Forecasting the number of species of asexually reproducing fungi (Ascomycota and Basidiomycota)

Asexually reproducing fungi play a significant role in essential processes in managed and wild ecosystems such as nutrients cycling and multitrophic interactions. A large number of such taxa are among the most notorious plant and animal pathogens. In addition, they have a key role in food production, biotechnology and medicine. Taxa without or rare sexual reproduction are distinguished based on their sporulating structures and conidiomata in traditional morphology-based taxonomy. The number, variation and diversity of asexually reproducing taxa are insufficiently known, even though fungi capable of asexual reproduction may provide an untapped, rich biological resource for future exploitation. Currently, ca. 30,000 asexual species belonging to ca. 3800 genera have been reported (including 1388 coelomycetous and 2265 hyphomycetous genera). Recent reports (2017–2020) reiterate that the number of asexually producing fungi is higher than the number of frequently sexually-reproducing fungi. With the advent of molecular tools and the abandonment of the dual nomenclature system for pleomorphic fungi, priority criteria were established and revisited in the latest outline of fungi and fungus-like taxa. However, species numbers and taxonomic boundaries of pleomorphic taxa and their synanamorphs or synasexual morphs have yet to be addressed. The number of species of speciose genera (e.g. Alternaria, Aspergillus, Cercospora, Fusarium, Phoma and Pseudocercospora), cryptic species, species of pleomorphic genera, less studied life modes (such as lichenicolous taxa, taxa from extreme environments) and species from biodiversity-rich areas still need evaluation to achieve more reliable estimates of their diversity. This paper discusses the current knowledge on the matter, with diversity estimates, and potential obstacles in several chapters on (1) speciose genera; (2) pleomorphic genera; (3) cryptic species; (4) well-studied but insufficiently resolved taxa, e.g. leaf inhabiting species, marine fungi, (5) less studied life modes, e.g. lichenicolous, rock-inhabiting fungi, insect-associated and yeast-forming taxa and (6) species from biodiversity-rich areas.