Special Interest Group Meeting

Evolution of arbuscular mycorrhizal (AM) fungi and the AM symbiosis

Arthur Schüßler, LMU Munich, Department Biology, Genetics, Grosshaderner Str. 4, 82152 Martinsried, Germany
Email: arthur.schuessler@lmu.de

Arbuscular mycorrhizal fungi (AMF, Glomeromycota) represent one of the earliest diverging terrestrial fungal lineages. As far as is known, they now all are obligate symbionts associated with >80% of vascular land plants, but also many bryophytes. The AMF provide inorganic nutrients and water to the plants and symbioses similar to extant arbuscular mycorrhiza (AM) type of were already formed by root-less plants some 400-450 Myr ago. Some new data from genome sequencings indicate that AMF may have a common ancestor with lineages that include the zygomycetes and not with the Dikarya. The latter was weekly supported by the SSU rRNA gene phylogeny. However, to date only for one AMF species are preliminary genome data available, and the monophyletic status at the phylum level, Glomeromycota, is unquestioned.

The AM symbioses likely is ‘homologous’ in all embryophytes (land plants), implicating that AM-like associations of bryophytes (liverworts, hornworts, mosses), lycophytes, ferns and seed plants probably originated at an early point of common ancestry of land plants. According to this concept, AM-less land plants (perhaps including most mosses) evolved from AM forming precursors. The molecular dating of AMF origin seems earlier than that for land plants, which leads to theories about even more basal photoautotrophic symbioses formed by ancient AMF lineages, such as with cyanobacteria.

It has been shown that AMF harbour ancestral bacterial endosymbionts (see talk of Paola Bonfante, this meeting), which are Gram-positive and called bacterium-like organisms (BLOs). Unexpectedly, the BLOs cluster within the Mollicutes whose symbiotic or parasitic members, in contrast to BLOs, lack cell walls. Perhaps BLOs maintained the Gram-positive trait, whereas the sister groups lost it. BLOs are vertically inherited, monophyletic and globally distributed endobacteria thriving exclusively in the AMF cytoplasm. They separated from their sister groups >400 Myr ago and colonized their fungal hosts already before main AMF lineages separated. This AMF-BLO symbiosis can, therefore, also be dated back at least to the times when the first land plants emerged. This adds an additional not yet understood level of complexity to the early evolution of the Glomeromycota and the evolution and functioning of the AM, which is a fundamental ‘driving’ component of nearly all recent terrestrial ecosystems.