Relationships among pathogenic and nonpathogenic isolates of Fusarium oxysporum based on the partial sequence of the intergenic spacer region of the ribosomal DNA.

Using PCR, we amplified and sequenced approximately 1,000 bp of the 5' end of the intergenic spacer (IGS) of the rDNA in 15 isolates of Fusarium oxysporum and one isolate of F. subglutinans. Isolates were selected to represent diversity in our collection based on differences in pathogenic race, vegetative compatibility group (VCG), mitochondrial DNA (mtDNA) haplotype, IGS haplotype, and DNA fingerprint. The objective of this research was to clarify the origin of virulence within F. oxysporum, the relationship between pathogenic and nonpathogenic strains, and the evolution of the different races of F. oxysporum f. sp. melonis. Bootstrapped parsimony analysis of the partial IGS sequence data identified a phylogenetic tree with highly significant branches. The two F. oxysporum f. sp. melonis VCGs, 0131 and 0134, were separated into distinct lineages. Race was not distinguished by significant IGS sequence differences within the pathogen VCGs. One exception was a race 1 isolate which was associated with VCG 0131 but, based on both mtDNA and IGS haplotype, had greater affinity with VCG 0134. Two IGS sequence types were found in this race 1 isolate, one suggesting an affiliation with VCG 0131 and the other similar to isolates in VCG 0134. This may have resulted from past somatic or sexual interactions between F. oxysporum f. sp. melonis, VCGs 0131 and 0134. Nonpathogens that were vegetatively compatible with the pathogen were not closely related to the pathogen based on IGS sequence data. Thus, nonpathogens and pathogens may share common alleles at vegetative compatibility loci by coincidence rather than because of recent clonal derivation from a common ancestor.