A multigene phylogeny of Olpidium and its implications for early fungal evolution

We concatenated sequences from the Ef-2, RPB1, RPB2 and actin loci for maximum likelihood and Bayesian analyses. In the resulting trees, Olpidium virulentus, O. bornovanus and non-flagellated terrestrial fungi formed a strongly supported clade. Topology tests rejected monophyly of the Olpidium species with any other clades of flagellated fungi. Placing Olpidium at the base of terrestrial fungi was also rejected. Within the terrestrial fungi, Olpidium formed a monophyletic group with the taxa traditionally classified in the phylum Zygomycota. Within Zygomycota, Mucoromycotina was robustly monophyletic. Although without bootstrap support, Monoblepharidomycetes, a small class of zoosporic fungi, diverged from the basal node in Fungi. The zoosporic phylum Blastocladiomycota appeared as the sister group to the terrestrial fungi plus Olpidium.This study provides strong support for Olpidium as the closest living flagellated relative of the terrestrial fungi. Appearing nested among hyphal fungi, