VARIABILITY OF CHLOROPLAST DNA AND NUCLEAR RIBOSOMAL DNA IN CASSAVA (MANIHOT-ESCULENTA CRANTZ) AND ITS WILD RELATIVES

Chloroplast DNA (cp) and nuclear ribosomal DNA (rDNA) variation was in vestigated in 45 accessions of cultivated and wild Manihot species. Te n independent mutations, 8 point mutations and 2 length mutations were identified, using eight restriction enzymes and 12 heterologous cpDNA probes from mungbean. Restriction fragment length polymorphism analys is defined nine distinct chloroplast types, three of which were found among the cultivated accessions and six among the wild species. Cladis tic analysis of the cpDNA data using parsimony yielded a hypothetical phylogeny of lineages among the cpDNAs of cassava and its wild relativ es that is congruent with morphological evolutionary differentiation i n the genus. The results of our survey of cpDNA, together with rDNA re striction site change at the intergenic spacer region and rDNA repeat unit length variation (using rDNA cloned fragments from tare as probe) , suggest that cassava might have arisen from the domestication of wil d tuberous accessions of some Manihot species, followed by intensive s election. M. esculenta subspp flabellifolia is probably a wild progeni tor. Introgressive hybridization with wild forms and pressures to adap t to the widely varying climates and topography in which cassava is fo und might have enhanced the crop's present day variability.