Abstract
Drought is the major sorghum production constraint in Ethopia which necessitates the identification of sorghum genotypes that carry genes (quantitative trait locus, QTL) associated with drought tolerance thereby developing drought tolerant sorghum varieties. The objectives of this study were to identify drought tolerant sorghum genotypes, map chromosomal regions (QTLs) associated with agronomically important traits and identify simple sequence repeat (SSR) markers tightly linked with these QTLs. One hundred and sixty (160) sorghum genotypes (152 landraces and 8 released varieties) were genotyped with 39 SSRs markers and evaluated in the field at Kobo in the off-season using an alpha lattice design replicated three times. Phenotypic data including days to 50% flowering, plant height, panicle weight, grain weight, grain weight per panicle, panicle harvest index, one thousand grain weight and number of grains per panicle were collected. Analysis of variance showed highly significant (P 0.2 for 33 evaluated loci pairs. Population structure analysis showed that there were four distinct clusters in the studied materials. A total of 10 marker-trait associations were identified using seven different SSR markers. The percentage of the total variation explained by the markers ranged from 2.6% (Xtxp114 with THGT) to 17.76% (Xtxp145 with PHT). The seven SSR markers (xcup53, bSbCIR223, Xtxp114, mSbCIR248, Xtxp145, Xtxp278, and gbsp123) were located on chromosomes 1, 2, 3, 5, 6, 7 and 8, respectively, each chromosome harboring one marker. Most of the identified markers were localized in chromosomal positions that have been previously reported as positions for drought tolerance-related traits, supporting the present findings. The results of this study can serve as initial effort for the association mapping studies in sorghum particularly in Ethiopia as the associated SSR markers are potential candidates for marker-assisted selection to improve drought tolerance in sorghum. However, as this study is the first attempt in the identification of QTLs for drought tolerance using association mapping, the identified QTLs need to be validated in independent or related populations and in different environments before their use in marker-assisted selection. Key words: Association mapping, drought, population structure, quantitative trait locus (QTLs), sorghum, simple sequence repeat (SSR).
Highlights
Sorghum [Sorghum bicolor (L.) Moench] is a largely selfpollinating (70-95%) monocot crop in the grass family of Poaceae with a diploid set of chromosomes (2n=2x=20) and an estimated genome size of 750 Mb (Doggett, 1976; Yonemaru et al, 2009)
In quantitatively inherited traits such as drought tolerance that are controlled by many genes each with small effects (QTLs), selection by conventional methods based on phenotypic variations is inefficient and challenging due to the complex nature of the trait and the complicating effects of the environment
Upadhyaya et al (2012) reported significant associations of five markers with maturity date and plant height on chromosomes 6, 9, and 10 using 242 sorghum accessions and 39 simple sequence repeat (SSR) markers. These works emphasize the possibility of genetic improvement in sorghum using the existing germplasm resources by evaluating in field; and identifying and mapping quantitative trait locus (QTLs) associated with desired traits and selecting the genotypes that carry favorable alleles for gene introgression through marker assisted selection
Summary
Sorghum [Sorghum bicolor (L.) Moench] is a largely selfpollinating (70-95%) monocot crop in the grass family of Poaceae with a diploid set of chromosomes (2n=2x=20) and an estimated genome size of 750 Mb (Doggett, 1976; Yonemaru et al, 2009). In Ethiopia, sorghum is largely cultivated in moisture stress areas that cover nearly 66% of the total area of the country (Tadesse et al, 2008) This necessitates evaluation and identification of genomic regions that confer resistance to drought stress at the reproductive stage thereby developing drought tolerant varieties. Upadhyaya et al (2012) reported significant associations of five markers with maturity date and plant height on chromosomes 6, 9, and 10 using 242 sorghum accessions and 39 SSR markers These works emphasize the possibility of genetic improvement in sorghum using the existing germplasm resources (including land races) by evaluating in field; and identifying and mapping QTLs associated with desired traits and selecting the genotypes (parents) that carry favorable alleles for gene introgression through marker assisted selection. No study to detect marker-trait association for moisture stress tolerance in sorghum have been previously reported in Ethiopia using association mapping strategy which initiated the present study
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