Wild Manihot species: botanical aspects, geographic distribution and economic value

A total of 98 Manihot species have been recognized in the genus. All of them are native to the tropics of the New World, particularly Brazil and Mexico. The cultigen, Manihot esculenta Crantz (cassava), grows throughout the lowland tropics. Wild species vary in growth habit from acaulescent or short shrubs to tree-like. Because of their adaptations to different conditions, they are gene reservoirs for tackling many abiotic and biotic stresses such as improving root quality and resistance to diseases. They have been used successfully by the first author for improving protein content, seed-fertility, apomixis, resistance to mealy bug, and tolerance to drought. A table of the most important species from an economic viewpoint is presented.

Anatomical alterations due to polyploidy in cassava, Manihot esculenta Crantz

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.

Cassava genetic resources and their utilization for breeding of the crop

Wild cassava relatives are perennials and vary in growth pattern from nearly acaulescent subshrubs to small trees. They have been used as a source of useful characters such as high protein content, apomixis, resistance to mealybug and mosaic disease, and tolerance to drought. Indigenous clones are a potential source of beta-carotene and lycopene. Apomixis genes have been transferred to the crop successfully through interspecific hybridization, and apomictic clones arising from these hybrids are now being grown at the Universidade de Brasília. Interspecific hybrids produced earlier were polyploidized and had their fertility restored. Different useful types of chimera were also produced.

Molecular analysis of apomixis in cassava

Cassava is the main staple for more than 800 million people in the tropics. It is propagated vegetatively by stem cuttings, which maintains superior genotypes but favors disease accumulation and spread. In this report, we present the results of the screening of the progeny and the second generation of the clone UnB 307 for apomixes using microsatellites. A total of 29 plants were screened, representing the maternal plant, its first and second generations that were left to open pollination. About 20% of the offspring were rated as genetically identical plants. This result confirms the facultative apomictic nature of cassava, with high environmental effect.

The synthesis of a new cassava-derived species, Manihot vieiri Nassar

A new species was synthesized artificially by chromosome doubling in a hybrid. The ensuing polyploid type exhibits an apomictic nature and maintains its morphological characteristics in the progeny. It showed a frequency of multiembryonic sacs of 29% in the ovules examined, whereas sacs were absent in the diploid type.