Corn breeding methods and precautions

Today I will tell you about the methods and approaches of corn breeding. Please take a good look:

1. Characteristics of corn breeding

The most important feature of corn is natural cross-pollination. The field composition of the natural pollination population is highly heterogeneous, and the genotype of the individual is highly heterozygous. This determines that in the natural pollination population of corn, the comparison of phenotypes between plants is not very meaningful, and a certain genotype selection process must be used to correctly decide on the selection; at the same time, due to the high heterozygosity of individual genotypes, phenotypic selection is unreliable, and a large number of individuals must be tested or offspring identified to confirm whether the phenotype is truly inherited. For these reasons, in the corn breeding process, it is generally necessary to go through multiple generations of selection and comparison to breed new inbred lines or varieties.

The mainstream of modern corn breeding is hybrid vigor breeding. The basic approach is to first select and breed homozygous parental inbred lines, then cross the parental inbred lines to breed hybrids with strong hybrid vigor. The hybrid vigor of the F1 generation is used in production. The breeding of inbred lines requires not only excellent traits, but also high combining ability. The selection of agronomic traits and combining ability of inbred lines is of equal importance and should not be neglected. This greatly increases the difficulty of breeding and prolongs the breeding cycle. Not only that, although the traits of inbred lines are related to those of hybrids, there is still a gap. At the hybrid level, agronomic traits must be selected and identified, and the heterosis levels of the main target traits must be compared to finally breed excellent hybrid varieties.

2. Breeding of Maize Inbred Lines

1. Good agronomic traits. Plant traits: mainly include plant type and lodging resistance. Plant traits are generally divided into compact, semi-compact, and flat types according to the posture of the leaves on the upper part of the ear; and divided into tall stalks, medium stalks, short stalks, and semi-dwarf stalks according to plant height.

Spike traits: Spike traits are generally composed of spike type, grain type, number of spike rows, etc. There are many types of spike types, the most obvious of which are long spike type, coarse spike type, barrel type, cone type, spindle type, etc.; grain type is divided into dent type and hard grain type and various transitional types. In the selection of inbred lines, it is best to take into account the selection of long spike type and coarse spike type. The long spike type has more rows of grains, but fewer rows of grains; the coarse spike type has more rows of grains, but the number of rows of grains cannot be very large. The number of rows of grains in the inbred line is generally 10 to 20 rows. 12 to 14 rows are more moderate. The choice of grain type should be determined according to the breeding goals. Generally, the hybrids of the inbred lines with hard grains have good commerciality, and the hybrids with dents are easy to produce with higher yields and starch content. In addition, the consistency of grain size, grain weight and grain color also needs to be considered.

Stress resistance: There should be a certain degree of resistance or tolerance to major corn diseases and natural disasters to ensure the stability of seed production and the stable development of hybrid vigor. Uniformity: The agronomic traits should be uniform in appearance, which is reflected in the fact that the genotype should be basically homozygous.

2. High combining ability. Combining ability refers to the potential ability of hybrid parents to play a role in the hybrid vigor of their hybrid offspring. The level of combining ability of inbred lines determines the yield-increasing ability and utilization value of future hybrids, and is therefore the most important trait of inbred lines. Combining ability is divided into general combining ability and special combining ability. The genetic basis of the former is derived from the additive effect of the genes of the parents, while the latter is determined by the non-additive effect of the genes of the parents. The two are relatively independent in genetics. Therefore, both cannot be neglected in the selection and breeding of inbred lines.

3. Strong seed production performance. Whether high-yield, high-quality, and resistant hybrids can be promoted or become main varieties depends on excellent seed production performance. This includes strong germination ability and vigor of parent seeds, good seedling growth, easy seedling protection, and coordination between male and female; good pollen dispersion of the male parent, large amount of pollen, and long flowering period; long male spike stalk of the female parent, easy male tassel, fast and uniform silking, good firmness, uniform grains, and high yield; small light and temperature response of the parents, easy coordination of flowering period, and easy success in seed production.

4. Basic materials for selecting inbred lines. Fundamentally speaking, the strong heterosis of corn hybrids comes from the sufficiently large genetic differences between the parents. Therefore, the basic materials for selecting inbred lines must be diverse in source and have a broad genetic basis. There are generally four sources of selection materials: the first is local germplasm, including old local variety groups, or inbred lines derived from local varieties. Under some special ecological conditions, there may be excellent resistance sources or excellent quality materials, which should be collected with special attention. The second is hybrids, which mainly refer to hybrids used in production or introduced, and also include hybrids used for selection with inbred lines for purposeful combination. Among them, the most widely used is single cross. It is generally believed that the use of hybrids widely used in local or foreign production is a shortcut to breeding excellent inbred lines. These selected lines generally have good agronomic traits and relatively high combining ability. The third is a population of comprehensive varieties or cyclical improvements. These populations generally have a higher frequency of excellent genes and a higher recombination rate of excellent genotypes, but the degree of aggregation of favorable genes may not be as good as that of single cross varieties. In the process of selecting lines, it is usually necessary to increase the amount of selection. The fourth type is the offspring of distant hybrids. There are many wild relatives of corn, which contain a wealth of excellent traits, but due to the difficulty of interspecific hybridization, the offspring are infertile or severely separated, and the stability is slow. It is difficult to directly use them to breed inbred lines, and they are rarely used now.

