Comparison of ribosomal DNA ITS regions among geographic isolates of Cenococcum geophilum

Cenococcum geophilum is an ecologically important mycorrhizal fungus with a global distribution and a wide host range. It has been difficult to study since it forms only sterile mycelia and, occasionally, sclerotial bodies. B ecause of its lack of morphological variability, its taxonomy and phylogene tic origins have until recently remained unclear. To better understand the genetic Variation and environmental adaptability of C. geophilum, a molecul ar phylogeny was constructed based on the nuclear ribosomal DNA internal tr anscribed spacers (ITS1 and ITS2) of 69 isolates from various hosts and hab itats. The results suggest DNA sequence conservation in the ITS regions. Co nsidering its broad geographic and host range, this ITS conservation was un expected. Our data imply that the ITS:! region is under evolutionary pressu re to maintain the RNA secondary structure (similar to the pressure on the CgSSU introns) involved in the post-transcriptional processing of rRNA. Als o, C. geophilum has very short ITS regions, thus limiting the number of mut able sites. This limited ITS variability suggests a recent radiation of C. geophilum, having been geographically distributed by a variety of efficient processes. C. geophilum appears to be a single taxonomic entity, possibly a single species. Therefore, it is an extremely adaptable, as well as ecolo gically valuable, taxon.