Characterization and association mapping for drought adaptation in Ethiopian sorghum [Sorghum bicolor (L.) Moench] germplasm

Abstract

Ethiopia is frequently affected by drought. As a result, drought is the major factor that affects sorghum production in Ethiopia. Knowledge of drought tolerance related traits and their mechanisms are the key component in selecting genotypes that withstand the effects of drought. The objectives of this study were to assess genetic variability among sorghum germplasm for root and shoot traits, to map chromosomal regions (QTL) associated with root and shoot traits related to drought adaptation and to identify SSR markers associated with drought adaptation traits.The experiment was carried out at Jimma University, College of Agriculture and Veterinary Medicine under greenhouse condition. One hundred thirty-six sorghum genotypes were characterized for twelve traits using a high throughput root phenotyping platform in a randomized complete block design with three replications. Among these genotypes, 108 of them were used for studying population structure and trait-marker association analysis using 39 SSR markers. The analysis of variance indicated that highly significant difference (P<0.01) were observed among the genotypes for all thestudied traits. The coefficients of correlation among different drought related traits showed that there was a significant positive and negative association among different drought related traits. The first three principal components (PCs) with eigenvalues greater than one accounted for 56.4% of the total genotype variation, the remaining 43.60% accounted for the last nine principal components.Moderate genotypic variation was exhibited for leaf area, shoot fresh weight, shoot dry weight, root to shoot ratio and root angle.All 136 genotype were grouped into four clusters whereby adifferent member within a cluster being assumed to be more closely related in terms of the trait under consideration with each other than that member in different clusters is. While highest phenotypic variation was exhibited for leaf area, shoot fresh weight, shoot dry weight, root dry weight and root to shoot ratio. Broad sense heritability ranged from 19.35% for root length to 71.08% for shoot fresh weight at seedling stage and high heritability was recorded for shoot fresh weight (71.08%), leaf area (70.22%) and root angle (66.22%). High heritability combined with high genetic advance was observed for shoot fresh weight, root angle, and leaf area. The 108 genotypes were grouped into three distinct subgroups. The plots of LD (r2 ) for pairs of loci, versus the genetic distance in cM, showed a clear trend on linkage disequilibrium decay in the studied genotypes and based on trend line it is around 15-20 cM. A total of 25 significant marker-trait associations/QTLs (P ≤ 0.05) were detected with 14 SSR markers and these markers were localized with previously identified QTL. As a future line of work, genotypes that showed desirable phenotypes such as narrow root angle need to be evaluated under field condition to verify their performance and thereby they can be used in the breeding programs. As this study is the first in Ethiopia, the identified QTLs need to be validated through repeated phenotypic measurement in independent or related populations. The SSR markers found to be associated with traits need to be validated before their use in marker-assisted selection.