try to be planted close to each other, and so that the flowering period to meet.

(2) hybrid nursery

Species hybrid progeny, the first generation of single-crossing does not occur in the segregation, do not choose a single plant; the first generation of double-crossing due to the occurrence of segregation should be selected as a single plant. Hybrid from the F2 generation (double cross from the F1 generation) began to occur strong trait separation, the emergence of different trait combinations of monocots, which is the hybrid of the greatest range of variation in a generation, so the planting of the individual to try to be more, and to give a larger area of nutrients and a better environmental conditions, so that a variety of traits can be fully expressed. The selection of the F2 generation is the most important, the merit of the selected single plant will have an impact on the subsequent generations, so it is necessary to carefully observe, first determine a good combination, and then select a good single plant within a good combination.

(3) Selection of seed nursery

From the F2 generation of good single plants selected into the hybrid F3 generation, F3 between the rows of traits have more obvious differences in the rows of plants are still separated, and only a very few rows of the performance of the more consistent. Therefore, F3 should focus on selecting good strains, and then select good single plants from the selected good strains. When selecting strains, it is preferable to select more plant coefficients and fewer single plants within each strain. Whether or not a hybrid combination has the hope of breeding a new variety can basically be determined at this point. If the F3 generation is not good strains, then the hope of breeding varieties is not very good, so in a good combination of appropriate selection of strains, the combination is too poor to be eliminated.

(4) Identification nursery (strain primary comparison test)

Planting of new strains upgraded in the selection nursery or strains selected from the previous year's identification nursery. Instead of selecting single plants in the identification nursery, the selected lines are compared to observe the stability of their traits and yield performance. Generally, the sequential arrangement of the intercomparison method is used, and a control is set up for every 5 or 10 plots, and repeated 2-3 times. After identification of good lines can be upgraded to varietal comparison test.

(5) Pinbi nursery

This is the last important part of breeding, it is in a larger area closer to the production conditions, more accurate yield comparison, and its comprehensive traits, cultivation techniques, such as comprehensive appraisal, sure that the new varieties. At the same time, multi-point demonstration, regional trials, production trials, etc.. Therefore, the test should be precise, comprehensive, detailed and reliable.

(6) regional trials

The cultivation of new varieties of medicinal plants, should also be carried out in the relevant areas of the regional trials to determine the appropriate promotion of the region of the good seed; some medicinal plants in the region, the breeding of varieties can be used in the local application, not necessarily for regional trials.

2. Mixed method

The genealogy method from the separation of generations (F2 or F1) from the beginning of the selection of a single plant, planted into the strain, the procedures are complicated, the workload is large, and from the F2 to F4 generation, many traits have not yet stabilized, but also to continue to isolate, thus increasing the workload of the strain to continue to choose. The hybrid method can greatly simplify the procedures.

The breeding program of the hybrid method is: F1 generation of planting and selection of the same genealogy method. F2 to F4 or F5 generation is not a single plant selection, but will be selected in the combination of mixed harvesting and sowing. When the F4 to F6 generations are basically genetically stable, and the pure individuals in the population reach about 80%, a single-plant selection will be started to plant a strain, and thereafter the procedure will be carried out according to the genealogy method.

3. Group selection method

This method is improved on the mixed method, so it is also called the improved mixed selection method. Although the mixed selection method simplifies the procedures, mixed planting can make the hybrid group under the action of natural selection, some traits to adapt to the natural conditions and cultivation conditions of the direction of development, but some other traits may be weakened, such as early maturity, fertilizer resistance, high yield, etc.. For example, early-maturing types may drop grains due to untimely harvesting, resulting in fewer progeny, and productive types may not develop sufficiently due to poor competitiveness. The group selection method can carry forward the advantages of the genealogical method and overcome the shortcomings of the hybrid method.

