Evidence on the origin of cassava: Phylogeography of Manihot esculenta

Cassava (Manihot esculenta subsp, esculenta) is a staple crop with great ec onomic importance worldwide, yet its evolutionary and geographical origins have remained unresolved and controversial. We have investigated this crop' s domestication in a phylogeographic study based on the single-copy nuclear gene glyceraldehyde 3-phosphate dehydrogenase (G3pdh), The G3pdh locus pro vides high levels of noncoding sequence variation in cassava and its wild r elatives, with 28 haplotypes identified among 212 individuals (424 alleles) examined. These data represent one of the first uses of a single-copy nucl ear gene in a plant phylogeographic study and yield several important insig hts into cassava's evolutionary origin: (i) cassava was likely domesticated from wild M. esculenta populations along the southern border of the Amazon basin; (ii) the crop does not seem to be derived from several progenitor s pecies, as previously proposed; and (iii) cassava does not share haplotypes with Manihot pruinosa, a closely related, potentially hybridizing species. These findings provide the dearest picture to date on cassava's origin. Wh en considered in a genealogical context, relationships among the G3pdh hapl otypes are incongruent with taxonomic boundaries, both within M. esculenta and at the interspecific level; this incongruence is probably a result of l ineage sorting among these recently diverged taxa, Although phylogeographic studies in animals have provided many new evolutionary insights, applicati on of phylogeography in plants has been hampered by difficulty in obtaining phylogenetically informative intraspecific variation. This study demonstra tes that single-copy nuclear genes can pro,ide a useful source of informati ve variation in plants.