An integrated taxonomic approach refines the Imperator torosus species concept and reveals the cryptic I. occultus sp. nov. (Boletales, Boletaceae)

Imperator torosus is a polymorphic, critical Friesian bolete, originally described from Central Europe, but with a convoluted taxonomic history which caused much uncertainty in delimiting it from similar-looking species. Previous molecular phylogenetic analyses confidently indicated that I. torosus , as presently circumscribed, is a polyphyletic species in need of taxonomic revision. Based on a two-locus (ITS and LSU) phylogeny, pairwise distance and percent nucleotide similarity analyses, as well as morphological and ecological evidence, I. torosus s. str. is defined and Imperator occultus described as a novel species. Besides unravelling the intricate taxonomic history of I. torosus s. l., detailed macro- and micro-morphological descriptions based on well-documented collections are provided for both I. torosus s. str. and I. occultus , accompanied by colour photographs taken in their natural habitats and line drawings of their main microscopic features. A comprehensive and circumstantial discussion upon the reasons why the recent epitypification of B. torosus is deemed untenable is presented. Morphological comparisons with xanthoid phenotypes of Imperator rhodopurpureus ( I. rhodopurpureus f. xanthopurpureus and I. xanthocyaneus ), as well as their taxonomic placement within the genus Imperator are also elucidated and discussed.