The specific practice of group selection method is: in the same combination of F2 generation, according to the contemporary performance of some of the main traits such as fertility, disease resistance, plant height, tuber size and shape and other characteristics of a number of types, according to different types of plants and types of mixed harvest and sowing, to F4 or F5 generation of selection of a single plant, and the subsequent breeding program with the genealogy method.

Medicinal plants are propagated and pollinated in different ways, their genetic characteristics are different, and thus there is a difference in the selection and breeding methods of hybrid progeny. Self-pollinated plants have hermaphroditic cells originating from the same body, which produces homogeneous combinations of conjugates, so the phenotypes are more consistent among individuals within the population. Two varieties for hybridization, the progeny through continuous self-crossing and segregation selection, with the increase of generations, the number of pure individuals gradually increased and quickly converge to purification, generally to the F6 generation no longer separate. At the same time does not reduce the viability due to self-crossing, so self-pollinated medicinal plants hybrid progeny selection and breeding of multiple monoculture selection using genealogical method.

Heteropollinated medicinal plants, such as rhubarb, astragalus, yanhousuo, etc., their progeny are heterozygous combinations, and the genotypes and phenotypes of the individuals in the group are very different, so the selected good individuals will also be segregation phenomenon. In order to separate out the inferior plants without reducing the viability, so the selection of heterogamous pollinated medicinal plants combines self and heterogamy, using the rotational selection method.

Asexual reproduction of medicinal plants such as dihuang, yuanhu, beimu, xuan shen, atractylodes macrocephala, maitake, mint, etc., which are reproduced with the nutritive organs of the mother, performs the same as the mother. But they can sexually reproduce under certain conditions, in the process of sexual reproduction, autoflowering, heteroflowering, often heteroflowering pollination are available, the population is heterozygous, the original genetic basis is more complex, the first generation of hybrids after the sexual hybridization of the segregation phenomenon. But the favorable variation and hybrid advantage of the hybrid generation can be fixed through asexual reproduction. Therefore, this kind of medicinal plants use the combination of sexual reproduction and asexual reproduction, which is essentially a special way to utilize the advantages of hybrids.

The Institute of Medicinal Plants Resources Development, Chinese Academy of Medical Sciences, carried out the seed selection of Ginseng in 1975. Scrophularia ningpoesis Hemsl is commonly used in asexual reproduction, but also sexual reproduction, the test shows that the ginseng is a heterogamous pollinated plant, from the ginseng plant seeds for the natural hybridization of the seeds within the species. 1975 sowing natural hybrid seeds, 1976 to get more than 3,000 seedlings in the fall, from which the 120 better single plants, propagated into asexual lines. 1978 from the natural hybridization of the seeds, 1976 to get more than 3,000 plants, from which 120 better single plants, propagated into the asexual line. In 1978, 20 strains were selected from 120 asexual strains, and their tuber yield was over 300g/plant, among which 6 strains were over 650g/plant. In 1978-1980, after the comparative test of strains, with Zhejiang Ginseng as the control, the area of the plot was 6m2, and the plot was replicated for three times (two times replicated in 1978), and the result was that the 6 strains had higher yield than the control. Yield increase, of which No. 3, No. 9 yield increase of 36-74%, determined by t, the difference reaches a significant level. No. 3 in Jiangsu Haimen trial planting 0.026mu, harvest 35kg, equivalent to 1347kg per mu, a significant increase in yield than the general mu yield of 600kg.

The above is an example of intra-species hybridization combined with asexual reproduction.

The breeding of the new varieties Beijing No. 1 and No. 2 is an example of the combination of inter-variety hybridization and asexual reproduction.

The breeding of new varieties of Yuanhu is an example of the combination of interspecific hybridization and asexual reproduction (see distant hybrid breeding).

Three, the remote hybridization breeding

The so-called remote hybridization refers to different species, genera, and even between different families of hybridization, sometimes in the geographic distribution of the distance, ecological type of large differences between the different types, varieties of hybridization is called ecological remote hybridization. Distant hybridization is more difficult, once the success of the results are also more prominent. However, distant hybridization often appears not mating, hybrid offspring infertility, sterility and strong, long-term separation, to bring difficulties in distant hybridization.

