GEOSIPHON PYRIFORME, A FUNGUS FORMING ENDOCYTOBIOSIS WITH NOSTOC (CYANOBACTERIA), IS AN ANCESTRAL MEMBER OF THE GLOMALES - EVIDENCE BY SSU RIBOSOMAL-RNA ANALYSIS

Geosiphon pyriforme inhabiting the surface of humid soils represents t he only known example of endocytobiosis between a fungus (Zygomycotina ; macrosymbiont) and cyanobacteria (Nostoc; endosymbiont). In order to elucidate the taxonomical and evolutionary relationship of Geosiphon pyriforme to fungi forming arbuscular mycorrhiza (AM fungi), the small -subunit (SSU) ribosomal RNA genes of Geosiphon pyriforme and Glomus v ersiforme (Glomales; a typical AM fungus) were analyzed and aligned wi th SSU rRNA sequences of several Basidiomycetes, Ascomycetes, Chytridi omycetes, and Zygomycetes, together with all AM-fungal (Glomales) sequ ences published yet. The distinct group of the order Glomales, which i ncludes Geosiphon, does not form a clade with any other group of Zygom ycetes. Within the Glomales, two main lineages exist. One includes the families Gigasporaceae and Acaulosporaceae; the other one is represen ted by the genus Glomus, the members of which are very divergent. Glom us etunicatum and Geosiphon pyriforme both form independent lineages a ncestral to the Glomales. The data provided by the present paper confi rm clearly that Geosiphon represents a fungus belonging to the Glomale s. The question remains still open as to whether or not Geosiphon is t o be placed within or outside the genus Glomus, since this genus is pr obably polyphyletic and not well defined yet. Geosiphon shows the abil ity of a Glomus-like fungus to form a ''primitive'' symbiosis with a u nicellular photoautotrophic organism, in this case a cyanobacterium, l eading to the conclusion that a hypothetical association of a Glomus-l ike fungus with a green alga as a step during the evolution of the lan d plants appears probable.