Phylogenetic Subgrouping of Rhizoctonia solani AG 2 Isolates Based on Ribosomal ITS Sequences

Abstract

In Rhizoctonia solani differences in the sequences of ribosomal DNA correlate with differences in habitat and virulence. All the current data agree for a completely conserved 5.8 S region, but show variation in both internal transcribed spacers (ITS) regions. This variation in size and nucleotide sequence, in general, is proportionately greater within isolates with greater differences in anastomosis compatibility, pathogenicity and habitat. Rhizoctonia solani AG 2 is geographically widespread and consists of several anastomosis subgroups with specific pathogenic behavior. To obtain insight in the phylogenetic relations among subgroups, and between subgroups and geographically separated isolates, the complete DNA sequences of the ITS regions of 31 isolates of Rhizoctonia solani AG 2 and one isolate each of AG BI and AG 1 were determined, aligned and processed for phylogenetic reconstruction. The results presented here confirm the previously established subgroups of AG 2–1, AG 2–2 and AG 2–3, support the division of AG 2–2 into at least two subgroups, and furthermore divide AG 2–1 into three phylogenetically distinct entities: AG 2–1 and Japanese AG 2–1 and Dutch AG 2-t isolates. The AG BI isolate was phylogenetically closer to the AG 2–1 complex than to AG 2–2 or AG 2–3. Apparently fusion frequency among isolates from different subgroups is not in direct correlation with the phylogenetic relationship.