Association mapping for drought tolerance in Sorghum [sorghum bicolor (l.) Moench]

Abstract

Sorghum [Sorghum bicolor {L.} (Moench] is the third most important cereal crop in Ethiopia next to maize and tef. Drought is the major sorghum production constraint in Ethiopia which requires identification of quantitative trait loci (QTLs) responsible for drought tolerance and thereby development of drought tolerant varieties. The objectives of this study were to identify drought tolerant genotypes, to map chromosomal regions (QTLs) associated with agronomically important traits including drought tolerance and identify SSR markers tightly linked with these QTLs. For this study, one hundred sixty sorghum genotypes (152 land races and 8 released varieties) were genotyped with 39 SSR and evaluated in field at Kobo in the off-season using an alpha lattice design replicated three times. Phenotypic data were collected 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. Analysis of variance showed highly significant (P<0.0001) differences among the genotypes for all characters. Grain weight per panicle ranged from 8.68 (210902) to 101.09 (210922) with an average value of 43.52. The first 17 best performers among the entries were the landraces indicating the presence of better performing genotypes in the land races than the existing released varieties and the five best performing genotypes were 210922, 73067, 69183, 69231, and 69241 in that order. Most of the characters showed moderate to high phenotypic and genotypic coefficient of variation. Heritability was high for all of the studied characters. High heritability coupled with high genetic advance as percent of mean was observed for plant height, panicle weight, grain weight per panicle, one thousand grain weight, and number of grains per panicle. Linkage disequilibrium (LD) analysis indicated that in all accessions, 107 loci pairs (32.92%) had a significant (p< 0.05) mean LD of 0.19, with an R2 > 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 7 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 were localized on chromosomes 1, 2, 3,5,6,7, and 8 harboring one marker each (xcup53, bSbCIR223, Xtxp114, mSbCIR248, Xtxp145, Xtxp278, and gbsp123 respectively). Most of the identified markers were localized with those previously identified as linked to drought tolerance-related traits using conventional QTL mapping supporting the reliability of the present findings. The results of this study can serve as initial effort for the association mapping studies in sorghum particularly in our country 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