Epistasis in cassava adapted to midaltitude valley environments

Little is known about the inheritance of agronomic traits in cassava (Manihot esculenta Crantz). The vegetative multiplication of cassava allows cloning of individual genotypes, and separates environmental from genetic variation in the within-family variation. Knowing the magnitude of between- and within-family variation is important for defining breeding strategies and for measuring different components of genetic variances, particularly the seldom-estimated epistasis. A group of nine genotypes, adapted to midaltitude environments, was used for a diallel study. Thirty clones were obtained from every F1 cross (each clone was represented by six plants), and planted in three replications at two locations. Statistical differences among crosses were found for fresh-root yield, harvest index, root dry-matter content, and reaction to mites (Mononychellus tanajoa Bondar) and to whiteflies (Aleurotrachelus socialis Bondar). General combining ability (GCA) was significant for all traits except fresh-root yield and dry-matter content, and specific combining ability (SCA) was significant for all traits except whitefly damage score. Fresh-root yield was the only trait with significant epistatic effects, which, combined with a large dominance variance, suggested a prevalence of nonadditive effects. The introduction of inbreeding would be one approach for the efficient exploitation of these nonadditive effects found for fresh-root yield. For the remaining traits, epistasis was negligible and current schemes exploiting additive effects may suffice.