Mostafa S. Elshahed

The anaerobic gut fungi (AGF, Neocallimastigomycota ) represent a basal zoosporic phylum within the kingdom Fungi . Twenty genera are currently described, all of which were isolated from the digestive tracts of mammalian herbivores. Here, we report on the isolation and characterization of novel AGF taxa from faecal samples of tortoises. Twenty-nine fungal isolates were obtained from seven different tortoise species. Phylogenetic analysis using the D1/D2 region of the LSU rRNA gene, ribosomal internal transcribed spacer 1, and RNA polymerase II large subunit grouped all isolates into two distinct, deep-branching clades (clades T and B), with a high level of sequence divergence to their closest cultured relative ( Khoyollomyces ramosus ). Average amino acid identity values calculated using predicted peptides from the isolates’ transcriptomes ranged between 60.80–66.21 % (clade T), and 61.24–64.83 % (clade B) when compared to all other AGF taxa; values that are significantly below recently recommended thresholds for genus (85%) and family (75%) delineation in the Neocallimastigomycota . Both clades displayed a broader temperature growth range (20–45 °C, optimal 30 °C for clade T, and 30–42 °C, optimal 39 °C for clade B) compared to all other AGF taxa. Microscopic analysis demonstrated that strains from both clades produced filamentous hyphae, polycentric rhizoidal growth patterns, and monoflagellated zoospores. Isolates in clade T were characterized by the production of unbranched, predominantly narrow hyphae, and small zoospores, while isolates in clade B were characterized by the production of multiple sporangiophores and sporangia originating from a single central swelling resulting in large multi-sporangiated structures. Based on the unique phylogenetic positions, AAI values, and phenotypic characteristics, we propose to accommodate these isolates into two novel genera ( Testudinimyces and Astrotestudinimyces ), and species ( T. gracilis and A. divisus ) within the order Neocallimastigales . The type species are strains T130A T ( T. gracilis ) and B1.1 T ( A. divisus ).