(A) The causes of mismatchlessness in distant hybrids and ways to overcome them

The causes of mismatchlessness mainly include genetic disorders (e.g., difficulties in chromosome pairing), reproductive segregation (including temporal and spatial segregation, segregation of floral structures and pollination methods), and physiological incongruity of sexual factors, etc. The causes of mismatchlessness in distant hybrids are as follows.

Methods to overcome mismatchability mainly include vector method, controlled pollination method (e.g., in a large amount of distant parent pollen, add a small amount of maternal pollen to stimulate the stigma, so that it is easy to accept the distant parent pollen), artificial treatment of the stigma method (shorten the stigma or applying the stigma mucus of the parent on the stigma of the female parent), asexual proximity, chromosome doubling, in vitro cultivation of zygote pollination, selecting the appropriate parental and combination methods, etc.

(2) The reasons for the premature death of distant hybrids and how to overcome them

The main reason for the premature death of distant hybrids is that the nuclei of the parental cells do not combine well. Sperm only form endosperm with the polar nucleus, but not with the egg, so the seed embryo is not fully developed, or sperm do not combine with the polar nucleus, although the seed embryo does not have endosperm to supply nutrients to the seed embryo, resulting in embryo death in the middle of the process.

The reason why hybrids are not easy to bear fruit or have low fertility is that the chromosome numbers and structures of different species are different, and the chromosomes of male and female gametes cannot be paired during the development process, so they cannot undergo normal meiosis.

The main methods to overcome the hybrid abortive or unfruitful are: doubling the chromosomes, backcrossing (generally the hybrid as the mother), prolonging the hybrid fertility (such as asexual reproduction to save the plant), pharmaceutical treatment (smearing the stigma spraying the plant, etc.), and strengthen the hybrid cultivation (petri dish germination with nutrient soil fine seedling) and other methods. Cultivation of isolated embryos or immature seeds of distant hybrids by tissue culture is also an important means to overcome the difficulty of seedling emergence of distant hybrids or the stoppage of embryo development in the middle of the process. For example, the Institute of Specialty Products of the Chinese Academy of Agricultural Sciences used tissue culture to cultivate immature seeds of interspecific hybrids of mussels and obtained hybrid plants.

(C) the selection of distant hybrids

The characteristics of the separation of distant hybrids are different from those of ordinary hybrids, which are separated in F1 or F2, but the separation of distant hybrids is not regular, and some of them are separated in the F2 generation or F3 generation, and even in the stabilized system, there is a strong separation again. Therefore, the progeny of distantly related hybrids should not be eliminated too early. Due to the complexity of the type of separation of distant hybrids, long time, low fruitfulness, poor seed fullness, so the selection and cultivation of the workload is very large, should pay attention to the following points:

1. Hybrids should have a large number of groups in the early generations, should not be eliminated too early in the hybrid progeny of the moment is not prominent.

2. Several selection methods can be combined and applied in the method, the hybrid material is less, the early generation can be used genealogical method, hybrid material is more, early can be used in the hybrid method, to be basically stabilized can be used in the method of single-plant selection. In order to introduce and isolate the hoped-for good traits, methods such as self backcrossing can be applied flexibly.

3. In order to overcome the problem of low seed fructification rate and poor fullness of distantly related hybrids, according to the experience of breeding in crops, selecting large seeds can increase the frequency of diploid plants in the population, and correspondingly increase the fructification rate. In advancing the process of hybrid generation, combined with double-crossing and backcrossing, enhanced breeding and selection can effectively improve the fruiting rate and seed fullness.

