TOWARD THE CONSTRUCTION OF A MOLECULAR MAP OF CASSAVA (MANIHOT-ESCULENTA CRANTZ) - COMPARISON OF RESTRICTION ENZYMES AND PROBE SOURCES IN DETECTING RFLPS

The construction of a detailed genetic map of cassava (Manihot esculen ta Crantz), classified as a tetraploid species, depends on the ability of cloned sequences to detect polymorphisms. As a first step in devel oping this map, 200 cloned nuclear sequences generated with different restriction enzymes were hybridized to total digested DNA from eleven cultivated lines and one wild Manihot species, M. aesculifolia. Polymo rphism was detected less frequently with both BamHI and EcoRI genomic probes than with PstI, HindIII and XbaI genomic probes. DNA digested w ith HpaII, DraI and TaqI displayed less polymorphism, whereas DNA dige sted with EcoRI and EcoRV displayed more polymorphism like that found in lettuce, rice and tomato (Landry et al., 1987; McCouch et al., 1988 ; Miller and Tanksley, 1990). Four-cutter restriction enzymes displaye d less frequency of polymorphism when compared with six-cutter restric tion enzymes. Polymorphism displayed by DraI was extremely low, indica ting that regions rich in adenine and thymine may not be hot spots for mutation in cassava. Polymorphism detected between cultivated genotyp es and M. aesculifolia was dramatically higher than that found among c ultivated genotypes.