Nagib M. A. Nassar

Conservation of the genetic resources of cassava (Manihot Esculenta) Determination of wild species localities with emphasis on probable origin

Wild species ofManihot are progenitors of cassava. They constitute valuable genetic reservoirs with genes of new characters. Study of geographic distribution of wildManihot species showed concentration of wild species in a number of centers in South and Central America. Considering Vavilov’s concept of centers of diversity based on the Age and Area hypothesis of Willis, and Harlan’s theory of Introgression, and by studying Indian immigrations in Pre-Columbian times, it is assumed that northern Amazonia is the place of domestication of cassava and that Goias is the primary center of diversity ofManihot species as a “biological group”.

Attempts to hybridize wild Manihot species with cassava.

Hybridization between the wildManihot species,anomala Pohl,M. oligantha Pax emend. Nassar subsp.nesteli, M. gracilis Pax andM. zehntneri Ule, and cassava have been tried. Seeds were obtained from all hybrids. Moreover, crosses ofM. oligantha subsp.nesteli and cassava were highly fertile.

Molecular and embryonic evidence of apomixis in cassava interspecific hybrids (Manihot spp.)

In cassava, apomixis could fix hetereozygosity and prevent the transmission of systemic pathogens which complicate vegetative propagation of the crop. Evidence from maternal inheritance of RAPD markers and the structure of the embryonic sac in large progeny sets of two distinct genotypes have further confirmed that apomixis occurs in cassava. Here, we have built on an earlier report of apomixis in cassava in four ways (1) we estimated the rate of faculative apomixis in the range of 2% (2) we detected the occurrence of apomixis in a second genotype, derived form a different interspecific cross; (3) apomictic behavior was demonstrated in an F1 individual and (4) embryonic evidence showed that the mode of apomixis is aposporic. Since apomixis was detected in an F1 interspecific hybrid it is possible that genes for apomixis could be transferred directly to cultivated cassava from a wild relative. Key words: Cassava, interspecific hybrid