Genetic Structure, Phenotypic Diversity and Salinity Tolerance Potential of Selected Sweet and Grain Sorghum Populations

Abstract

Sorghum (Sorghum bicolor L. Moench), a diploid with 2n = 20, has a genome estimated to contain 811Mbp of DNA. Sorghum Moench belong to the botanical family Poaceae and sub-generic Eusorghum. It is a subject of plant population genetics research based on its importance as one of the world's leading grain crops and of desirable syrup. Sorghum cultivars (S. bicolor) classified within primary gene pool, are genetically impoverished, therefore determination of genetic divergence based on phenotypic diversity, salinity tolerance potential, heritability and molecular diversity, is timely and crucial. The findings, as revealed in this study could be utilized in selection of heterotic parents for crossbreeding. Seven qualitative and 16 quantitative traits were measured and the phenotypic data analyzed. A number of fitness traits including seedling dry weight, germination percentage, root length, shoot length, shoot length and wet weight were measured under different levels of selection pressure. The hydroponic growth medium was contaminated with NaCl at different (EC5, EC 10, and EC 15) concentration levels. The phenotypic variance, broad sense heritability (h2) and genotypic variance for all characters based on salinity levels was calculated. Molecular genetic diversity study involved use of microsatellite markers where cetyltrimethyl ammonium bromide (CTAB) protocol was used for extraction of DNA. Gel electrophoresis, DNA quantification, polymerase chain reaction and SSR markers optimization was done. The data obtained was analyzed using statistical softwares; GenStat v.12, DARwin v.6 and PowerMarker v.3.25. The data were standardized prior to cluster analysis. Statistical methods employed clearly discriminated these sorghum genotypes into distinct groups. Phenotypic diversity analysis distinctively demarcated grain sorghums from sweet sorghums. The first four principal components accounted for 97.19 % of the total variation with PC4 scoring 1.14Eigen value. Both the biplot and dendrogram formed through (HCA) classification with UPGMA technique sorted genotypes into 3 main clusters. Three sweet sorghums 3018, 3022 and 3008 gathered in one sub-cluster. Genetic distance matrix based on phenotypic traits ranged from 2.36 to 8.80. The phenotypic diversity index ( ) for traits studied recorded a mean value of 0.70 and a maximum 0.95.Based on tolerance index Seredo (83.31%) was identified as a tolerant population and Serena (74.09%) as a considerably sensitive tolerant population. Tolerance index (TI) showed a decrease in TI (5 dS/m>10 dS/m >15 dS/m) with increase in salinity stress levels. The biplot display showed that Kivila, a landrace population had a close relationship with 3008 and 3022 sweet sorghums. The traits were projected by vectors thereby indicating wide salinity tolerance diversity. The genetic distance between Seredo (2.88) and Serena showed a narrow genetic base indicating that they could have descended from a common ancestral population. Minimum distance coefficient (1.14) of Manhattan dissimilarity was recorded between Kivila and 3022. Coefficient of variance values as well as scores for heritability was higher at control than that of respective selection pressures (5dS/m, 10dS/m and 15dS/m). Heritability in broad sense values ranged from 38.57 % for wet weight to 62.58 % germination percentage at 15dS/m. The SSR markers used were polymorphic (50%) and informative with PIC value ranging from 0.35 to 0.37 with a mean of 0.36. The frequency of the major alleles at different SSR loci ranged from 0.50 to 0.64. Recorded expected heterozygosity (He) ranged from 0.46 to 0.50 with an average of 0.47 whereas heterozygosity (HO) values observed ranged from 0.43 to 0.89 scoring a mean of average of 0.66. Hardy-Weinberg equilibrium computation produced a highly significant p value (0.047) for locus Xtxp 021. Coordinate (Jaccard similarity coefficient) calculated for the two first axes had positive Eigen values, 0.08517 and 0.06173 for the first and second coordinates in that order. PCoA provided a better diversity structure than the dendrogram. Ultimately the patterns of genetic diversity and relationships of sorghum revealed in this study is insightful in providing useful information for identification and utilization of the genetic resource in breeding programmes to break performance barriers in sorghum.