GEOSIPHON PYRIFORME, A FUNGUS FORMING ENDOCYTOBIOSIS WITH NOSTOC (CYANOBACTERIA), IS AN ANCESTRAL MEMBER OF THE GLOMALES - EVIDENCE BY SSU RIBOSOMAL-RNA ANALYSIS
H. Gehrig et al., GEOSIPHON PYRIFORME, A FUNGUS FORMING ENDOCYTOBIOSIS WITH NOSTOC (CYANOBACTERIA), IS AN ANCESTRAL MEMBER OF THE GLOMALES - EVIDENCE BY SSU RIBOSOMAL-RNA ANALYSIS, Journal of molecular evolution, 43(1), 1996, pp. 71-81
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.