Discussion on the main methods of plant seed breeding

Principle (1): Gene recombination (separating excellent traits or bringing all kinds of excellent traits together through gene separation, free combination or linkage exchange).

(2) Method: Self-pollination and continuous seed selection.

(3) For example:

It is known that tall stalk (D) is dominant for short stalk (D), rust resistance (R) is dominant for susceptible rust (R), and two pairs of traits are inherited independently. At present, there are two pure varieties: rust-resistant and susceptible to short stalks. It is necessary to cultivate new varieties with excellent characters through cross breeding.

How to operate: (see the right)

(1) F 1 is obtained by crossing pure wheat with high resistance to stem rust and dwarf rust;

② F 1 selfed to obtain F2;

(3) selecte F2 medium-short rust-resistant wheat and selfing to obtain F3;

(4) Leave the dwarf disease-resistant individuals without trait segregation in F3, and repeat steps (3) and (4) for trait segregation in F3.

(4) Characteristics: The breeding cycle is long, and it is necessary to constantly select the superior and eliminate the inferior in order to breed the required types.

(5) Description:

(1) This method is often used for:

A. between individuals of different varieties of the same species, as in the above example;

B between individuals of different species with close genetic relationship (in order to make offspring fertile, chromosome doubling is needed to obtain individuals who are allopolyploids), such as the cultivation of octoploid triticale and the hybridization of radish and cabbage.

(2) If an organism reproduces its offspring through sexual reproduction, it must choose a purebred with excellent traits to prevent the offspring from character separation; If an organism produces offspring through asexual reproduction, it only needs to obtain excellent traits, and purebred and hybrid do not affect the expression of offspring traits.

2. Mutation breeding

(1) principle: gene mutation

(2) Methods: Physical factors (such as X-ray, γ-ray, ultraviolet ray, laser, etc. ) or chemical factors (such as nitrous acid, diethyl sulfate, etc. ) is used to treat organisms, making them make mistakes in DNA replication during cell division, thus causing gene mutation.

(3) Example: Space breeding, obtaining high-yield penicillin strains.

(4) Characteristics: improve the mutation rate, create the mutation types needed by human beings, and select and cultivate excellent biological varieties from them. However, this breeding method is blind because of non-directional mutation.

(5) Description: This method is often used in microbial breeding and crop mutation breeding.

3. Haploid breeding

(1) principle: chromosome variation

(2) Methods: Haploid plants were obtained by anther culture in vitro, and then the number of chromosomes was artificially induced to double.

(3) For example:

It is known that tall stalk (D) is dominant for short stalk (D), rust resistance (R) is dominant for susceptible rust (R), and two pairs of traits are inherited independently. At present, there are two pure varieties: rust-resistant and susceptible to short stalks. It is required to cultivate new varieties with excellent characters through haploid breeding.

How to operate: (see the figure below)

(1) F 1 is obtained by crossing pure wheat with high resistance to stem rust and dwarf rust;

②f 1 anthers were cultured in vitro to obtain haploids;

(3) Treating haploid seedlings with colchicine to double chromosomes, and selecting individuals resistant to dwarf disease as the required type.

(4) Characteristics: Because all the obtained individual genes are homozygous, there is no trait segregation in self-bred offspring, so the breeding cycle is obviously shortened compared with cross breeding.

(5) Description:

① This method is generally applicable to plants.

(2) This breeding method is sometimes combined with cross breeding, and anther culture in vitro needs the support of tissue culture technology.

4. Polyploid breeding:

(1) principle: chromosome variation

(2) Methods: The germinated seeds or seedlings were treated with colchicine, so that the number of chromosomes in the cells was doubled, and the cells with doubled chromosome numbers continued to undergo normal mitosis, thus developing into polyploid plants.

(3) For example:

① Cultivation of triploid seedless watermelon (autotriploid cultivation)

Flowchart: refer to the picture on page 55 of the second volume of the compulsory textbook for senior two.

