Taxonomy of the anaerobic gut fungi (Neocallimastigomycota): a review of classification criteria and description of current taxa

Members of the anaerobic gut fungi (Neocallimastigomycota) reside in the rumen and alimentary tract of larger mammalian and some reptilian, marsupial, and avian herbivores. The recent decade has witnessed a significant expansion in the number of described Neocallimastigomycota genera and species. However, the lack of a centralized culture collection, coupled with difficulties associated with their isolation and maintenance has greatly complicated comparative studies to resolve inter- and intra-genus relationships. Here, we provide an updated outline of Neocallimastigomycota taxonomy. We critically evaluate various morphological, microscopic, and phylogenetic traits previously and currently utilized in Neocallimastigomycota taxonomy, and provide an updated key for quick characterization of all genera. We then synthesize data from taxa description manuscripts, prior comparative efforts, and molecular sequence data to present an updated list of Neocallimastigomycota genera and species, with emphasis on resolving relationships and identifying synonymy between recent and historic strains. We supplement data from published manuscripts with information and illustrations from strains in the authors’ collections. Twenty genera and thirty-six species are recognized, but the status of ten species in the genera Caecomyces, Piromyces, Anaeromyces, and Cyllamyces remains uncertain due to the unavailability of culture and conferre (cf.) strains, lack of sequence data, and/or inadequacy of available microscopic and phenotypic data. Six cases of synonymy are identified in the genera Neocallimastix and Caecomyces, and two names in the genus Piromyces are rejected based on apparent misclassification. &nbsp