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Cassava (Manihot esculenta Crantz) is one of the two most important food crops in sub-Saharan Africa. This area accounts for most of the root harvest worldwide, followed by Asia and Latin America – the centre of origin for Manihot species. In Africa and Latin America, cassava is mostly used for human consumption, while in Asia and parts of Latin America it is also used commercially for the production of animal feed and starch-based products. Cassava is regarded as a crop adapted to drought-prone environments, where cereals and other crops do not thrive, and it also grows well in poor soil. There are about 100 wild Manihot species, which provide an important genetic endowment for cassava breeding. Professional cassava breeding started in the 20th century and was spurred on by increasing population demands. The main breeding goals are high yield per unit area, particularly in marginal or pest-prone environments. The most notable results from cassava breeding are seen today in sub-Saharan Africa, where it has been transformed from a poor man's crop to an urban food, and in Southeast Asia, where it has changed from a subsistence crop to an industrial cash crop. Long-term research by many international and national partners has led to breeding high-yielding cassava cultivars that increased crop yields up to 40%. Manipulation of genes from wild species has led to new cultivars that resist prevailing diseases and pests, allowing the avoidance of large-scale famine in sub-Saharan Africa. Cassava improvement continues to tap genetic variation through conventional breeding (including the use of wild species) and biotechnology, because many pathogens still take their toll and occasionally epidemics affect farmer fields significantly. However, new sources of variation are needed to genetically enhance the nutritional quality of this important food crop in Africa and other areas in the tropics of the developing world.

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AbStRACt. Information on anatomical structure is needed by breeders working on improvement for drought tolerance. For studying the effect of polyploidy on cassava anatomy and its significance to tolerance to drought, we induced a polyploidy type of a selected clone (UnB 530) by applying an aqueous solution of 0.2% colchicine on lateral buds for a period of 12 h. The stem identified as tetraploid was propagated to produce the whole plant. Free-hand cross-sections of the median portion between stem internodes were made. They were clarified using 50% sodium hypochlorite solution, stained with 1% safranin-alcian blue, passed through an ethanol series and butyl acetate and mounted in synthetic resin. The tetraploid type showed more prismatic and druse crystals in the cortical parenchyma, and its pericycle fibers had thicker walls. The secondary xylem of tetraploid types was wider than diploid ones, having thinner walls and less starch.

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Wild species of Manihot are progenitors of cassava. They constitute valuable genetic reservoirs presenting genes that show new characters. Screening of these species showed some of them to have a notably high percentage of protein combined with a low percentage of hydrocyanic acid. Study of natural habitats revealed resistance to drought and excessive soil aluminum toxicity as well as adaptation to low temperature. Some of the hybrids obtained showed high root productivity and resistance to stem borers. Apomixis was discovered in the wild and transferred successfully to the cultivate species.*Dedicated to Clibas Vieira, a tireless student of crop improvement on his 70th anniversary.