SSR-based and grain yield-based diversity of hybrid maize in Thailand

Commercial hybrid maize varieties are often used to extract new inbred lines in hybrid breeding programs. Seven commercial hybrid maize varieties were crossed in a diallel design, and the resulting hybrids and selfed progeny were evaluated for yield and adaptability grown in six locations in Thailand. The parental varieties were analyzed with a set of 64 Simple Sequence Repeat (SSR) loci, spanning all 10 chromosomes. The average number of alleles per SSR locus was 4.98 with a range of 2–11. The polymorphic information content (PIC) varied from 0.24 to 0.89, with an average of 0.69. Using SSR-based genetic distance (GD), an UPGMA dendrogram showed seven field corn varieties classified into three distinct groups. Troyer's genetic distance (TGD) calculated from yield data, which is a function of inbreeding depression, varied from 0.493 to 1.015 between different parental combinations. A significant positive correlation was found between the GD and TGD across the six locations (0.66^**). The GD correlated positively with specific combining ability (SCA) as well across all locations (r = 0.76^**) heterosis was significant for grain yield and SCA, accounting for 47% of the sum of the square among entries. Therefore, we suggest that broad base populations derived from each of the distinct groups can be formed using combined criteria of SSR-based GD, TGD, grain yield, GCA and SCA.     

Detection and integration of gene mapping of downy mildew resistance in maize inbred lines though linkage and association

Association mapping relies on the variation and extent of linkage disequilibrium within a species to identify genes of interest; this is an alternative to linkage mapping in traditional biparental population, which exploits only the variation in the two parents of the mapping population. This study was designed to identify association between 48 SSR markers and downy mildew (DM) resistance using a set of 60 public and private maize inbred lines in Thailand. Genetic diversity and population structure in the set were calculated. A total of 489 alleles with an average gene diversity of 0.70 revealed two subpopulations among the 60 maize inbred lines. Analysis of variance resulted in significant effects of phenotypic values of tested entries, with significant effects of inbred lines, locations, and their interaction. In addition, the analysis of variance for broad sense heritability also indicated high heritability (0.97), and association analysis revealed three significant SSR-trait associations (P < 0.05). These three significant SSR loci have not been reported in previous linkage mapping studies. Our results suggest that new allelic variants associated with DM resistance in these germplasm collections should be useful to help identify new lines carrying alleles for DM resistance in breeding marker-assisted selection programs.     

Detection of candidate R genes and single nucleotide polymorphisms for downy mildew resistance in maize inbred lines by association analysis

Downy mildew (DM) is a plant disease that strongly limits maize production. The resistance (R) genes for this disease have been identified and characterized and serve as important tools for plant disease evaluation. In this study, partial genes for PIC15, PO145579, and zmcf5 were identified and analyzed in a panel of 60 public and private maize inbred lines using bioinformatics and statistics tools. These candidate R genes exhibited an average nucleotide diversity value of 0.015. The negative Tajima’s D values for these genes suggested that purifying selection has played a role in the evolution of maize DM resistance genes. In addition, linkage disequilibrium among these genes occurred across an extent of ~ 200 bp. Using association analysis methods, a general linear model, and a mixed linear model, five polymorphisms within the partial PIC15 and PO145579 genes were detected and associated with DM disease. These are new polymorphisms that have not previously been reported in association with DM disease traits. Therefore, correlations between these new polymorphisms and DM disease in these germplasm collections could advance the development of a functional marker for marker-assisted selection of DM disease resistance in maize breeding programs.     

泰国玉米杂种优势群和杂交利用模式构建及研究进展

泰国的玉米研究,农业合作部农业厅开始于1950年,大学部肯色萨大学开始于1958年。国家玉米和高粱研究中心,通称苏湾农场和农业厅大田作物研究所下属从事玉米研究的Nakhon Sawan大田作物研究中心,通称沙湾中心,分别成立于1965,1963年。著名的SW1于1969-1972年间选育成功,与SW1种源有高度配合力的广基综合种KS23于1987-1988合成,作为与KS23相配对的、相互轮回选择的广基综合种KS24,于1992-1993年合成。截至目前,已合成和改良了下列群体,硬粒型群体有SW2,SW3,SW5,Sawan-complex,NSDR;马齿型群体有KS6,KS27,CaripenoDMR,KS28,Pop.24,Pop.28。Ki21,Ki45,Kei0102等自交系则分别来源于Pacific 9,Pioneer 3013等。接着,在1982-2006年间发展和(或)应用了6个杂交利用模式,即SW1C4×SW1C4;SW1C4×Caripeno DMRC1;(SW1C4、C8,SW3C2、C3,KS6C2的选育系)×Mo17热带种质衍生系Ki21,Ki45;SW1×Pop.28;(SW1,non-SW1选育系)×优良杂交种Pioneer 3013二环系-Kei0102和KS24×KS23(即SW1×non-SW1)。