Abstract

The incidence of root rot diseases partly contribute to the currently observed low percentage increase in the yield of cassava. We estimated gene diversities and identified putative hybridizing parents for root rot resistance using 18 simple sequence repeats loci in 43 improved genotypes of cassava. Root rot was measured over 2 years as the percentage proportion of rotten roots to the total number of roots harvested at 12 month after planting. Estimated rot ranged from 1.2 to 21.2% with a mean of 5.7±0.5. Rank-sum analysis generated 8 rot classes and identified TMS 96/1089A as best genotype resistant to root rot. Gene diversity analysis revealed expected heterozygosity that ranged from 0.701 for very highly susceptible genotypes to 0.781 for moderately resistant and susceptible. Genetic differentiation ranged from -0.0178 (resistant and susceptible) to 0.0523 (very highly resistant and highly resistant genotypes). A total heterozygosity of 0.764 was estimated and was largely due to within class diversity (0.755). DNA analysis representatives for window (DARwin) identified 10 hybridizing groups with a dissimilarity coefficient that ranged from 0.18 to 0.81 on a mean of 0.60. The results obtained from the present study are useful for the genetic improvement of cassava against root rot disease.

Key words: Cassava genotypes, gene diversity, heterozygosity, resistant genotypes, root rot disease.

Abbreviation

Abbreviations: A, Number of alleles; áº¬, mean number of allele; AR, allelic richness; CMD, cassava mosaic disease; DARwin, dis-similarity analysis representatives for windows; D_ST, between class heterozygosity; Freq.,frequency; FSTAT, F-statistics; F_ST, (theta) fixation index, estimator for genetic differentiation; G_ST, proportion of heterozygosity between class; He, heterozygosity;  Ä¤e, expected heterozydosity; H_O, observed heterozygosity;H_S, within class heterozygosity; H_T, total heterozygosity; HS, highly susceptible; HZG, hybridizing group; IITA,international institute of tropical agriculture; MR, moderately resistant; MS, moderately susceptible; NR_tR, number of rotten roots; NTSYS, numeric taxanomy system of statistics; P, polymorphic loci; R, resistant; Rootrot, root rot; S, susceptible; SAS, statistical analysis system; SSR, simple sequence repeat; Stdzdmean, standardized mean; TNR, total number of roots harvested; VHR, very highly resistant;  VHS, very highly susceptible.