Advances in Glomeromycota taxonomy and classification

Abstract

Concomitant morphological and molecular analyses have led to major breakthroughs in the taxonomic organization of the phylum Glomeromycota. Fungi in this phylum are known to form arbuscular mycorrhiza, and so far three classes, five orders, 14 families and 29 genera have been described. Sensulato, spore formation in 10 of the arbuscular mycorrhiza-forming genera is exclusively glomoid, one is gigasporoid, seven are scutellosporoid, four are entrophosporoid, two are acaulosporoid, and one is pacisporoid. Spore bimorphism is found in three genera, and one genus is associated with cyanobacteria. Here we present the current classification developed in several recent publications and provide a summary to facilitate the identification of taxa from genus to class level.

Keywords: Archaeosporomycetes; Gigasporales; Glomerales; Glomeromycetes; Paraglomeromycetes; VA mycorrhiza; endomycorrhizas; evolution; phylogeny.

Fig. 1.

Representative tree of the phylum Glomeromycota based on molecular (SSU, ITS region, partial LSU of the rRNA gene, and partial β-tubuline gene) and morphological analyses (spore wall structures, structures of the spore bases and subtending hyphae, germination, and germination shield structures). Adapted from (Oehl et al. 2008, –d). The drawings in the central columns show the spore formation types of the genera, and the typical germination shields for those genera which form persistent shields already during spore formation.

Figs 2–11.

Characteristic germination shields in Gigasporales with germ pore (gp) as connection between spore cell contents and shields that are positioned on the surface of the germinal wall; germ tubes emerge from germ tube initiations (gti). Fig. 2. Orbispora pernambucana (isotype, ZT Myc 641) with mono-lobed, hyaline germ shield (orb). Figs 3–4. Scutellospora calospora (photo taken at INVAM) and S. dipurpurescens (holotype OSC #83343) have bi-lobed, violin-shaped, hyaline shields. Figs 5–8. Racocetra coralloidea (type, OSC #31026), R. castanea (ex type, ZT Myc 4377), Cetraspora nodosa (isotype, DPP, Szczecin, Poland) and C. helvetica (isotype, ZT Myc 3038) have wavy-like, multiply lobed, hyaline shields. Figs 9–11. Dentiscutataceae shields are yellow brown to brown. Fig. 9. Dentiscutata reticulata (photo taken at INVAM) shields with multiple small compartments. Fig. 10. Quatunica erythropa (photo taken at INVAM) is assumed to be the only known species in Glomeromycota with four spore walls. Fig. 11. Fuscutata heterogama (ex type, ZT Myc 642) has a bi-lobed, oval to ovoid shield.

Figs 12–21.

Characteristic spore bases and subtending hyphae (sh) in Glomeromycetes genera with glomoid spore formation. Figs 12–13. Glomus ambisporum (Oehl collection, from Bolivia) and G. aureum (type, ZT Myc 822) with two wall layers (SWL1 and SWL2), marked introverted wall thickening at sb and in sh, and a small, bridging septum (sp). Fig. 14. Funneliformis coronatus (ex type, Oehl collection) with funnel-shaped sh and conspicuous sp; introverted wall thickening is lacking. Fig. 15. Septoglomus constrictum (Oehl collection, from Switzerland) with conspicuous septum that sometimes resembles a plug. Fig. 16. Simiglomus hoi (Oehl collection, specimen mounted at York university) with cylindrical sh; sh wall thickened over long distances; several septae are regularly observed within the hyphae; no introverted wall thickening at sb, pore at sb generally opened. Fig. 17. Claroideoglomus etunicatum (Oehl collection, from Bolivia) with funnel/bill-shaped, white sh; all Entrophosporaceae (syn. Claroideoglomeraceae) with characteristic color change of structural wall layer at sb, if spores are pigmented. Fig. 18. Albahypha drummondii (type, DPP) with slightly funnel-shaped, white sh. Fig. 19. Viscospora viscosa (ex type, photo taken at INVAM) with cylindrical, white hypha; sp within sh in some distance of sb; introverted wall thickening of sh at sp position, here not that obvious as usually found for this species; viscose spore surface. Fig. 20. Diversispora versiformis with short, fragile sh that is principally continuous with semi-persistent outermost spore wall layer (SWL1) but not with structural layer SWL2 (Oehl collection, from Tibet). Fig. 21. Redeckera fulva (Oehl ex Trappe collection) with inflating sh and conspicuous broad sp exactly at spore base.