Nuclear male sterility is very common in plants, and its sterility is controlled by nuclear genes. According to the dominant and recessive relationship between sterile gene and fertile gene, it can be divided into recessive genic male sterility and dominant genic male sterility. Genic male sterile plants mainly come from natural mutation. According to public reports, so far, there are four nuclear sterile materials found in cabbage, three of which are male sterility controlled by recessive genes (Nishi Heping Gang,1958; Rundfeldt， 1960； Etc., 1984), and the rest 1 is male sterility controlled by dominant gene (Fang et al., 1995). These four genic male sterile materials are all derived from natural mutation and are all sterile types controlled by single gene locus. Dominant genic male sterility mutation is a unique type independently discovered in China and has been successfully applied to the utilization of heterosis in cabbage (Fang et al.,1997; Fang Zhiyuan et al., 1997).
1. Single gene recessive nuclear sterile germplasm resources
The genotype of genic male sterile materials controlled by a pair of recessive genes is msms, and the genotype of fertile plants is MSms or MSMS. The fertile plants of these two genotypes are their restorers, while recessive genic male sterility can not find a typical maintainer. Only 1/4 sterile plants can be obtained from the self-bred progeny of F 1 hybrid fertile plants (MSms), or nearly 1/2 can be obtained from the progeny by backcrossing F 1 hybrid fertile plants with sterile plants. Through backcross screening, a male sterile "dual-purpose line" with a stable sterile plant rate of about 50% was obtained, also known as dual-purpose line. Fang Zhiyuan et al. (1983) discovered a male sterile material 83 12 1ms controlled by a recessive single gene from the natural population of cabbage. The leaves of sterile plants are normal at low temperature, the sterile flowers are small but the flowers are normal, the stamens are degenerated without pollen, the pistil is normal, the nectaries are developed and rich, and the seed pods are normal after pollination. If this material is used to prepare hybrids, 50% of fertile plants must be pulled out, which is labor-consuming and time-consuming, so it is difficult to apply it in practice (Fang Zhiyuan et al., 200 1).
2. Single-gene dominant nuclear sterile germplasm resources
Dominant nuclear male sterility controlled by a single gene is usually the product of single gene mutation. Because the frequency of dominant single gene mutation is extremely low, it is difficult to find this material, and no complete restorer line or complete maintainer line can be found. The genotypes of sterile plants are MSms and MSMS, but the genotypes of sterile plants exist in theory, but there is generally no way to obtain them. The genotype of the fertile plant is msms, the fertile plant is mated with the sterile plant, and the sterile plant of the offspring is separated from the fertile plant 1: 1. Therefore, a male sterile "dual-purpose line", also known as dual-purpose line B, with a sterile plant rate of about 50% can be obtained only through cross test screening. Fang Zhiyuan et al. obtained the male sterile plant 79-399-3 from the natural population of the original cabbage material 79-399 in 1979, which was first reported publicly in 1993. The sterile plants of this material have good economic characters, normal leaf color, no yellowing at low temperature, normal flowering of sterile flowers, stamen degeneration, completely normal pistil and developed nectaries and flowers. The sterile material 79-399-3 was systematically studied from the aspects of genetics, cytology, molecular biology, breeding and utilization of sterile lines. Through research, it has been confirmed that the major genes controlling sterility in this material are a pair of dominant nuclear genes, which are sensitive to temperature under certain genetic background and environmental conditions, indicating the existence of modified genes (Fang Zhiyuan et al., 1993). Some male sterile plants of dominant male sterile materials are environmentally sensitive, that is, under certain genetic background and environmental conditions, some sterile plants can produce fertile trace pollen, and dominant male sterile plants with homozygous sterile genes can be separated from the self-bred offspring of such trace pollen sterile plants. At present, more than 70 dominant sterile homozygous plants and more than 10 excellent dominant sterile lines with good combining ability, sterile plant rate and sterility degree reaching 100% have been identified and screened, and have been used to prepare hybrids.
(2) Cytoplasmic Male Sterile Germplasm Resources
Spontaneous mutation of cytoplasmic male sterility is rare in nature. Cytoplasmic male sterility (CMS) is usually considered as a genetic trait controlled by the interaction between nuclear male sterility gene and cytoplasmic male sterility gene. That is to say, only when the nuclear sterile gene and cytoplasmic sterile gene coexist, will it lead to male sterility. This sterility can not only screen maintainer lines, but also find restorer lines and realize the "three lines" matching. Since Ogura(Ogura, 1968) discovered the first source of cytoplasmic male sterility in radish, many types of cytoplasmic male sterility from different sources have been discovered and cultivated in cruciferous crops at home and abroad (Liu et al., 200 1), among which Ogu CMS (radish cytoplasmic male sterility) and Polcms (Polma) are the most studied and widely used. Since 1970s, China has mainly introduced the cytoplasmic sterile germplasm resources of cabbage from abroad. The main introduced materials are Ogu CMS and CMS line CMSN7809 1 of black mustard cytoplasmic cabbage, among which Ogu CMS sterile source is mainly used. The cabbage research group of Vegetable and Flower Research Institute of Chinese Academy of Agricultural Sciences introduced Ogu CMS from different sources three times, namely CMS-SR 1, CMS-SR2 and CMS-SR3 (Fang Zhiyuan et al., 2004).
