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Geosiphon pyriforme inhabiting the surface of humid soils represents the 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 versiforme (Glomales; a typical AM fungus) were analyzed and aligned with SSU rRNA sequences of several Basidiomycetes, Ascomycetes, Chytridiomycetes, and Zygomycetes, together with all AM-fungal (Glomales) sequences published yet. The distinct group of the order Glomales, which includes Geosiphon, does not form a clade with any other group of Zygomycetes. Within the Glomales, two main lineages exist. One includes the families Gigasporaceae and Acaulosporaceae; the other one is represented by the genus Glomus, the members of which are very divergent. Glomus etunicatum and Geosiphon pyriforme both form independent lineages ancestral to the Glomales. The data provided by the present paper confirm clearly that Geosiphon represents a fungus belonging to the Glomales. The question remains still open as to whether or not Geosiphon is to be placed within or outside the genus Glomus, since this genus is probably polyphyletic and not well defined yet. Geosiphon shows the ability of a Glomus-like fungus to form a "primitive" symbiosis with a unicellular photoautotrophic organism, in this case a cyanobacterium, leading to the conclusion that a hypothetical association of a Glomus-like fungus with a green alga as a step during the evolution of the land plants appears probable.

The genus Clusia is notable in that it contains arborescent crassulacean acid metabolism (CAM) plants. As part of a study of CAM in Clusia, titratable acidities were measured in 25 species and í i^c values were measured for 38 species from Panamá, including seven undescribed species, and 11 species from Colombia, Costa Rica and Honduras. CAM was detected in 12 species. Clusia flava, C rosea and C. uvitana exhibited Í l^C values or diurnal fluctuations in acidity indicative of strong CAM. In C. croatii, C cylindrica, C. fructiangusta, C lineata, C odorata, C pratensis, C quadrangula, C. valerioi and C. sp. D diurnal fluctuations in acidity were consistent with weak CAM but the Í i^C values were Cß-like. AU of the species that exhibited strong or weak CAM were in the C. flava or C. minor species groups. CAM was not detected in any member of the C. multiflora species group. Strong CAM species were not collected at altitudes above 680 m a. s.l. On the basis of ^^^C values, the expression of CAM was similar in terrestrial, hemiepiphytic and epiphytic species and did not differ between individuals of the same species http://www.springerlink.com/media/ECXX5U63F83JUG8CA...butions/LA'/N/J/LVNJBJQ2KGNTAVP7_html/fulltext.html (1 of 39)6/5/2004 1:09:18 PM 10.1007/s00468-004-0342-y that exhibited different Hfe-forms. This study indicates that phylogenetic affihation may be a predictor of an ability to exhibit CAM in Clusia species from the Panamanian region, and that weak CAM is probably a common photosynthetic option in many Clusia species. í l^C value is not a particularly good indicator of a potential of Clusia species growing in the field to exhibit CAM because it appears that the contribution in most species of CAM to carbon gain is generally rather small when integrated over the life-time of leaves.

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