Breeding good varieties by hybridization methods, or using hybrid advantages, is called hybrid breeding. Hybridization, which can transfer an organism's genetic material from one population to another, is an important way to increase biological variability.
Hybridization of different types of parents can obtain a recombination of variant traits, and there may be a combination of excellent traits of both parents in the hybrid offspring, and even a combination of excellent traits of superparents (e.g., crossbreeding of Founder silver carp and Jiujiang red carp), of course, there may also be combinations of dominant traits of both parents that are not obvious, or disadvantage traits that are not present in both parents (e.g., crossbreeding of bighead carp and silver carp).
The breeding process is to select individuals who meet the breeding goals from the repeated types of hybrid offspring to further breed, and even obtain new varieties with excellent traits and stable traits.
Hybridization can increase the variability and heterogeneity of the offspring of hybrid species, synthesize the excellent traits of the parents, produce some new traits that the parents do not have, enable the offspring to obtain greater genetic improvement, the emergence of available hybrid advantages, and play a huge role in the improvement and production of fish varieties, which is one of the basic ways of fish breeding and is also an effective measure for the cultivation of high-quality fish species.
The most commonly used hybridization between varieties (refers to the hybridization between two varieties) is the most commonly used in hybrid breeding. The second is distal hybridization (hybridization between individuals of different species and genera).
Practice shows that hybridization between varieties, such as silver carp and bighead carp hybridization, cannot be popularized and applied in the production field, and the key is caused by the instability of the combined traits after hybridization.
Traits are not excellent traits, from the first-age fish breeding growth rate, bait coefficient and silver carp, bighead carp comparison is no difference, but there are obvious differences in the second-instar growth period, slow growth rate, small individuals, serious shadow. It can be seen that not all hybridizations can create good varieties.
Hybrid germplasm advantage refers to the phenomenon that the first generation of hybrids produced by the hybridization of two parents with different genetic compositions is superior to their parents in terms of growth potential, viability, fertility, stress resistance, yield and quality.
Hybrid germplasm dominance is the comprehensive performance of many traits, and the size of hybrid germplasm dominance often depends on the relative differences and complementarity between parental traits. In general, the greater the difference in kinship, ecological type and physiological characteristics, the advantages and disadvantages of the relative traits between the parents can complement each other; the stronger the heterosis advantage, the higher the degree of homozygote of the parents, and the more uniform the heterosis can be obtained.
Hybrid germplasm dominance is often expressed in economically meaningful traits, so hybridization that produces and utilizes hybrid advantages is often referred to as economic hybridization. At present, the most common application of fish hybrid breeding is economic hybridization, with the second generation of children began to gradually decline, if the second generation of children is allowed to self-inbrance or continue, let their generations of free mating, the result will be a gradual reduction in heterozygous, hybrid advantage tends to decline or even die.
At present, in the production of freshwater fisheries, the utilization of hybrid germplasm advantages has become one of the important measures to increase yield and improve quality. At present, the utilization of hybrid germplasm such as carp family, Lotus carp [lotus red carp (female) × Yuanjiang carp (male)], Feng carp [Xingguo jing carp (female) × scattered scale mirror carp (male)], Hibiscus carp [scattered scale mirror carp (female) × Xingguo red carp (male)] approved by the National Fine Breed Approval Committee can bring huge economic benefits, such as heterogeneous silver carp, which only grows much faster than the parents, and has strong stress resistance, strong disease resistance, high survival rate, high breeding yield, excellent quality and other economic advantages.
The choice of parents is the key, first of all, as far as possible to select the parents with good comprehensive traits, many advantages, few disadvantages or excellent traits that can complement each other, but also pay attention to the selection of parent hybrids with large ecological types and distant relatives, such as the heterogeneous silver carp is the Chinese Academy of Sciences (Institute of Biology) the earliest is the Fangzheng silver carp developed in the Shuangfeng Reservoir of Fangzheng County, Heilongjiang Province, and the Xingguo red carp in Jiangxi Province is the offspring of the heterogeneous triploid female nucleus developed by hybridization, and the growth rate has obvious advantages.
