Septoglomus mexicanum, a new species of arbuscular mycorrhizal fungi from semiarid regions in Mexico

Septoglomus mexicanum is here described as a new species of arbuscular mycorrhizal fungi (AMF; Glomeromycota) based on morphological and phylogenetic analyses. It was isolated from rhizospheric soil of two endemic Mexican legumes: Prosopis laevigata and Mimosa luisana, which grow in semiarid regions of central Mexico. Septoglomus mexicanum is characterized by forming globose spores of (154.5–)202.8(–228.9) µm diam and a spore wall consisting of four layers (SWL1–SWL4): outer wall layer (SWL1) hyaline, evanescent, (1.7–)3.2(–4.3) µm thick; SWL2 laminate and smooth, orange to reddish orange, (3.1–)4.5(–6.1) µm thick; SWL3 laminate, smooth, reddish orange to reddish brown, (4.1–)5.1(–5.7) µm thick; and SWL4 hyaline, semiflexible, (0.93–)1.2(–1.4) µm thick. None of the spore wall layers stain with Melzer’s reagent. The subtending hypha has a color from yellowish to golden and presents a septum on spore base. Septoglomus mexicanum can be distinguished from all other Septoglomus species by spore size and color, by spore wall structure (four layers), and by color change of the subtending hypha. Phylogenetic analysis based on the AMF extended DNA barcode covering a 1.5-kb fragment of the small subunit (SSU), internal transcribed spacer region (ITS1-5.8S-ITS2), and the large subunit (LSU) of rRNA genes places S. mexicanum in the genus Septoglomus, separated from other described Septoglomus species, especially S. turnauae, with whom it could be confused morphologically. All available sequences in public databases suggest that this new fungal species has not yet been previously detected. Thus, there are currently 149 Glomeromycota species registered in Mexico, representing 47.4% of the known species worldwide.