Redefining<i>Humicola sensu stricto</i>and related genera in the<i>Chaetomiaceae</i>

Abstract

The traditional concept of the genus <i>Humicola</i> includes species that produce pigmented, thick-walled and single-celled spores laterally or terminally on hyphae or minimally differentiated conidiophores. More than 50 species have been described in the genus. Species commonly occur in soil, indoor environments, and compost habitats. The taxonomy of <i>Humicola</i> and morphologically similar genera is poorly understood in modern terms. Based on a four-locus phylogeny, the morphological concept of <i>Humicola</i> proved to be polyphyletic. The type of <i>Humicola</i>, <i>H. fuscoatra</i>, belongs to the <i>Chaetomiaceae</i>. In the <i>Chaetomiaceae</i>, species producing humicola-like thick-walled spores are distributed among four lineages: <i>Humicola sensu stricto</i>, <i>Mycothermus</i>, <i>Staphylotrichum,</i> and <i>Trichocladium</i>. In our revised concept of <i>Humicola</i>, asexual and sexually reproducing species both occur. The re-defined <i>Humicola</i> contains 24 species (seven new and thirteen new combinations), which are described and illustrated in this study. The species in this genus produce conidia that are lateral, intercalary or terminal on/in hyphae, and conidiophores are not formed or are minimally developed (micronematous). The ascospores of sexual <i>Humicola</i> species are limoniform to quadrangular in face view and bilaterally flattened with one apical germ pore. Seven species are accepted in <i>Staphylotrichum</i> (four new species, one new combination). Thick-walled conidia of <i>Staphylotrichum</i> species usually arise either from hyphae (micronematous) or from apically branched, seta-like conidiophores (macronematous). The sexual morph represented by <i>Staphylotrichum longicolleum</i> (= <i>Chaetomium longicolleum</i>) produces ascomata with long necks composed of a fused basal part of the terminal hairs, and ascospores that are broad limoniform to nearly globose, bilaterally flattened, with an apical germ pore. The <i>Trichocladium</i> lineage has a high morphological diversity in both asexual and sexual structures. Phylogenetic analysis revealed four subclades in this lineage. However, these subclades are genetically closely related, and no distinctive phenotypic characters are linked to any of them. Fourteen species are accepted in <i>Trichocladium,</i> including one new species, twelve new combinations. The type species of <i>Gilmaniella</i>, <i>G. humicola</i>, belongs to the polyphyletic family <i>Lasiosphaeriaceae</i> (<i>Sordariales</i>), but <i>G. macrospora</i> phylogenetically belongs to <i>Trichocladium</i>. The thermophilic genus <i>Mycothermus</i> and the type species <i>My. thermophilum</i> are validated, and one new <i>Mycothermus</i> species is described. Phylogenetic analyses show that <i>Remersonia</i>, another thermophilic genus, is sister to <i>Mycothermus</i> and two species are known, including one new species. <i>Thermomyces verrucosus</i> produces humicola-like conidia and is transferred to <i>Botryotrichum</i> based on phylogenetic affinities. This study is a first attempt to establish an inclusive modern classification of <i>Humicola</i> and humicola-like genera of the <i>Chaetomiaceae</i>. More research is needed to determine the phylogenetic relationships of "humicola"-like species outside the <i>Chaetomiaceae</i>.