Procedures Results and Discussion

Chromosomal variation was assessed by multilocus enzyme electrophoresis. Plasmid content was visualized via a modified Eckhardt procedure. Clustering from a matrix of pairwise mismatched distances was performed by the UPGMA method, both for the chromosomal variation and the plasmid content. The degree of non-random association of chromosomal alleles or plasmids was estimated from the VJVe ratio. A high genetic diversity (H-0.501) was detected and among the 126 isolates were 43 distinctive multilocus genotypes (ETs). Fifty-six percent of the isolates belonged to three ETs, indicating a high degree of genotype dominance. Of 33 different plasmid profiles, three were found in 49% of the isolates, indicating also a dominant plasmidic structure. The MLEE and plasmid profile dendrograms grouped the isolates into the same two genetic divisions. None of the ETs or plasmid profiles was shared by the divisions. Within the genetic divisions, isolates of the same ET could have different plasmid profiles. Also, a plasmid profile could be found in different ETs, showing lateral transfer of plasmids within divisions. The linkage disequilibrium analysis showed the random association of chromosomal alleles within genetic groups, but not for the entire population. The linkage disequilibrium analyses confirmed the random association of plasmids both within genetic groups and for the entire population, indicating enough plasmid flow to break the linkage disequilibrium, although exclusive combinations exist for either chromosomal division. These results point toward a reticulated and epidemic genetic structure at the chromosomal level, and an epidemic plasmidic structure for this R. etli bv. phaseoli population.

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