Detailed Tutorial on the Latest Technical Methods of Corn Breeding
Seed is the most important means of production, and new varieties are the most core foundation of agricultural scientific and technological progress. Corn breeding and production practice show that 43% to 60% of the increase in corn yield is achieved through excellent varieties and their promotion. Therefore, breeding and promoting new corn varieties is of decisive importance for improving corn yield per unit area and comprehensive grain production capacity.
Characteristics of Corn Breeding
The most important characteristic of corn is natural cross-pollination. The field composition of a naturally pollinated population is in a highly heterogeneous state, and the genotypes of individuals are in a highly heterozygous state. This determines that in a naturally pollinated corn population, comparisons of phenotypes among plants are of little significance, and selection and rejection can only be correctly decided through a certain process of genotype selection. At the same time, because individual genotypes are highly heterozygous, phenotypic selection is unreliable. A large number of individuals must be subjected to testcrossing or progeny identification to confirm whether the phenotype is truly inherited. For these reasons, in the process of corn breeding, it is generally necessary to go through multiple generations of selection and comparison before a new inbred line or variety can be developed.
The mainstream of modern corn breeding is heterosis breeding. The basic approach is to first breed homozygous parent inbred lines, then cross the parent inbred lines to breed hybrids with strong heterosis. What is used in production is the heterosis of the F1 generation. The breeding of inbred lines requires not only that their own traits be excellent, but also that their combining ability be high. The selection of agronomic traits and combining ability of inbred lines is equally important and neither can be neglected. This greatly increases the difficulty of breeding and prolongs the breeding cycle. Moreover, although the traits of inbred lines are related to the traits of hybrids, there is still a gap. At the hybrid level, agronomic traits still need to be selected and identified, and at the same time the level of heterosis for major target traits must be compared, ultimately breeding an excellent hybrid variety.
Breeding of Corn Inbred Lines
1. Good agronomic traits
① Plant traits: mainly including plant architecture and lodging resistance. Plant traits are generally divided according to the posture of the leaves above the ear into compact type, semi-compact type, and flat type. According to plant height, they are divided into tall stalk, medium stalk, short stalk, semi-dwarf stalk, etc.
② Ear traits: ear traits are generally composed of ear type, kernel type, number of kernel rows, etc. There are many ways to classify ear type, the most obvious being long-ear type, thick-ear type, cylindrical type, conical type, spindle type, etc. Kernel type is divided into dent type and flint type and various transitional types. In inbred line breeding, it is best to give consideration to both long-ear type and thick-ear type selection. The long-ear type has more kernels per row, but fewer kernel rows. The thick-ear type has more kernel rows, but the number of kernels per row cannot be very large. The number of kernel rows in inbred lines is generally 10 to 20 rows. 12 to 14 rows is relatively moderate. The choice of kernel type should be determined according to the breeding goal. Generally, hybrids assembled from inbred lines tending toward the flint type have good commercial quality, while hybrids easily assembled from those tending toward dent type have relatively high yield and starch content. In addition, kernel size, kernel weight, and consistency of kernel color also need to be considered.
③ Stress resistance: there must be a certain degree of resistance or tolerance to major corn diseases and natural disaster conditions, so as to ensure the stability of seed production and the stable expression of heterosis. Uniformity and consistency: agronomic traits are required to be uniform and consistent in appearance, which is reflected in the genotype as basically homozygous.
2. High combining ability
Combining ability refers to the potential ability of hybrid parents to play a role in the heterosis of their hybrid progeny. The level of combining ability of an inbred line determines the yield-increasing ability and utilization value of the hybrid to be bred in the future, and is therefore the most important trait of an inbred line. Combining ability is divided into general combining ability and specific combining ability. The genetic basis of the former is the additive effect of genes derived from the parents, while the latter is determined by the non-additive effect of parental genes. The two are relatively independent genetically. Therefore, in the breeding of inbred lines, neither can be neglected.
3. Strong seed production performance
Whether a hybrid with high yield, good quality, and good resistance can be promoted or become a main planted variety depends to a very important extent on having excellent seed production performance, including strong germination ability and germination vigor of parent seeds, good seedling growth, easy stand establishment, and coordinated male and female flowering. The male parent sheds pollen well, has a large amount of pollen, and has a longer flowering period. The female parent has a long tassel stalk, easy tassel emergence, fast and uniform silking, good seed setting, uniform kernels, and high yield. The parents have low sensitivity to light and temperature, flowering is easy to coordinate, and seed production is easy to succeed.
4. Basic materials for breeding inbred lines
Fundamentally speaking, the strong heterosis of corn hybrids comes from sufficiently large genetic differences between parents. Therefore, the basic materials for breeding inbred lines must come from diverse sources and have a broad genetic basis. The sources of materials for line selection generally fall into 4 aspects: the first is local germplasm, including old local variety populations, or inbred lines derived from local varieties, etc. Under some special ecological conditions, there may be excellent resistance sources or excellent quality materials, and special attention should be paid to collecting them. The second is hybrids, mainly referring to hybrids used in production or introduced, and also including hybrids deliberately assembled from inbred lines for line selection, among which single-cross hybrids are the most widely used.
