Intraspeci c ITS Variability in the Kingdom Fungi as Expressed in the International Sequence Databases and Its Implications for Molecular Species Identi cation

The internal transcribed spacer (ITS) region of the nuclear ribosomal repeat unit is the most popular locus for species identi cation and subgeneric phylogenetic inference in sequence-based mycological research. The region is known to show certain variability even within species, although its intraspeci c variability is often held to be limited and clearly separated from interspeci c variability. The existence of such a divide between intraand interspeci c variability is implicitly assumed by automated approaches to species identi cation, but whether intraspeci c variability indeed is negligible within the fungal kingdom remains contentious. The present study estimates the intraspeci c ITS variability in all fungi presently available to the mycological community through the international sequence databases. Substantial differences were found within the kingdom, and the results are not easily correlated to the taxonomic af liation or nutritional mode of the taxa considered. No single unifying yet stringent upper limit for intraspeci c variability, such as the canonical 3% threshold, appears to be applicable with the desired outcome throughout the fungi. Our results caution against simpli ed approaches to automated ITS-based species delimitation and reiterate the need for taxonomic expertise in the translation of sequence data into species names.