Phylogenetic placement and host specificity of mycorrhizal isolates belonging to AG-6 and AG-12 in the Rhizoctonia solani species complex

Rhizoctonia solani isolates belonging to anastomosis group 6 (AG-6) and AG- 12 are mycorrhizal with the Australian orchid Pterostylis acuminata. In thi s study, phylogenetic analyses were conducted using sequences of the ITS (i nternal transcribed spacer) region of the rRNA gene from all known anastomo sis groups of R. solani. The mycorrhizal isolates of AG-6 and AG-12 were pl aced on independent branches, despite their functional and geographic simil arities. The mycorrhizal AG-6 isolates were positioned most closely to the pathogenic AG-6 isolates from Southern Africa. The remaining AG-6 isolates froth Japan and the USA were positioned in a distinct and independent clust er, suggesting that various sub-groups of AG-6 have evolved independently w ithin R. solani. Germination tests were conducted using AG 6 isolates obtai ned froth pathogenic and saprophytic interactions, as well as isolates belo nging to the pathogenic AG 3, 4, 5, 7, and 8. All AG-6 isolates promoted ge rmination of P. acuminata seed, regardless of their origin, although they d id so more slowly than the AG-12 isolates. All other AGs either parasitized or disregarded the orchid seed, indicating that the ability to form mycorr hizal interactions is restricted in R. solani. This is the first phylogenet ic analysis to include all known AG with a range of host associations and g eographic origins including the newly described mycorrhizal AG, and will as sist in understanding the evolutionary history and host specificity of this fungus.