The genetic variability of 20 isolates of Fusarium oxysporum, nine nonpathogenic and 11 causing common bean (Phaseouls vulgaris) wilt, was analyzed on the basis of the distribution of the transposable element impala. The presence of transposable elements belonging to subfamilies D and E of impala was determined through polymerase chain reaction (PCR) using specific primers for each subfamily. The presence of members of the two subfamilies was observed in most of the isolates, suggesting that it is an old component in the F. oxysporum f. sp. phaseoli genoma. Hybridization of total DNA of each isolate, digested with EcoRI, with impala fragments of subfamily E produced a highly variable band pattern in the nonpathogenic isolates, indicating the possible activity of these elements. On the other hand, in pathogenic isolates, the band patterns were more homogeneous and some isolates showed very similar patterns, indicating that these impala copies have lost their capacity to transpose. These inactive copies are suitable as genetic markers. Among the pathogenic isolates, endogenous copies of impala were not detected in Fus4; therefore, this isolate could be used in experiments of insertional mutagenesis with the pNI160 plasmid, which harbors the active W impala element disrupting the niaD (nitrate reductase) gene.
transposon; bean wilt; PCR; genetic variability; pathogenicity
