Redefining Humicola sensu stricto and related genera in the Chaetomiaceae. Redefining Humicola sensu stricto and related genera in the Chaetomiaceae

The classic concept of the genus Humicola includes species that produce spores normally referred to as chlamydospores or aleurioconidia. These spores are usually produced laterally or terminally on hyphae or on minimally differentiated conidiophores, and are pigmented, thick-walled and single-celled. Over 50 species have been described in the genus. Species commonly occur in soil, indoor environments and composting habitats. The taxonomy of Humicola and morphologically similar genera is poorly understood in modern terms. Based on a four-locus phylogeny and morphological data, the genus Humicola proved to be polyphyletic. The type of Humicola, H. fuscoatra, belongs to the Chaetomiaceae. In the Chaetomiaceae, species producing humicola-type thick-walled spores are distributed over four lineages: Humicola, Mycothermus, Staphylotrichum and Trichocladium. In the genus Humicola, asexual and sexually reproducing species are known. The re-defined genus Humicola contains 24 species (seven new and thirteen new combinations) and those are described and illustrated in this study. The species in this genus produce chlamydospores laterally, intercalary or terminally on/in hyphae, as conidiophore formation is absent. The ascospores of sexual Humicola species are limoniform to quadrangular in face view and bilaterally flattened with one apical germ pore. Seven species are accepted in Staphylotrichum (four new species, one new combination). Thick-walled conidia of Staphylotrichum species usually arise either from hyphae (micronematous) or from apical branched seta-like conidiophores (macronematous). Staphylotrichum longicolleum (= Chaetomium longicolleum) represents the sexual morph of Staphylotrichum, and produces ascomata with long necks composed of a fused basal part of the terminal hairs, and ascospores that are broadly broad limoniform to nearly globose, bilaterally flattened, with an apical germ pore. The Trichocladium lineage has a high morphological diversity in both asexual and sexual structures. Phylogenetic analysis revealed four subclades in the lineage. However, these subclades are genetically closely related, and no phenotypic characters could be linked to any of those subclades. Fifteen species are accepted in Trichocladium (one new species, thirteen new combinations). The type species of Gilmaniella, G. humicola, belongs to the polyphyletic family Lasiosphaeriaceae (Sordariales). Gilmaniella macrospora phylogenetically belongs to Trichocladium and is combined in this genus. The thermophilic genus Mycothermus and the type species My. thermophilum are validated, and one new Mycothermus species is described. Phylogenetic analyses show that Remersonia, another thermophilic genus, is a sister genus of Mycothermus and two species are present in this genus (incl. one new species). Thermomyces verrucosus produces humicola-like conidia and is combined with Botryotrichum on the basis of phylogenetic affinities. The present study is a first attempt to establish an inclusive modern classification of Humicola and humicola-like genera of the Chaetomiaceae. More studies are needed to determine the phylogenetic relationships of “humicola”-like species outside the Chaetomiaceae.