Structural and phylogenetic analysis of the rDNA intergenic spacer region of Verticillium dahliae

The nuclear ribosomal intergenic spacer (IGS) region was structurally analyzed and exploited for molecular discrimination and phylogenetic analysis of vegetative compatibility groups (VCGs) of Verticillium dahliae. A structural study of 201 available IGS sequences of the fungus was performed, and four classes of ubiquitous repetitive elements, organized in higher-order repetitive structures or composite blocks, were detected in a variable IGS subregion. This subregion was amplified from an international collection of 59 V. dahliae isolates covering all VCGs, together with nine representative V. albo-atrum and V. longisporum isolates, and sequenced. Structural and phylogenetic analyses of the sequences of this polymorphic IGS subregion were consistently informative and allowed the identification of two main lineages in V. dahliae, that is, clade I including VCGs 1A, 1B, 2A, 4B, and 3 and clade II containing VCGs 2B, 4A, and 6. Analysis of IGS sequences proved a highly suitable molecular tool for (a) rapid interspecific differentiation, (b) intraspecific discrimination among VCGs of V. dahliae, facilitating high-throughput VCG confirmation and prediction/profiling, and (c) phylogenetic analysis within and among V. dahliae VCGs.