Fine endophytes (Glomus tenue) are related to Mucoromycotina, not Glomeromycota

Abstract

Fine endophytes (FE),Glomus tenue, are traditionally considered to be arbuscular mycorrhizal fungi (AMF) with distinctive microscopic morphology when stained. FE have fine hyphae (c. 1.5 lm diameter) which branch intra- cellularly in a distinctive fan-like pattern (Gianinazzi-Pearson et al., 1981; Abbott, 1982). The hyphae contain small swellings along their length, sometimes referred to as vesicle-like swellings (Hall, 1977). FEform arbuscules (or arbuscule-like structures) with fine elements in a tapered, conical shape (Greenall, 1963; Merryweather & Fitter, 1998). Spores of FE are very small (< 20 lm) compared to the majority of Glomeromycota, and colourless (Hall, 1977). Morphological variations indicate that FE may consist of multiple species (Thippayarugs et al., 1999), hence we use the term FE to indicate a species group.

Within the kingdom Fungi, both morphological and genetic characteristics are used to determine taxonomic classification (Stürmer, 2012). In 2001, all AMF were placed within the phylum Glomeromycota (Schüßler et al., 2001). In the listing of glomeromycotan species by Schüßler & Walker (2010), some members of the genus Glomus were not revised due to insufficient taxonomic knowledge, and this included FE. A key reason for classifying FE within the Glomeromycota was the presence of arbuscules, considered apomorphic for the phylum (Morton, 1990). However, the morphological features of root colonization by FE are distinct from other, coarse, AMF so their placement within the genus Glomus and the Glomeromycota was questioned (Hall, 1977; Schüßler & Walker, 2010), and their status as mycorrhizal fungi is ambivalent.