Genetic diversity of Fusarium oxysporum strains from common bean fields in Spain

Abstract

Fusarium wilt is an endemic disease in El Barco de Avila (Castilla y León, west-central Spain), where high-quality common bean cultivars have been cultured for the last century. We used intergenic spacer (IGS) region polymorphism of ribosomal DNA, electrophoretic karyotype patterns, and vegetative compatibility and pathogenicity analyses to assess the genetic diversity within Fusarium oxysporum isolates recovered from common bean plants growing in fields around El Barco de Avila. Ninety-six vegetative compatibility groups (VCGs) were found among 128 isolates analyzed; most of these VCGs contained only a single isolate. The strains belonging to pathogenic VCGs and the most abundant nonpathogenic VCGs were further examined for polymorphisms in the IGS region and electrophoretic karyotype patterns. Isolates belonging to the same VCG exhibited the same IGS haplotype and very similar electrophoretic karyotype patterns. These findings are consistent with the hypothesis that VCGs represent clonal lineages that rarely, if ever, reproduce sexually. The F. oxysporum f. sp. phaseoli strains recovered had the same IGS haplotype and similar electrophoretic karyotype patterns, different from those found for F. oxysporum f. sp. phaseoli from the Americas, and were assigned to three new VCGs (VCGs 0166, 0167, and 0168). Based on our results, we do not consider the strains belonging to F. oxysporum f. sp. phaseoli to be a monophyletic group within F. oxysporum, as there is no correlation between pathogenicity and VCG, IGS restriction fragment length polymorphism, or electrophoretic karyotype.

FIG. 1

Gel electrophoresis analysis showing the patterns obtained when the amplified IGS DNAs from FOP-SP5 (IGS-A), AN6 (IGS-B1), AB17 (IGS-B2), AB16 (IGS-B3), FOP-CL25 (IGS-B4), and ATCC 18131 (IGS-B5) were digested with restriction endonucleases AvaI (lanes 1), AccI (lanes 2), ClaI (lanes 3), and XhoI (lanes 4). Lane M, marker DNA fragments (lambda DNA digested with EcoRI and HindIII); lane L, 100-bp DNA ladder. Only fragments larger than 600 bp are represented. In IGS-B3 lane 3, the top band is partially digested DNA. Numbers on the left and right are kilobase pairs.

FIG. 2

EKs of F. oxysporum f. sp. phaseoli and representative nonpathogenic F. oxysporum isolates separated by pulsed-field gel electrophoresis and visualized by ethidium bromide staining. Molecular size markers are chromosomes from H. wingei, S. pombe (Sp), and S. cerevisiae (Sc). Run conditions were as described in Materials and Methods.