The hybridization method usually used is that after the parental determination, the hybrid combination method is directly related to the success or failure of breeding, and the hybridization methods such as singlebred, composite hybridization and backcrossing are usually used.
Hybridization (singlebred) between two varieties is denoted by x×y, and the offspring of their hybrid germplasm become a generation of hybrids, such as the germplasm of the Founder silver carp (female) × Xingguo red carp (male). Because it is simple and easy to operate, it is the most widely used in production, generally mainly using the first generation of hybrid germplasm, such as heterogeneous silver crucian carp and lotus carp.
Breeding methods for more than two varieties that have been crossed twice or more. If singlebring fails to meet the trait requirements expected by breeding, compound hybridization is often used, with the aim of creating primitive populations of hybrids with a rich genetic base, from which better individuals can be selected.
Composite hybridization can be divided into triplet, bibred and so on. Triscrossing is the rebreeding of a single hybrid with another and can be expressed as (x×y) ×y. For example, (pocketed red carp× Yuanjiang carp) × scattered mirror carp = triplet carp; bibred is a hybrid of two different monobred species, which can be expressed as (a×b) × (c×d) or (a×c) × (b×c). For example, (horned tilapia × nile) × (Mozambican × nile).
Hybrid offspring continue to rebreed to one of their parents to enhance the breeding method for a particular parent trait in the hybrid generation. Backcrossing breeding may be used when the purpose of breeding is to attempt to introduce one or more economic traits from a group b into another group a population. For example, mud carp has many excellent traits, but cannot tolerate low temperatures and needs genetic improvement. It can be used to tolerate low temperature Xianghua carp for multiple backcrossing, and the attention to the selection of backcrossing offspring must be focused on the target trait of cold resistance, so as to eventually breed a new species of mud carp with excellent cold resistance.
First of all, familiarize yourself with the reproductive habits of various fish. On the one hand, it is necessary to understand the habits of the fish's own reproductive maturity season, reproductive season, egg nature, sexual cycle, egg laying period for the protection of eggs and offspring, etc.; on the other hand, it is necessary to understand the requirements of fish for external conditions during the reproductive period, such as water temperature, light, flow rate, dissolved oxygen volume of water, substrate of spawning grounds, fish nests, feed, etc. Secondly, it is necessary to regulate the age of parental maturity, through staging, creating different external conditions and other means, it is necessary to make uncrossed parents meet in the reproductive season.
In the early stage of hybridization, when the sexual maturity and reproductive season are approaching, it is necessary to raise female and male fish in separate ponds to avoid self-mating. The fertilization method is determined according to the specific situation, and the hybridization between varieties generally adopts natural spawning and fertilization, and the artificial insemination is used for natural reproduction and hybridization.
Hybridization with different species of fish must be recorded or registered, including parental traits and related circumstances, such as hybridization method, hybridization time, fertilization rate, survival rate, etc., for reference and backup. After the fry are discharged from the pond, the test pool needs to be listed and registered, indicating the date of fertilization and the name of the parents. Management should be strengthened during breeding, and special attention should be paid to avoiding mixing between different impurities. If hybrids are accidentally released into natural waters such as rivers and lakes, it will cause serious consequences of hereditary mixing of natural populations and seriously affect the effective use of natural resources.
From hybridization to breeding and promotion of new varieties, it often takes 10 generations, so it is necessary to accelerate the breeding process. Ex situ generation can be used, greenhouse and artificial climate to generation can also be used, and the breeding process can be accelerated when the size of the required breeding base is not large.
When using the individual selection method, the selection generally starts from the second generation of the child, and the second generation of factors has the largest range of variation, and it is expected to select a satisfactory variant from it, and it is basically possible to judge whether there is a hope of breeding varieties by the second generation of the child. By the time of the fifth generation, most traits and lineages are fairly stable, and the identification of varieties can be carried out at this stage.
When using the group selection method, individual selection is generally not made before five generations, only group selection and polyculture are carried out. In the fifth to eighth generations of the sub-generations, individual selection, the establishment of families and the determination of varieties, the real selection of new varieties with excellent traits to lay a good foundation for the production of pollution-free fish.