Description:

A. Why should diploid watermelon be pollinated after triploid watermelon seeds are planted?

Watermelon triploid plants can't form seeds because of the disorder of synapses during meiosis and can't form normal germ cells. However, after the diploid watermelon pollen is inoculated on the triploid plant, when the pollen germinates on the stigma, the enzyme system that converts tryptophan into indoleacetic acid is secreted into the ovary of the triploid watermelon plant, which makes the ovary synthesize a lot of auxin. Secondly, a small amount of auxin in diploid watermelon pollen can also diffuse into the ovary after pollination, and both sources of auxin can make the ovary develop and bear fruit (triploid seedless watermelon).

B. If diploid watermelon is the female parent and tetraploid watermelon is the male parent, that is, reverse hybridization will make the seed coat formed by integuments thick and hard, thus affecting the quality of seedless watermelon.

② Cultivation of octoploid triticale (allopolyploid cultivation):

Common wheat is hexaploid (AABBDD), and its somatic cells contain 42 chromosomes, belonging to the genus Triticum. Rye is diploid (RR), and its somatic cells contain 14 chromosomes, belonging to the genus Rye. It is generally difficult for two species of different genera to cross, but there are also a few common wheat varieties that contain hybridizable genes and can accept rye pollen. The filial generation after hybridization contains four sets of chromosomes (ABDR) and is sterile. Must be artificially doubled to produce offspring. After chromosome doubling, a single cell contains eight sets of chromosomes (AABBDDRR), which come from different genera and species, so it is called allooctoploid triticale.

(4) Features: The plants obtained by this breeding method have stout stems, large leaves, large fruits and seeds, and increased contents of nutrients such as sugar and protein.

(5) Description:

① This method is often used in plant breeding;

(2) sometimes with cross breeding.

(2) Breeding according to "engineering principle"

1. "Genetic Engineering" Breeding

Principle (1): DNA recombination technology (belonging to the category of gene recombination)

(2) Methods: According to people's wishes, the individual genes of one organism were copied and modified, and put into the cells of another organism to directionally transform the genetic traits of the organism. The operation steps include: extracting the target gene, combining the target gene with a vector, introducing the target gene into a recipient cell, detecting and expressing the target gene, etc.

(3) Examples: Escherichia coli strains, insect-resistant cotton, transgenic animals, etc. that can secrete human insulin were obtained.

(4) Features: strong purpose and short breeding cycle.

(5) Description: For microorganisms, this technology must be closely coordinated with fermentation engineering to obtain the products needed by human beings.

2. "Cell Engineering" Breeding

Somatic hybrid nuclear transfer of main plants

Methods Two kinds of somatic cells from different plants were fused into hybrid cells, and the hybrid cells were cultured into new plants. The operation steps include: removing cell wall by enzymolysis, inducing protoplast fusion by inducer, and tissue culture of hybrid cells. It is to transplant the nucleus of one organism into the enucleated egg cells of another organism, and then culture the cells into new biological individuals. The operation steps include: nuclear aspiration, transplantation to enucleated eggs, culture (possibly using embryo transfer technology) and so on.

Examples are "tomato and potato" hybrid plants, carp and crucian carp, nuclear transplanted fish, cloned animals and so on.

These characteristics can overcome the incompatibility obstacle in distant hybridization and greatly expand the range of parental combinations that can be used for hybridization.

It shows that this method must be supported by technical means such as plant tissue culture. This method is sometimes supported by technical means such as embryo transfer.

(3) Breeding by using plant hormones

1. Principle: Proper concentration of auxin can promote fruit development.

2. Methods: A certain concentration of auxin analogue solution was coated on the unpolluted pistil stigma, and the ovary developed into seedless fruit.

3. Example: Cultivating Seedless Tomatoes

4. Characteristics: Because auxin can promote fruit development and can't lead to the change of plant genotype, this variation type is not inherited.

5. Description: This method is suitable for plants.