Abstract: Haploid generally refers to individuals developed from gametes produced by organisms, and the number of chromosomes is half that of normal organisms. Haploid breeding is one of the important breeding methods in modern crop breeding. Haploid is induced by anther culture, so that its single chromosome doubles and becomes a homozygote with vitality and normal fruiting, thus cultivating new crop varieties. Since most plants are diploid, the advantage of this breeding method is that homozygous normal plants can be obtained quickly.
Key words: haploid genome haploid breeding sterility? Chromosome variation in shaking propagation year
High school biology textbooks focus on haploid and haploid breeding and their similarities and differences with other breeding methods. When it comes to this part of knowledge, there are many theoretical problems worth discussing. The main issues are discussed as follows:
1. Haploids all come from gamete development.
Usually, the haploid mentioned in textbooks comes from the development of male nucleus (parthenogenesis) or female nucleus (parthenogenesis), that is, from gametes. However, the development of male nucleus is generally completed under artificial conditions, such as haploid obtained by anther culture in vitro.
In fact, there are other sources of haploid in nature.
(1) spontaneous. This source is often related to polyembryony, and the most likely reason is the sudden change of temperature or the stimulation of heterogeneous and exotic pollen.
② False fertilization. That is, pollen or male nucleus stimulates unfertilized female gametes to produce haploid plants.
③ Semi-fertilization. Male and female gametes participate in embryogenesis, but do not fuse, thus producing chimera plants with parental origin.
In addition to the most commonly used anther and pollen culture in vitro, there are many other methods to obtain haploids artificially.
① Pollination with distant heterogenous pollen: Stirring the stigma to make the egg cells in the embryo sac develop into seeds;
(2) Weakening pollen pollination: pollen is pollinated after artificial storage for a period of time. Because of its weak germination ability, it can not complete normal fertilization, but it can make egg cells develop into seeds;
③ Chemical treatment: such as treating the stigma with 2,4-D, gibberellin and colchicine;
④ Pollen pollination by high-dose radiation;
⑤ Abnormal temperature treatment and mechanical stimulation of ovary;
⑥ Cross-pollinate related crops, and after a period of time, put the young embryos on the culture medium for in vitro culture. In the early stage of embryonic development, one chromosome disappears and haploid plants can be obtained.
It can be seen from the above facts that although some haploids are not directly developed from gametes, almost all haploids lose chromosomes from male gametes or female gametes for some reason during embryogenesis, that is, haploids only contain genetic material from one parent.
Second, the reasons why haploids can survive
Haploid organisms have half as many chromosomes as normal organisms. Chromosomes in a complete genome are a group of non-homologous chromosomes, which have different shapes and functions, but contain all the information that controls the growth, development, inheritance and variation of organisms. Haploid contains at least one complete genome, so it has genetic information of biological growth, development and genetic variation, and all required traits can be controlled by the expression of relevant genetic information.
3. Why are haploids common in plants but rare in animals?
Most plants have no sex chromosomes, most are monoecious, some are hermaphroditic, and only a few plants are determined by XY sex. Gametes in plant pollen generally contain a whole set of genetic information of the species, and there is almost no lack of any genes needed for growth and development. Haploid is easy to obtain under natural conditions or after in vitro culture, and it is easy to survive.
Most animals are hermaphroditic, with sex chromosomes and gametes often lacking some genes necessary for growth and development. Even if larvae can be obtained, they often cannot become adults. Just like bees and ants in animals, their sex is not determined by sex chromosomes, but by the number of "autosomes", so haploids can be produced. But why the haploids in bees and ants are male is still unknown. Of course, there are other artificially cultured haploid animals, but they often show many genetic defects. In nature, these haploid animals are difficult to survive.
4. Is haploid really sterile?
Usually, haploids contain only one genome. When meiosis forms gametes, normal gametes cannot be formed because there is no synaptic pairing of homologous chromosomes. Haploids with odd chromosome numbers are all the same.
