| In recent years, the lack of germplasm resources has become the limitation of maize breeding. It has attracted the attention of breeders at home and abroad that relative single and vulnerability of germplasm resources. It has become the inevitable trend of breeding research that improvement and innovation of new germplasm.We found a dwarf mutant with natural variation in the process of developing common maize inbred lines M676 in 2007 winter. This mutant could be genetically stable, and the main performance of this mutant is the plant height was declined without separated. So we named it dm676. In this paper, the experiments of phenotypic description, genetic analysis, hormone response, tissue cytology observation, gene mapping, allelic analysis, and combining ability analysis were given to explain the biological and genetic characteristics of the dwarf mutant and evaluate its utilization value in breeding. The main results were as follows:1. The biological characteristics of dm676 was as follows.The average plant height of dm676 was 86.7 cm, which was lower than the normal M676 by 96.7cm; the average ear height of dm676 was 25.6 cm, which was lower than the normal M676 by 50.2 cm. The internodes length was significantly or very significantly shortened. The following internodes length of the ear of dm.676 was shorter than that of M676 by 49.3 cm, and the above section of the ear of dm676 was shorter than that of M676 by 26.4 cm. The internodes of mutant was shorted, and the mutant has Small blade spacing and dwarfing characteristics. Compared with M676, the variation of ears was small, so the mutant dm676 had certain research and application value.2. The genetic studies of dwarf mutants were analyzed. The population of F1, F2 and backcross population backcrossed to dm676 derived from the cross among dm676 and Yul193, Yu1193-9, Yu8954 and M676 were constructed, and the plant height were measured. The distribution of all the F2 population were in accordance with 3:1 by Chi square test. There was no significant difference between the positive and negative F1 strains, which proved that Dwarfing traits controlled by nuclear genes, and the distribution of all the backcross population backcrossed to dm676 were in accordance with 1:1. These all proved that the mutation gene is recessive single gene inheritance.3. The sensitivity test of exogenous hormone and cytological observation of stem tissue were studied. The dwarf mutant was treated with three exogenous hormones including gibberellins (GA3), growth hormone (IAA) and Brassinosteroids (BR) at different stages 35 days after maize seeding. The plant height at different period was measured and the data was analyzed. The results showed that three kinds of hormones could not restore the plant height of mutant dm676 to the level of wild type M676.Ear internodes and ear internodes up and under 1 ear internodes of M676 and mutant dm676 were sampled for the cytological comparison of paraffin sections. The results showed that the longitudinal ear cells of dm676 arranged more regular, which length was shorter than that of M676, and the longitudinal cells under 1 ear internodes of dm676 arranged disorder, which length was shorter than that of M676. So we speculated that the dwarf of dwarf mutant was cased by shorten of cell length of ear stem and 1 ear internodes.4.Fine mapping and allele test of dm676. F2 population from the cross between mutant dm676 and 18-599 was constructed. There was 2397 high plants and 835 dwarf plants including total 3232 plants. The separation ratio of high plants and low plants was 3:1, which in accord with 3:1.261 pairs of polymorphic markers were screened out by using 1171 pairs of SSR markers by extracting and detecting DNA of dwarf plants, some high plants and two parents. The DNA of recessive individual plant was genotyped by 261 polymorphic primers through PCR and gel electrophoresis band pattern analysis. So we mapped the gene dm676 on chromosome 1. Fifteen markers with clear genotype were selected from 41 pairs of polymorphic markers on chromosome 1. The gene was closely linked to umc1035 by analysis these markers. So ten markers flanking umc1035 were designed, including umc2569, umc1335, umc1709, umc1335, umc1924, umc2396, umc2236、umc1925, umc2237 and umc1486. Four markers including umc2396,umc1335. umc2569 and umc1486 had polymorphs between two parents.655 dwarf markers were genotyped by four markers. The molecular linkage map was constructed using mapmaker3.0 software, the gene dm676 was mapped to umc2396 and umc1486 with the genetic distance was 0.3 cM and 11.93 cM. In order to narrow down the mapping interval, eight SSR markers including umc1586, umc1492, umc1356, umc2238, umc1374, umc1556, umc1044 and umc1883was found. The markers such as umc2236. umc2238 and umc1356 had polymorphism between both parents.90 plants was selected from 670 dwarf plants and genotyped by markers umc1335, umc2236, umc2396, umc2238, umc1356 and umc1486. The results was analyzed by mapmaker3.0 software. Finally, the gene dm676 was mapped to the position between SSR umc2396 and umc1356 with a distance of 4.17Mb.Bioinformatics analysis showed that the mapping interval contained the dwarf gene br2. So the mutant dm676 was crossed with plants NA360 with gene br2. The plant height of F1 was significantly higher than that of the parents, which showed that gene dm676 and gene br2 were allelic gene with different variance. The plant height of F2 population was in accord with normal distribution with the range between 38 to 152 cm. The plant height of some plants in F2 population showed Obvious super affinity, which also showed that gene dm676 and gene br2 were allelic gene with different variance.5. The breeding potential of dm676 was analyzed. Cross combinations was designed according to genetic mating design according to NCⅡ when dm676, M676 and other inbred lines as parent. Two years combining ability analysis showed that the GCA for yield traits of dwarf mutant dm676 was significant positive effect. The GCA for 100 grain weight was significant positive effect in both years. The GCAs for plant height, ear height, ear row number were significant negative effects in both years. The GCAs for bald tip length, ear length were not inconsistent for two years. The GCA for yield traits of dwarf mutant dm676 were significantly higher than that of M676 in both years. Compared with other 8 tested lines, the GCA for yield traits of dwarf mutant dm676 was larger than that of 7 tested lines, while the GCA was smaller than that of 12HN145. Compared with M676, the mutant dm676 has a large breeding potential that not only increase the production but also decreased the plant height and ear height of cross combination designed. |
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