1. Cytoplasmic sterile material CMSN7809 1
CMSN7809 1 was introduced from the Netherlands on 1980. Medium-early maturity, subglobose. At low temperature, the sterile plants grow normally and the leaf color does not turn yellow. The stamens of sterile flowers degenerate without pollen, the pistil is normal, and the artificial pollination is medium, but most sterile flowers are semi-open, and the nectaries are small or absent, which affects the fruit setting under the conditions of insect pollination and free pollination. In the progeny of sterile materials, the rate of sterile plants remained between 33.7% and 60%. Therefore, this material is difficult to be used in the actual seed production of cabbage.
2. Cytoplasmic Male Sterility of Radish
Ogu CMS was discovered by Kokura in Japanese radish breeding farm on 1968. This material is completely sterile. A large number of experiments have proved that its sterility is controlled by cytoplasmic gene and two pairs of recessive nuclear genes * * * *, and the male abortion is complete, and the sterility degree and sterile plant rate are 100%. Sterility is very stable and is not affected by environmental conditions. When this sterile source was first transferred to Brassica vegetables such as cabbage, due to the genetic distant effect, the cytoplasmic genetic material was not in harmony with the nuclear genetic material, resulting in the defects of yellowing leaves, few nectaries and abnormal pistil in the transferred offspring (Bannerot, 1977). Among them, the yellowing of leaves seriously hindered photosynthesis, which made the plant grow slowly, delayed maturity and reduced yield. At the same time, due to nectary degeneration and pistil deformity, the pollination rate is low in natural state. Therefore, this material has not been used in actual cabbage seed production.
3. The cytoplasmic male sterile line SR 1 of radish cytoplasmic cabbage type.
1978, the Institute of Vegetables and Flowers of Chinese Academy of Agricultural Sciences introduced three radish cytoplasmic cabbage sterile materials, namely CMSR 1409, cmsr141and cmsr14/3. Among them, CMSR 1409 is early-maturing spherical, and the other two are late-maturing flat-headed. The sterility of these three materials is very stable, and all cabbage materials can be their maintainers. However, the leaf color of their plants, especially the heart leaves are severely yellowed at low temperature, the nectary is underdeveloped, and the pistil is abnormal, which affects their growth rate and normal fruit setting. Therefore, this material is also difficult to be used in the actual seed production of Chinese cabbage.
4. Improvement of CMS line CMSR2 in radish cytoplasmic cabbage.
Foreign scholars such as the United States and France obtained improved OugCMS without yellowing at low temperature at seedling stage through protoplast fusion. Institute of Vegetables and Flowers, Chinese Academy of Agricultural Sciences 1994 Improved CMS materials CMSR295 1 1 and CMSR29556 were imported from Cornell University in the United States. They are all late-maturing, with irregular economic characters, but their sterility is very stable, and all cabbage materials are its maintainers. Compared with the three radish cytoplasmic sterile materials introduced from 1978, the biggest improvement of these two sterile materials is that the leaf color of the plants does not turn yellow at low temperature, but there are still many problems in flowering and fruiting. Most flowers are semi-open, many flowers have abnormal pistils, underdeveloped nectaries and quite a few pod deformities.
5. Improve CMS CMS CMSR3 of radish.
It is an improved radish cytoplasmic sterile material introduced from the United States by Institute of Vegetables and Flowers, Chinese Academy of Agricultural Sciences 1998. Among them, CMSR3625 is early-maturing spherical, and CMSR3629 is late-maturing cabbage. The sterility of these two materials is also very stable, and all cabbage materials can maintain their sterility. Compared with the two radish cytoplasmic male sterile materials introduced by 1994, its outstanding advantages are that most sterile flowers can open normally, and the pistils are almost all normal. The dead buds of the two materials are about 20% at the early flowering stage and gradually decrease at the late flowering stage, which has good economic characteristics and combining ability. This material has a good application prospect. In the hybrid combinations with them as female parents, several combinations showed early maturity and disease resistance in autumn, but the uniformity was not good, indicating that sterile materials needed to be transferred through one-step backcross.
To sum up, there are six main sources of cabbage male sterile materials studied and utilized at present, and their main characteristics are shown in table 10-6.
Table 10-6 Sources and main characteristics of several cabbage male sterile materials