The ability of agriculture to meet the challenges of an increasing world population rests on the ability of plant scientists to introduce new traits into existing crops. Increasing yield and nutrition are particularly important for the primarily food crops of the developing world where the expanding human population places extreme pressure on agricultural capacity. Biotechnology offers one approach to this challenge. An alternative approach uses the natural diversity of a crop's wild relatives in a traditional program of plant breeding. The value of this approach has been documented in rice, where crosses between cultivated rice, Oryza sativa, and wild relatives have yielded progeny that vary in an astounding array of agriculturally important traits.

The evolutionary relationships between domesticated species and their wild relatives serve as a guide to the most appropriate wild germplasm for such breeding programs. Unfortunately, for some crops the relationship among wild species and their domesticated crops are poorly resolved. Here I will discuss my laboratory's recent work on the evolutionary relationships within the genus Manihot. Manihot esculenta, cassava, is an important root crop of the tropics, feeding over 600,000 million people worldwide and serving as the primary source of calories in sub-Saharan Africa. We are in the process of reconstructing the evolutionary relationships within the genus by a molecular phylogeny. The phylogeny reconstruction has concluded that cassava is of South American origin and not from Mexico as previously hypothesized. We have identified the wild progenitor of cassava using several different gene sequences. A molecular population genetics study of gene genealogies and microsatellite variation has pinpointed the region of the progenitor species range where cassava was domesticated, the southern gallery forests on the edge of the Amazon and Cerrado regions of Brazil. Anther study examines genetic variation of land races of cassava cultivated by the peoples of Amazonia. Agriculturally important traits such as high vitamin and sugar content have been identified. These studies point to the importance of germplasm conservation for crop improvement.



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