A phylogenetic census of global diversity of gut anaerobic fungi and a new taxonomic framework

Abstract

Obligate anaerobic fungi of the phylum Neocallimastigomycota play a key role in digesting fibrous feeds in the gut of herbivores, but little is known about their global diversity. In this study, the collective diversity of gut anaerobic fungi was examined using all curated internal transcribed spacer 1 (ITS1) sequences of anaerobic gut fungi available in GenBank. The 262,770 quality-checked fungal ITS1 sequences downloaded from GenBank were assigned to 274 operational taxonomic units (OTUs) at the approximate species level. Of these approximate species-equivalent (Sp-eq) OTUs, 119 were represented by at least five ITS1 sequences, with 38 containing known species and 81 containing no known species. Based on a rarefaction analysis, the currently available ITS1 sequences represent nearly all the major species of gut anaerobic fungi, but much more sequencing effort is needed to assess the actual richness of minor OTUs. One dataset of ITS1 reference sequences (referred to as AF-RefSeq) and one comprehensive taxonomic framework are also presented, and they are shown to be suitable for taxonomic classification of most of the ITS1 sequences in GenBank. The results of the present study may help guide future studies involving taxonomic and phylogenetic analysis of ITS1 sequences of anaerobic fungi and targeted isolation and characterization of new anaerobic fungi.