It is generally believed that using hybrids widely used in local or non-local production for line selection is a shortcut to breeding excellent inbred lines. These selected lines generally have good agronomic traits and relatively high combining ability. The third is synthetic varieties or recurrently improved populations. These populations generally have relatively high frequencies of excellent genes and recombination rates of excellent genotypes, but the degree of aggregation of favorable genes may not be as good as that of single-cross hybrids, and in the line selection process the selection quantity usually needs to be increased. The fourth is distant hybrid progeny. Corn has many wild relatives, which contain abundant excellent traits, but because interspecific hybridization is difficult, progeny are sterile or segregate severely, stabilization is slow, and direct use for breeding inbred lines is difficult, so it is rarely used now.
5. Methods for breeding inbred lines
① Conventional breeding method: this breeding method is to select individual inbred lines from a segregating original population, and after several generations of visual selfing selection, determine combining ability, and finally select excellent inbred lines. The problem with the conventional line selection method is that it is very difficult to define the scale of early selection. At present, some breeders do not plant selected ears as ear rows in early generations, but instead plant equal amounts of selected ears mixed together, and only in later generations do they strictly select individual plants to grow as ear rows for combining ability determination. This method effectively increases the number of selected genotypes and at the same time increases selection pressure on individual plants, which may help improve resistance and obtain target genotypes. When the breeding scale is not large, the conventional breeding method relies more on the breeder's experience.
② Haploid line selection method: its basic principle is to use naturally or artificially induced and cultivated haploid plants, 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 requires only two years to obtain homozygous inbred lines, shortening the breeding cycle compared with conventional methods, and is very attractive to breeders. However, haploid breeding omits the step of field selection, and traits such as resistance usually still need to be selected after doubling.
③ Radiation and chemical mutation line selection method: the radiation mutation line selection method uses radiation energy generated by radioactive isotopes or even cosmic rays to induce structural variation in chromosomes or allele mutation, and selects useful individuals according to phenotypic variation to breed inbred lines. The mutation mechanism of this method is mainly damage recovery, and favorable variation is discovered during the recovery process. Therefore, when selecting original materials, both a rich genetic basis and strong damage resistance should be considered. In the line selection process, attention should be paid to eliminating various deformed plants and unfavorable variations, and selecting healthy plants with obvious mutant traits. In recent years, this method has been transplanted by some people as space breeding or aerospace breeding.
④ Molecular line selection method: the so-called molecular line selection method is inbred line breeding carried out at the DNA level, including transgenic line selection and molecular marker-assisted line selection. Transgenic line selection is the use of genetic transformation technology to introduce some favorable exogenous genes from other animals and plants into inbred lines or hybrids, breeding excellent inbred lines with traits controlled by exogenous genes. This method is crucial for modifying certain target traits that do not exist in the corn gene pool. Molecular marker-assisted line selection first determines molecular markers tightly linked to certain excellent genes, then uses these molecular markers as indirect selection traits to select target traits, and finally breeds excellent inbred lines.
⑤ Improvement of inbred lines: after some inbred lines are developed, it is found that they have some shortcomings that must be overcome. After some inbred lines have been used for a period of time, it is found that some important traits have degenerated, or infection by new physiological races of diseases has occurred, or they are not adapted to the environment of a new promotion region. At this time, the shortcut to solving the problem is to modify individual traits of the original inbred line and breed a new improved line.
The most commonly used method for improving inbred lines is the backcross transfer method. In the backcross transfer method, the inbred line to be improved is used as the recurrent parent, and the donor parent of the target gene is used as the non-recurrent parent. After more than 5 generations of backcross improvement and 1 to 2 generations of selfing stabilization, a new improved inbred line can be bred. The effectiveness of backcrossing is related to the inheritance mode of the target trait. Generally, the backcross method is most effective for simple traits or traits controlled by a single gene and for cytoplasmic inherited traits, and less effective for complex traits. When selecting donor parents in the backcross method, attention should be paid to ensuring that the main target traits do not have linkage drag, and also to selecting excellent inbred lines with the same heterosis orientation, so as to prevent improving the original undesirable traits while bringing in new undesirable traits, as well as problems such as reduced combining ability. Backcross progeny should also be tested for combining ability, and among different sister lines those with high combining ability should be selected for use.
Breeding of Corn Hybrids
The breeding of hybrids is to assemble inbred lines or other excellent parent materials into hybrid combinations, and through testcross identification, variety comparison, and various variety comparison trials, breed strong-heterosis hybrids that meet the requirements of the breeding goal:
1. Breeding of inbred lines for hybrids: the breeding of inbred lines has already provided a large amount of reference materials and information for hybrid breeding. Before using a group of excellent inbred lines to breed hybrids, a relatively detailed understanding of the parents to be used is needed, and assembly among parents should follow certain principles: distant kinship and large geographical differences, for example, the assembly model of foreign lines x domestic lines has been widely recognized by domestic breeders. Types should differ and traits should be complementary. Differences among types usually reflect differences in genotypes, and trait complementarity is even more important, especially important resistance traits, which should exist in at least one of the parents. Agronomic traits and seed production performance should be good, which is extremely important for the application of new hybrids. Combining ability should be high, which is one of the most basic conditions, especially high combining ability for yield traits.
2. The hybrid breeding technology is simple: in assembling single-cross hybrids, in addition to breeders selecting and matching parents according to their own experience, the two most commonly used methods are generally:
① Diallel crossing method: number of single-cross combinations = P(P-1)/2, where P is the number of parents.
② Backbone line method: number of single-cross combinations = M×N, that is, inbred lines are divided into two groups, M and N. Possible crosses are made between the groups, with one group composed of backbone inbred lines and the other group composed of new inbred lines. This method is usually the most effective.