ITS Variability and Population Differentiation in Fomes fomentarius and Fomes inzengae

One of the unresolved issues in modern fungal molecular systematics is the lack of criteria for distinguishing intraspecific from interspecific variability of genetic markers. This paper discusses the results of an analysis of 69 original ITS sequences obtained from fungi collected in the Altai, the Urals, and northern Kazakhstan regions, and 42 sequences retrieved from GenBank and UNITE, belonging to two cryptic, sympatric polypore species: Fomes fomentarius (L.) Fr. and Fomes inzengae (Ces. & De Not.) Cooke. The sequences were divided into geographic groups reflecting a wide range of climatic conditions (from subarctic to subtropical) and geographic distances between sampling locations. Both species are characterized by low intragroup nucleotide diversity of ITS sequences and high nucleotide similarity, indicating low individual variability. A high, nearly complete identity in the ITS region of rDNA is also characteristic at the intergroup level, expressed in very low nucleotide divergence and high nucleotide similarity of ITS sequences from different geographic groups. The fixation index, calculated from the ratio of nucleotide diversity to divergence of ITS sequences, indicates a complete absence of genetic differentiation between local populations of F. inzengae and moderate differentiation in F. fomentarius. However, between populations of these two species, even in areas of their joint distribution, complete or very high genetic isolation occurs, confirming the validity of dividing F. fomentarius sensu lato into two cryptic sympatric species—F. fomentarius and F. inzengae.