Heterogeneity in electrophoretic karyotype within and between anastomosis groups of Rhizoctonia solani

Isolates of the soil borne basidiomycete Rhizoctonia solani vary in cultural appearance, in growth characteristics and in pathogenicity towards plants. Isolates of R. solani can be divided into anastomosis groups (AGs) and this division accommodates part of the observed diversity. However, a clear link between AGs and relevant phytopathological characters is missing and a well-described genetic basis for AGs is lacking. To provide genetic information, we examined polymorphism of the internal transcribed spacer (ITS) of the ribosomal RNA genes and determined the electrophoretic karyotype by means of pulsed field gel electrophoresis (PFGE). ITS polymorphism confirmed the genetic isolation of AGs and indicated that genetically isolated subgroups exist within AGs. PFGE showed that R. solani isolates have at least 11 chromosomes ranging in size from 0.6 to 6 Mb. The R. solani genome size is estimated to be between 37 and 46 Mb. The karyotype of R. solani is highly variable and although the observed heterogeneity in karyotype within an AG is smaller than between AGs, karyotype and AG are not obviously correlated. Rather it can be concluded that the variation in electrophoretic karyotype is reflected in the phenotypical variation of R. solani, with generally less diversity within an AG than between AGs.