4. Combining distant hybridization and asexual propagation is a shortcut for medicinal plant breeding.

Most of the important species of medicinal plants are perennial and have long reproductive cycles. With distant hybridization and sexual reproduction, it takes a long time for the progeny to stabilize and separate wildly. However, some species of important medicinal plants can reproduce asexually, can fix the hybrid advantage without separation, the Chinese Academy of Medical Sciences Institute of Medicinal Plant Resource Development to cultivate a new type of Yuanhu hybrid is a typical example.

Zhejiang Yuanhu high content of active ingredients, reproduction coefficient is larger (a plant can grow tubers 5-8), but small. Northern wild Yuanhu tuber large, adapted to the northern environment, robust growth, but low reproduction coefficient (a plant can grow 2-3 tubers) in order to synthesize the advantages of the two kinds of Yuanhu, so that the hybrid, the resulting hybrids planted, from 1981 onwards, the tubers of the plants began to asexually propagate, after nine years, to 1987 has been obtained asexual lineage of sixth-generation, the best hybrid 9 has been propagated from the A solid seedling was propagated to 650 tubers weighing 3296 g. The total alkaloids of Yuanhu and dl-tetrahydropalmatine (Yuanhuetin), which has the strongest analgesic effect, were higher than those of the genuine Zhejiang Yuanhu (Table 8-1).

Table 8-1 Comparison of alkaloid content between hybrid Yuanhu and Zhejiang Yuanhu

The results of the comparison between the hybrid Yuanhu and Zhejiang Yuanhu under the same conditions in 1987 showed that the reproduction coefficient of the hybrid Yuanhu was close to that of the Zhejiang Yuanhu, and the weight increase times exceeded that of the Zhejiang Yuanhu, and the yield per unit area was 2.88 times of that of the Zhejiang Yuanhu, and the weight of a single tuber was 4-5 times of that of the Zhejiang Yuanhu (Table 8-1). Hu 4-5 times (Table 8-2).

Table 8-2 Comparison of yield between hybrid Yuanhu and Zhejiang Yuanhu

Note: There is no duplication due to the limitation of hybrid plant.

Considering that the plant of Zhejiang Yuanhu is small, the spacing between plants can be appropriately reduced, under normal circumstances, Zhejiang Yuanhu uses 35-50kg of seeds per mu, in order to give full play to the characteristics of the individual in the test, but also in order to be consistent with the spacing between plants of the hybrid Yuanhu, the amount of seeds used in Zhejiang Yuanhu is only equivalent to 31.7kg per mu, even if the amount of seeds used in Zhejiang Yuanhu is increased by a factor of two, the yield is also increased by a factor of two, to reach the yield of the hybrid Yuanhu. Even if the amount of seeds used in Zhejiang Yuanhu is doubled, the yield is also doubled, reaching 318.3kg per mu, and the hybrid Yuanhu can still increase the yield by 44% compared with Zhejiang Yuanhu, which shows that the breeding method of combining distant hybridization and asexual reproduction is effective.

Four, the utilization of hybrid advantage

The so-called hybrid advantage, refers to the two genetic composition of the parents of the hybrid hybrid first generation, in the growth, vitality, fertility, yield and quality than the parents of the phenomenon of superiority. "Self-fertilization is degraded, hybridization is beneficial", which is a characteristic of heterogamous pollinated plants, and hybridization is also beneficial for self-pollinated plants. Therefore, the use of hybrid advantage is an important means of increasing yield and improving quality of medicinal plants.

Any organism that reproduces sexually, regardless of whether it is an intraspecific or interspecific hybridization, the hybrid offspring are advantageous. Although the hybrid advantage as early as in the 18th century in the middle of the plant breeder's extensive attention, but the real application to the production of the last four or five decades. 30 years the United States first in the application of maize, and later extended to other crops. China is the first to complete the rice "three lines" supporting the earliest application of rice hybrid advantage of the country. "Three lines" supporting hybrid rice, an average yield increase of more than 50kg per mu, the use of asexual reproduction to fix the role of hybrid advantage has been shown in the breeding of medicinal plants.