5. Methods for selecting inbred lines.

(1) Conventional breeding method. This breeding method is to select individual inbred lines from a separate original population, and after several generations of visual self-pollination selection, the combining ability is measured to finally select excellent inbred lines. The problem with the conventional selection method is that it is difficult to define the scale of early selection. At present, some breeders do not plant in rows in the early generations, but mix the selected ears in equal amounts. Only in the late generations do they strictly select individual plants to plant in rows and measure the combining ability. This method effectively increases the number of selected genotypes, while increasing the selection pressure on individual plants, which may help improve resistance and obtain target genotypes. When the breeding scale is not large, conventional breeding methods rely more on the experience of breeders.

(2) Haploid selection method. The basic principle is to use naturally or artificially induced and cultivated haploid plants to obtain homozygous diploids through artificial chromosome doubling or natural doubling, and then select excellent individual plants from them to become inbred lines. Haploid breeding generally only takes two years to obtain homozygous inbred lines, which shortens the breeding cycle compared to conventional methods and is very attractive to breeders. However, haploid breeding omits the step of field selection, and traits such as resistance usually require double selection.

(3) Radiation and chemical mutagenesis selection method. Radiation mutagenesis selection method uses the radiation energy generated by radioactive isotopes or even cosmic rays to induce structural variation or allele mutation in chromosomes, and selects useful individuals to form inbred lines based on phenotypic variation. The mutagenesis mechanism of this method is mainly based on damage recovery, and favorable variations are found during the recovery process. Therefore, when selecting raw materials, it is necessary to consider both rich genetic basis and strong damage resistance. In the process of selecting lines, it is necessary to pay attention to eliminating various deformed plants and unfavorable variations, and select healthy plants with obvious mutant traits. This method has been transplanted by some people into aerospace breeding or space breeding in recent years.

(4) Molecular selection method. The so-called molecular selection method is the inbred line selection carried out at the DNA level, including transgenic selection method and molecular marker-assisted selection method. Transgenic selection is to use genetic transformation technology to introduce some beneficial exogenous genes from other animals and plants into inbred lines or hybrids to breed excellent inbred lines with exogenous gene-controlled traits. This method is crucial for transforming certain target traits that are not available in the corn gene pool. Molecular marker-assisted selection is to first determine the tightly linked molecular markers of certain excellent genes, then use these molecular markers as indirect selection traits to select the target traits, and finally breed excellent inbred lines.

(5) Improvement of inbred lines. After some inbred lines are bred, some shortcomings that must be overcome are found. After some inbred lines are used for a period of time, some important traits are found to degenerate, or new disease physiological subspecies are infected, or they are not adapted to the environment of the new promotion area. At this time, the shortcut to solving the problem is to transform individual traits of the original inbred lines and breed new improved lines.

The most commonly used method for improving inbred lines is backcrossing. Backcrossing is to use the improved inbred line as the recurrent parent and the target gene donor parent as the non-recurrent parent. After more than 5 generations of backcrossing and 1-2 generations of self-pollination, a new improved inbred line can be bred. The effectiveness of backcrossing is related to the inheritance mode of the target trait. Generally, backcrossing is most effective for simple traits or traits controlled by a single gene and cytoplasmic genetic traits, and has poor effects on complex traits. When selecting donor parents for the backcrossing method, attention should be paid to the main target traits not to have linkage drag, and attention should also be paid to selecting excellent inbred lines with the same hybrid advantage orientation to prevent the improvement of the original undesirable traits from bringing about new undesirable traits and causing problems such as decreased combining ability. The combining ability of backcross offspring should also be measured, and the application of high combining ability should be selected from different sister lines.

3. Breeding of corn hybrids

The breeding of hybrids is to combine inbred lines or other excellent parent materials into hybrid combinations, and select strong superior hybrids that meet the breeding objectives through test cross identification, variety comparison and various variety comparison tests.

1. Inbred line breeding for hybrids. The breeding of inbred lines has provided a lot of reference materials and information for hybrid breeding. Before using a group of excellent inbred lines to breed hybrids, it is necessary to have a detailed understanding of the parents used and to combine the parents according to certain principles: distant kinship and large geographical differences. For example, the combination mode of foreign lines x domestic lines has been widely recognized by domestic breeders; there are differences in types, and the traits must be complementary. The differences between types usually reflect the differences in genotypes. Trait complementarity is more important, especially important resistance traits, which must exist in at least one of the parents; good agronomic traits and seed production performance, which are extremely important for the application of new hybrids; high combining ability, which is one of the most basic conditions, especially the combining ability of yield traits.

2. The breeding technology of hybrids is simple. In addition to breeders selecting parents based on their own experience, there are two most commonly used methods for single-cross breeding: one is the round-robin method: the number of single-cross combinations = P(P-1)/2, where P is the number of parents; the other is the backbone line method: the number of single-cross combinations = M×N, that is, the inbred lines are divided into two groups, M and N. The possible hybridization is done between the groups, one group is composed of the backbone inbred lines, and the other group is composed of the new inbred lines. This method is usually the most effective.