The latest course of Maize breeding techniques and methods

Seeds are the most important means of production, and new varieties are the core foundation of agricultural scientific and technological progress. Maize breeding and production practice show that 43% to 60% of the increase in maize yield is achieved through excellent varieties and their popularization. therefore, the cultivation and promotion of new maize varieties play a decisive role in improving the level of maize per unit yield and the comprehensive production capacity of grain.

Characteristics of Maize breeding

The main characteristic of maize is natural cross-pollination, the field composition of natural pollination population is in a highly heterogeneous state, and the individual genotype is in a high heterozygous state, which determines that in the population of natural pollination of maize, the comparison of phenotypes among plants is of little significance, and the choice can be correctly decided through a certain process of genotype selection. At the same time, because of the high heterozygosity of individual genotypes, the phenotypic selection is unreliable, so a large number of individuals must be tested or identified in order to confirm whether the phenotype is really inherited. For these reasons, in the process of maize breeding, it is generally necessary to go through multi-generation selection and comparison in order to develop new inbred lines or varieties.

Heterosis breeding is the mainstream of modern maize breeding, the basic way is to select homozygous parent inbred lines, and then cross parent inbred lines to select hybrids with strong heterosis. The heterosis of F1 generation is used in production. The breeding of inbred lines requires not only good characters, but also high coordination ability. It is of equal importance to the selection of agronomic characters and combining ability of inbred lines and should not be neglected. This greatly increases the difficulty of breeding and prolongs the breeding cycle. Moreover, although the characters of inbred lines are related to those of hybrids, there is still a distance between them. At the level of hybrids, agronomic characters should be selected and identified, and the heterosis levels of main target characters should be compared. finally, a good hybrid variety was bred.

Breeding of Maize inbred Lines

1. Good agronomic characters

① plant characters: mainly include plant type and lodging resistance. The plant characters are generally divided into compact type, semi-compact type and peaceful spreading type according to the extended posture of the upper leaves of the ear. According to plant height, it can be divided into tall stem, middle stem, dwarf, semi-dwarf and so on.

② panicle traits: panicle traits are generally composed of panicle type, grain type, number of rows per ear and so on. There are many kinds of panicle types, the most obvious are long spike type, thick spike type, cylinder type, cone type, spindle type and so on. The grain type can be divided into horse tooth type and hard grain type and their various transition types. It is best to give consideration to the selection of long panicle type and thick panicle type in the breeding of inbred lines. The number of grains per row of long panicle type was more, but the number of grain rows was less. The number of grains per row of thick panicle type is more, but the number of grains per row can not be many. The grain number of inbred lines is generally 10-20 rows. Lines 12-14 are more moderate. The selection of grain type should be determined according to the breeding objectives. generally, the hybrids of inbred lines with hard grain type have good commercial quality, and the hybrids with partial horse teeth are easy to produce higher yield and starch content. In addition, the consistency of grain size, grain weight and grain color also need to be considered.

③ stress resistance: it should have a certain resistance or tolerance to the main maize diseases and natural disaster conditions to ensure the stability of seed production and the stable play of heterosis. Neatness and consistency: agronomic characters are required to be neat and consistent in appearance and basically homozygous in genotype.

2. High combining ability

Combining ability refers to the potential ability of hybrid parents to play a role in the heterosis of their hybrid progenies. the combining ability of inbred lines determines the yield-increasing ability and utilization value of hybrids in the future, so it is the most important character of inbred lines. combining ability can be divided into general combining ability and special combining ability. The genetic basis of the former comes from the gene additive effect of parents, while the latter is determined by the non-additive effect of parent genes. They are relatively independent in heredity. Therefore, the two should not be neglected in the breeding of inbred lines.

3. Strong seed production performance.

Whether the hybrids with high yield, high quality and good resistance can be popularized or become the main varieties, a very important condition is to have excellent seed production performance, including strong germination ability and germination potential of parent seeds, good seedling growth, easy to protect seedlings, and coordination between male and female. The male parent has good pollen distribution, large amount of pollen and long flowering period. The female parent has long tassel stalk, easy heading, fast and neat silking, good fruiting, uniform grain and high yield. The response of parents to light and temperature is small, the coordination of florescence is easy, and seed production is easy to be successful.

4. Basic materials for breeding inbred lines.

Fundamentally speaking, the strong heterosis of maize hybrids comes from sufficient genetic differences among parents. Therefore, the basic materials for breeding inbred lines must come from various sources and have a wide genetic basis. There are four sources of selection materials: the first is local germplasm, including the old local variety population, or inbred lines from local varieties, etc. There may be good resistance sources or good quality materials under some special ecological conditions, so special attention should be paid to collection. The second kind is hybrids, which mainly refers to the hybrids applied or introduced in production, and also includes the hybrids used for line selection with inbred lines, of which the single cross is the most widely used.