(a) the performance of hybrid advantage

1. fast growth rate. The nutrient organs of the hybrid are well developed, the plant is tall, and the rate of organic matter accumulation and total yield exceeds that of the parent.

2. Strong reproductive organs, large embryo, high fruiting rate, high seed yield, and the ability of seedling topsoil, seedling resistance and plant fertility are enhanced.

3. The quality is also superior to the parent.

(2) Genetic causes of hybrid dominance

The causes of hybrid dominance have not yet been unanimously concluded, and there are currently three hypotheses:

1. Dominant Factor Hypothesis

The central idea is that the favorable traits of the parents are controlled by the dominant genes, and the unfavorable traits are controlled by the recessive genes, and that the dominant genes are concentrated in the hybrids after the crosses are made, and that the cumulative effect of the dominant genes will be more effective. After the cross, the dominant gene is concentrated in the hybrid, and due to the cumulative effect of the dominant gene, the suppression of the recessive gene by the dominant gene at each locus and the interaction of the non-allelic genes, the F1 generation of the hybrid has a strong advantage.

2. Hyperdominant hypothesis

Also known as the allelic heterozygosity hypothesis. The core of this hypothesis is that an allele point, not only has dominant and recessive differences, but also can be differentiated into many smaller alleles with different physiological effects. Through hybridization, heterogeneous combinations of more subtle alleles with different effects can increase interactions between genes, thus allowing many traits to outperform the respective homogeneous combinations of the parents and show heterozygous dominance.

3. Life mechanics

This is Darwin through a lot of research concluded that self-pollination is harmful and heterogamy is beneficial. Heterogamous pollination parent genetic basis of the disparity, the contradiction is prominent, the interaction increases, so the advantage is strong, and vice versa is weak.

The above assumptions have some truth, but they are limited and one-sided. In recent years, domestic and foreign scholars have conducted extensive research on the physiological and metabolic characteristics of hybrids, which will help further reveal the reasons for the formation of hybrid dominance.

(C) Estimation of hybrid dominance

The following methods are commonly used:

1. Mean dominance

That is, the hybrid generation (F1) is more dominant than the average value of the two parents (P1, P2), which is also known as the mean dominance.

2. True hybrid dominance

that is, the hybrid F1 generation than the better of the two parents of a parent's advantage, so also known as super-parental dominance.

3. Competitive hybrid advantage

that is, the advantage of hybrid F1 over the control (the best local promotion varieties). This advantage has value in production.

(D) the utilization of hybrid advantage

The method of utilizing hybrid advantage in production varies according to the mode of propagation and pollination of medicinal plants.

1.asexual reproduction of medicinal plants as long as the hybrid generation through hybridization, select the strong hybrid advantage of a single plant asexual reproduction can be bred into new varieties.

2. Sexually reproduced medicinal plants, depending on the pollination method.

(1) self-pollinating plants As a result of long-term self-pollination, the genes are basically pure, can be obtained through hybridization between varieties of hybrid advantage, but self-pollinating plants and male and female flowers, to produce a large number of hybrid seeds, must be studied to solve the problem of the method of de-masculation. There are three ways to solve the problem, namely, artificial desexing, chemical desexing and the use of male sterile lines.

(2) heterogamous pollinated medicinal plants, the genetic basis is more complex, although you can use inter-species hybridization to gain advantage, but because the parent gene is not pure, the advantage of the F1 generation is limited, should first cultivate self-fertilizing lines, and then hybridization, in order to get the strong advantage of the F1 generation. Cultivation of self-inherited lines is generally bag self-crossing, but some medicinal plants self-crossing does not bear fruit, such as the groundnut, Yuanhu, Astragalus, etc., available haploid breeding technology to cultivate self-inherited lines. So far, the medicinal plants that have been cultivated haploid plants are dihuang, coix lacryma, wolfberry, ginseng, aconite, and these haploids can be obtained by simply doubling their chromosomes to obtain pure self-inherited lines.