Genetic Diversity and Differentiation of Brazilian Bitter and Sweet Manioc Varieties (Manihot esculenta Crantz, Euphorbiaceae) Based on SSR Molecular Markers

Manioc (Manihot esculenta Crantz) originated in Amazonia and is the main staple for more than 800 million people worldwide; it also had a fundamental role as a source of calories for many pre-Columbian peoples, especially in Amazonia, where it was domesticated. There are two major groups of manioc varieties: sweet varieties have low amounts of toxic substances (cyanogenic glycosides) and may be consumed with minimum processing, while bitter varieties have a high degree of toxicity and must be detoxified to be safe before consumption. These groups are outcomes of divergent selective pressures. Natural selection probably maintains large amounts of cyanogenic glycosides to serve as a plant defense when in cultivation. Human selection may reduce the toxicity of the plants when roots are directly consumed, but may be neutral when the roots are consumed after some kind of processing. Although farmers recognize the distinction of the two groups of varieties, the variation of cyanogenic glycosides is continuous among different varieties. Genetic differentiation between sweet and bitter varieties was detected with molecular markers, as well as different patterns of groupings of varieties from different regions of Brazil. The genetic distinctions suggest that the sweet varieties originated during the initial domestication in southwestern Amazonia and bitter varieties arose later during cultivation in Amazonia, as hypothesized by Arroyo-Kalin in a recent paper. They also suggest that these groups of varieties were dispersed independently, even though they are cultivated complementarily today, with sweet varieties in home-gardens and bitter varieties in swiddens.