So can even haploids in the genome produce normal gametes? The key is to see if there are homologous chromosomes. For example, haploid plants cultivated from the pollen of octoploid triticale contain four sets of chromosomes, but they cannot obtain normal gametes, that is, they are sterile. Another example is plant A with genotype AA, which can become a new species B (genotype AAAA) after chromosome doubling. If haploid plants (AA) are cultivated from B pollen, haploid plants are fertile.
In fact, a haploid containing only one genome is still fertile in theory. For example, if a haploid has only ***8 chromosomes in a genome, then the probability of producing a normal gamete from this haploid is (1/2)8, and it is generally infertile due to the extremely small probability. Therefore, the textbook only emphasizes that haploid is highly sterile, not completely sterile.
5. Haploid breeding can definitely shorten the breeding cycle.
The biggest advantage of haploid breeding method is that it can obviously shorten the breeding cycle, which is mainly relative to cross breeding. The main reason is that most of the new varieties obtained are heterozygotes (that is, using the advantages of * species) or dominant homozygotes. However, in order to obtain new varieties of recessive individuals, the time required for haploid breeding should not be shorter than that for cross breeding. If the plants of aabbdd genotype are used as parents to cultivate the offspring plants of AaBbDd, haploid breeding and cross breeding will take two years. That is, haploid breeding can significantly shorten the breeding time when there are dominant traits in breeding.
Six, haploid breeding results must be diploid or homozygous.
Haploid breeding is a process of selecting the desired varieties by anther culture in vitro and inducing chromosome doubling from the pollen of normal plants. Because most plants are diploid, the varieties obtained from haploid breeding must be diploid and homozygous. If the pollen comes from polyploid, there will be very different results. If the pollen of tetraploid plants (genotype Aaaa) is cultured in vitro, haploid plants have three genotypes, namely aa, aa and AA. Obviously, haploid plants with genotype Aa were induced, and the obtained plant genotype was AAaa, which was not homozygous or diploid.
Seven, haploid breeding process does not produce new genes.
Take the topic as an example. The following is the eighth question in the Biology Volume of the 2008 Guangdong College Entrance Examination:
The inappropriate method to improve rice varieties lacking some disease resistance is (? Shake)
A. Mutation breeding? Shake B. Haploid breeding? Shake C. Genetic engineering breeding? Shake D. cross breeding
The reference answer is B.
Some scholars think that this issue is not rigorous, and suggest to increase relevant restrictions. For example, only one breeding method is used to improve rice varieties lacking a certain disease resistance, and the inappropriate method is (? Shake it? Shake)
A. Mutation breeding? Shake it? Shake B. Haploid breeding? Shake it? Shake C. Genetic engineering breeding? Shake it? Shake D. cross breeding
Because of the addition of "only using the following breeding methods", there is no debate about whether haploid breeding can introduce genes. The answer is B.
So is it complete after adding restrictions? Actually, it is not. As far as haploid breeding is concerned, the anthers (pollen) of normal plants should be cultured in vitro. In the process of culture, sperm cells dedifferentiate to form callus, which has strong division ability, is prone to mutation and may produce new characters. One of the applications of mutant in the elective textbook "Practical Application of Plant Cell Engineering" in the third volume of senior high school biology new curriculum standard published by People's Education Press is the utilization of mutant. The original text is as follows: "In the process of plant tissue culture, because the cultured cells are in a constant meristem state, they are easily influenced by culture conditions and external pressures (such as rays and chemicals) to produce mutants. Mutants that are useful to people can be screened from these mutated individuals and then cultivated into new varieties. "
Eight, the principle of haploid breeding variation is only chromosome variation.
According to the basic flow of haploid breeding, the only heritable variation produced by this method is chromosome variation. But at present, many scholars believe that haploid breeding can be divided into narrow sense and broad sense. Narrow haploid breeding only refers to collecting anthers (pollen) from a plant (usually diploid) and finally obtaining homozygous plants; Haploid breeding in a broad sense also includes the process of obtaining plants that provide pollen (varieties that meet the production requirements), that is, the narrow process only involves chromosome variation, and the broad process includes gene recombination besides chromosome variation. According to the analysis in point 7, gene mutation may occur at some time in the generalized process.
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