| Most important agronomic traits in crops are quantitatively inherited. Improvement of these quantitative traits is one of the major objectives in crop breeding programe. Since maize whole genome has been sequenced completely, map-based cloning is becoming one of the important methods in QTL cloning. NIL construction of major QTL and fine mapping is the precondition of map-based cloning. In this stuty, NILs for two QTL of 100-grain weight (qGW1 and qGW3) and one NIL for QTL of popping fold (qPF1) were constructed using Marker-assisted selection (MAS) according to the QTL identified in F2:3 population, respectively. And then, qGW1 and qPF1 were fine mapped using BC3F2 population developed from QTL-NILs, respectively. The result of this stuty using NILs and its secondary segregation population confirmed the results of QTL for 100-grain weight and popping fold identified in F2:3 population, and the precise of QTL mapping were improved, which set the basis for cloning these QTLs.The main results of this study are as below follows:1. According to the QTL for 100-grain weight identified in F2:3 population, qGW1 was in the interval of umc1395～umc2237 on chromosome 1, explaining 16.7% of the phenotypic variation. Using GY220, a high oil inbred line, as the recurrent parent and a normal inbred line 8622 as donor parent, a near-isogenic line for 100-grain weight was developed by MAS in GY220 background after 3 generations of backcrossing. Thirty nine SSR markers were used in genetic background selection. The average interval of markers on chromosome was 55.47 cM. Recovering ratio of genetic background (RRGB) of the selected BC3F1 plant was 96.2%. QTL analysis was conducted in the BC3F2 population derived from its NIL, and found that the QTL was located on umc1601～umc1754 (bin 1.05～1.06) and explained 20.94% of the phenotypic variation with LOD score of 9.78. The location of QTL found in BC3F2 population was the same as that detected in the F2:3 population, but the interval was narrowed and its contribution for phenotypic variation increased. Plants in BC3F3 with heterozygous genotype and high RRGB were planted. Twenty two pairs of primers were developed by SSRHUNTER and primier5.0 based on the BAC sequences of maize, and 7 pairs of primers exhibited polymorphisms between 8622 and GY220 were used to detect the recombined plants. Consequently, qGW1 was narrowed down to 5.096 Mb region between markers B67-1 and umc1754 using the chromosome fragment substitution approach. 2. According to the QTL for 100-grain weight identified in F2:3 population, qGW3 was in the interval of bnlg1447～phi029 on chromosome 3, explaining 16.7% of the phenotypic variation. Using GY220 as the recurrent parent and a normal inbred line 8984 as donor parent, a near-isogenic line for 100-grain weight was developed using the same method as NIL-qGW1. Forty four SSR markers were used in genetic background selection, and the average interval of markers was 55.47 cM. RRGB of the selected BC3F1 plant was 95.5%. QTL analysis was conducted in the BC3F2 population derived from its NIL, and found that the QTL was located on umc1392～umc1655 (bin 3.04) and explained 14.94% of the phenotypic variation with LOD score of 9.76. But there is a minor difference between the location of QTL found in this BC3F2 population and that in the F2:3 population, It is interesting that there were high positive correlation between 100-grain weight, ear diameter and plant height, and two QTL for ear diameter and plant height were also detected in the same interval of umc1392～umc1655 as qGW3 , which was not found in the F2:3 population.3. According to the QTL for popping fold identified in the F2:3 population, qPF1 was detected in the interval of bnlg1643～umc1885 on chromosome 1, explaining 7.88% of the phenotypic variation. Using Dan232, a dent corn inbred, as the recurrent parent and an elite popcorn indred N04 as donor parent, a near-isogenic line for popping fold was constructed using the same method as qGW1. Thirty one SSR markers were used in genetic background selection and the average interval of markers was 53.55 cM. RRGB of the selected BC3F1 plant was 93.5%. QTL analysis was conducted in the BC3F2 population derived from its NIL, and found that the QTL was located in the maker interval umc2240～umc2047 (bin 1.08～1.09) and explained 14.16% of the phenotypic variation with LOD score of 2.29. The location of QTL found in BC3F2 population was the same as that detected in F2:3 the population, but the interval was narrowed and its contribution for phenotypic variation increased. One QTL for popping volume was also found at the same location, which explained 15.28% of the phenotypic variation with LOD score of 2.65. Using chromosome fragment substitution approach as qGW1, the interval for qPF1 was narrowed down to 5.94 Mb between markers B67-1 and umc1754.
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