| Mechanical grain harvesting is the developing trend of maize harvesting technology.Higher moisture content in the grains at mature period is not suited for mechanical harvest.The grain moisture content at physiological maturity and grain dehydration rate after physiological maturity are key factors determining grain moisture content at harvest time.Maize hybrids Zhengdan 958(Zheng 58 x Chang 7-2)and Xianyu 335(PH6WC x PH4CV)are widely cultivated in China,showing differences in grain moisture content(GMC)and grain dehydration rate(GDR).In this study,a mixed near isogenic lines(NILs)population was constructed by crossing PH6 WC and PH4 CV with Zheng 58 and Chang 7-2,and then by consecutively backcrossing and selfing.Genotypes of these NILs were identified by maize 55 K SNP chips.After two years of repeated experiments in 2017 and 2018,the agronomic traits were measured.GMCs and GDRs at different times after pollination were obtained and used for genome-wide association studies(GWAS).Molecular markers related to GMC and GDR were identified and developed.The major research results are as follows:(1)The population genotypes and population structure of NILs were obtained.Based on the parents of Zhengdan 958(Zheng 58,Chang 7-2)and Xianyu 335(PH6WC,PH4CV),we constructed a mixed NILs containing about 601 lines.The genotypes of these lines were obtained by maize 55 K SNP chip.After quality control,31095 SNPs were used for population structure analysis.These lines can be divided into four subgroups: Z58 NILs,C7-2 NILs,PH6 WC NIL and PH4 CV NILs.The genetic distance is smaller among different lines in the same subgroup,whereas the genetic distance is larger among different subgroups,indicating that different subgroups retain most of genetic characteristics from its parent.(2)The NILs constructed in this study are suited for GWAS analysis.Ear cob color is controlled by quality traits.Here the colors of different 553 NILs were identified and then used for GWAS analysis.There were four significant SNPs located on chromosome 1,which were closely related to ear cob color.Zm00001d028842(P1)gene was identified through scanning maize genome sequences closed to the four SNP sites.The identification of this gene controlling cob color is consistent with previous study.The results showed that our NILs have genetic diversity and can be stably inherited,and can be used for gene location identification and molecular marker development conveniently.(3)GWAS analyses of GMCs at different stages after pollination were carried out to identify significant SNPs.GWAS analyses of GWCs were carried out by using twoyear repeated experimental data.After pollination for 35 days,13 significant SNPs related to GMC were identified in 2017,mainly distributed on chromosomes 1,4,5,9 and 10.In 2018,14 significant SNPs were identified,mainly distributed on chromosomes 1,2,3,8,9 and 10.After pollination for 42 days,11 significant SNPs were identified in 2017,mainly distributed on chromosomes 1,3,4,5 and 9.In 2018,18 significant SNPs were identified,mainly distributed on chromosomes 2,3,5 and 10.After pollination for 49 days,9 significant SNPs were identified in 2017,mainly distributed on chromosomes 1,2,4,6 and 10.In 2018,10 significant SNPs were identified,mainly distributed on chromosome 9.After pollination for 56 days,16 significant SNPs were identified in 2017,mainly distributed on chromosomes 1,3,6,8 and 9.In 2018,14 significant SNPs were identified,mainly distributed on chromosomes 1,3,4,7 and 8.(4)GWAS analyses of GMCs at physiological maturity and 7 days after physiological maturity.The formation of black layer was used as a marker of grain physiological maturity,and GWAS analysis of GMC at physiological mature stage was carried out.A total of 13 significant SNPs were identified,mainly distributed on chromosomes 1,5 and 6,with 7 SNPs on chromosome 1.Seven days after physiological maturity,21 significant SNPs were identified,almost all of which distributed in chromosome 4.(5)GWAS analyses of GDRs before and after physiological maturity.GWAS analyses were carried out on GDRs 7 days before physiological maturity and 7 days after maturity by suisng Z58 NILs.GWAS analysis of GDR 7 days before physiological maturity identified 16 SNPs,mainly distributed on chromosomes 1,2,3,4 and 6.GWAS analysis of GDR 7 days after physiological maturity identified 13 SNPs,mainly distributed on chromosomes 5,6,8,9 and 10.(6)Screening of molecular markers related to GMC and GDR.Based on the identified significant SNPs related to GMC and GDR after physiological maturity,and SNPs related to GMC at 49 days and 56 days after pollination,we identified 13 SNPs with significant correlation with moisture content at maturity and dehydration rate before maturity,distributed on chromosomes 1,4 and 6.And there were 13 SNPs significantly correlated with GMC and GDR 7 days after physiological maturity,most of which distributed on chromosomes 4,5,6,8 and 10.(7)Selection of lines with lower GMC or higher GDR using SNP markers.To select lines with lower GMC or higher GDR,SNP markers identified from GWAS analyses of lower GMC or higher GDR were used to screen the 601 lines.And 15 inbred lines with higher GDR were selected.When comparing with the control PH4 CV,the success rate of screening is about 57.14%-80%.When comparing with the control Chang 7-2,the success rate of screening is 71.42%-100%.Our results proved that the molecular markers related to GMC and GDR are relatively successful and have high application value,and can be used for screening inbred lines to improve breeding efficiency.In summary,we constructed a mixed NIL population containing 601 lines,and genotyping these lines by using maize 55 K SNP chip.GWAS analyses of GMC and GDR were conducted and identified some SNP markers related to GMC and GDR.Our results of this study may help to accelerate the breeding processes for high-quality and fast-dehydrated excellent hybrids,meeting the requirements of the rapid development of maize mechanical harvesting in China. |
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