A new species concept for the clinically relevant <i>Mucor circinelloides</i> complex

<i>Mucor</i> species are common soil fungi but also known as agents of human infections (mucormycosis) and used in food production and biotechnology. <i>Mucor circinelloides</i> is the <i>Mucor</i> species that is most frequently isolated from clinical sources. The taxonomy of <i>Mucor circinelloides</i> and its close relatives (<i>Mucor circinelloides</i> complex - MCC) is still based on morphology and mating behaviour. The aim of the present study was a revised taxonomy of the MCC using a polyphasic approach. Using a set of 100 strains molecular phylogenetic analysis of five markers (ITS, <i>rpb1</i>, <i>tsr1</i>, <i>mcm7</i>, and <i>cfs</i>, introduced here) were performed, combined with phenotypic studies, mating tests and the determination of the maximum growth temperatures. The multi-locus analyses revealed 16 phylogenetic species of which 14 showed distinct phenotypical traits and were recognised as discrete species. Five of these species are introduced as novel taxa: <i>M. amethystinus</i> sp. nov., <i>M. atramentarius</i> sp. nov., <i>M. variicolumellatus</i> sp. nov., <i>M. pseudocircinelloides</i> sp. nov., and <i>M. pseudolusitanicus</i> sp. nov. The former formae of <i>M. circinelloides</i> represent one or two separate species. In the MCC, the simple presence of well-shaped zygospores only indicates a close relation of both strains, but not necessarily conspecificity. Seven species of the MCC have been implemented in human infection: <i>M. circinelloides</i>, <i>M. griseocyanus, M. janssenii, M. lusitanicus</i>, <i>M. ramosissimus, M. variicolumellatus</i>, and <i>M. velutinosus</i>.