ORIGIN AND DIVERSIFICATION OF ENDOMYCORRHIZAL FUNGI AND COINCIDENCE WITH VASCULAR LAND PLANTS

AMONG the Eukaryota, the true fungi comprise four divisions (Chytridio mycota, Zymogomycota, Ascomycota and Basidiomycota) that constitute a natural group which is thought to have diverged about 1 billion (10(9) ) years ago, believed also to be the time of divergence between metaph yta and metazoa lineages1. The endosymbionts responsible for the most prevalent plant root symbiosis, the vesicular-arbuscular mycorrhizae ( VAM) or, more appropriately, arbuscular mycorrhizae, comprise 130 spec ies of fungi classified in the Zygomycotina, order Glomales2-4. The ar buscular endomycorrhizae are considered to be ecologically important f or most vascular plants5-8 in view of their beneficial effects on plan t growth and survival. They are one of the few plant-fungus associatio ns with a fossil record and may even have facilitated the origin of la nd flora. But the biochemical and genetic characterization of these mi crosymbionts has been hampered by the inability to grow them in pure c ulture. To investigate the origin and clarify the phylogenetic relatio nships of these organisms, we have sequenced ribosomal DNA genes from twelve species. Our phylogenetic analyses confirm the existence of thr ee families within arbuscular fungi on the basis of morphological char acters. We obtain approximate dates for the divergence of major branch es on the phylogenetic tree. These include an estimate for the origin of VAM-like fungi of 353-462 Myr ago, which is consistent with the hyp othesis that VAM were instrumental in the colonization of land by anci ent plants.