AFLP assessment of genetic variability in cassava accessions (Manihot esculenta) resistant and susceptible to the cassava bacterial blight (CBB)

Cassava bacterial blight (CBB) is caused by Xanthomonas axonopodis pv. mani hotis (Xam). Resistance is found in Manihot esculenta and, in addition, has been introgressed from a wild relative, M. glaziovii. The resistance is th ought to be polygenic and additively inherited. Ninety-three varieties of M . esculenta (Crantz) were assessed by AFLPs for genetic diversity and for r esistance to CBB. AFLP analysis was performed using two primer combinations and a 79.2% level of polymorphism was found. The phenogram obtained showed between 74% and 96% genetic similarity among all cassava accessions analys ed. The analysis permitted the unique identification of each individual. Tw o Xam strains were used for resistance screening. Variation in the reaction of cassava varieties to Xam strains was observed for all plant accessions. The correlation of resistance to both strains, had a coefficient of 0.53, suggesting the independence of resistance to each strain. Multiple correspo ndence analysis showed a random distribution of the resistance/susceptibili ty response with respect to overall genetic diversity as measured by AFLP a nalysis. A total heterozygosity index was calculated to determine the diver sity within clusters as well as among them. Our results demonstrate that re sistance to CBB is broadly distributed in cassava germplasm and that AFLP a nalysis is an effective and efficient means of providing quantitative estim ates of genetic similarities among cassava accessions.