Using ITS2 secondary structure to create species-specific oligonucleotide probes for fungi

Oligonucleotide microarray based on ITS2 rDNA sequences would be extremely useful in identifying fungi within soil samples. However ITS2 contains phylogenetic information and duplication of sequences among taxa make false positive detections likely unless a way could be found to identify taxon-specific portions of the ITS2 sequence a priori. Examination of component ITS2 sequences suggested one method of identifying species-specific probes. Analysis of 168 fungal ITS2 sequences showed that all 168 ITS2 rRNA sequences could be folded to produce similar secondary structures of 3-4 loops. Unique probes occurred most often in the second loop. While the loop 2 sequence was unique in all taxa, there were partial congeneric and intergeneric duplicates. Evidence for a decrease in duplicates with increasing phylogenetic distance was mixed. From the evidence, 2 or 3 disjunct oligonucleotide probes from the loop 2 sequence might be sufficient to identify most fungal species. This combination appears minimally susceptible to false positives and conceivably could be extended to design probes to identify any eukaryotic species.