It is generally believed that the selection of hybrids widely used in local or remote production is a shortcut to develop excellent inbred lines, which have good agronomic characters and high combining ability. The third is the comprehensive species or recurrent improved population, the frequency of good genes and the recombination rate of good genotypes in these populations are relatively high, but the degree of aggregation of favorable genes may not be as good as that of single cross, so it is usually necessary to increase the amount of selection in the process of line selection. The fourth is the offspring of distant hybrids. There are many wild related species of maize, which are rich in excellent characters, but because of the difficulty of interspecific hybridization, serious sterility or segregation of offspring and slow stability, it is difficult to select inbred lines directly, so it is rarely used now.

5. the method of selecting inbred lines.

① conventional breeding method: this breeding method is to select individual inbred lines in a separate original population, and then determine the combining ability after several generations of visual inbred selection, and finally select excellent inbred lines. The problem with the conventional system selection method is that the scale of early selection is difficult to define. At present, some breeders did not divide the ear row in the early generation, but mixed the same amount of selected ears, and strictly selected a single plant into ear row in the late generation to determine the combining ability. This method effectively increases the number of selected genotypes and increases the selection pressure on a single plant, which may be helpful to improve resistance and obtain target genotypes. In the case of small breeding scale, conventional breeding methods rely more on the experience of breeders.

② haploid selection method: its basic principle is to use natural or artificial induction and cultivation of haploid plants through artificial chromosome doubling or natural doubling to obtain homozygous diploid, and then select excellent individual plants to become inbred lines. Haploid breeding generally takes only two years to obtain homozygous inbred lines, which shortens the breeding cycle than conventional methods and is very attractive to breeders. However, haploid selection omits the step of field selection, and double selection is usually required for traits such as resistance.

③ radiation and chemical mutagenesis selection method: radiation mutation selection method is to use radiation energy produced by radioisotopes and even cosmic rays to induce chromosome structural variation or allele mutation, and select useful sports inbred lines according to phenotypic variation. The mutagenic mechanism of this method is mainly based on damage recovery, and favorable variations are found in the process of restoration, so it is necessary to take into account the rich genetic basis and strong anti-damage ability in the selection of original materials, and pay attention to eliminate all kinds of malformed plants and unfavorable mutations in the process of line selection. select healthy plants with obvious mutation characters. This method has been transplanted into space breeding or space breeding by some people in recent years.

④ molecular line selection method: the so-called molecular line selection method is inbred line selection at the DNA level, including transgenic line selection method and molecular marker-assisted line selection method. Transgenic line selection is to introduce some favorable foreign genes of other animals and plants into inbred lines or hybrids with the help of genetic transformation technology to produce excellent inbred lines with traits controlled by foreign genes. this method is important for transforming some target characters that are not available in maize gene pool. Molecular marker-assisted selection is a closely linked molecular marker determined by some excellent genes at first, then these molecular markers are used as indirect selection traits to select the target traits, and finally excellent inbred lines are bred.

Improvement of ⑤ inbred lines: some inbred lines found some shortcomings that must be overcome after being bred. Some inbred lines found that some important characters degenerated after being used for a period of time, or new disease races occurred, or did not adapt to the environment of the new extension area. At this time, the shortcut to solve the problem is to transform the individual characters of the original inbred lines and breed new improved lines.

The most commonly used method of improved inbred lines is backcross breeding, which takes the improved inbred lines as recurrent parents and the target gene donor parents as non-recurrent parents. After more than 5 generations of backcross improvement and 1-2 generations of self-cross stability, a new improved inbred line can be bred. The effect of backcross is related to the genetic mode of target traits. General backcross method is most effective for simple traits or traits controlled by single gene and cytoplasmic genetic traits, but not for complex traits. In the selection of donor parents, backcross should pay attention to the selection of good inbred lines with the same heterosis orientation without linkage encumbrance, so as to prevent the improvement of original bad characters and the decrease of combining ability. The combining ability of backcross progenies should also be determined, and the application with high combining ability should be selected in different Zi-Mei lines.

Breeding of Maize hybrids

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

1. Breeding of inbred lines of hybrids: the breeding of inbred lines has provided a large number of reference materials and information for hybrid breeding. Before using a group of excellent inbred lines to select hybrids, it is necessary to have a detailed understanding of the parents used and to match them according to some principles: distant genetic relationship and great geographical differences. For example, the combination model of foreign lines and domestic lines has been widely recognized by domestic breeders. There are differences in types, characters should complement each other, the differences between types usually reflect the differences of genotypes, and character complementarity is more important, especially the important resistance traits, at least in one of the parents. The agronomic characters and seed production performance are good, which is very important for the application of new hybrids. The combining ability should be high, which is one of the most basic conditions, especially for yield traits.

2. The breeding technique of hybrids is simple: in addition to the parents selected by the breeders according to their own experience, there are two most commonly used methods:

① rotation method: the number of single cross combinations = P (Pmur1) / 2, where P is the number of parents.

② backbone line method: the number of single cross combinations = M × N, that is, inbred lines were divided into M and N groups. Possible crosses were made between groups, one of which was composed of backbone inbred lines and the other was composed of new inbred lines. This method is usually the most effective.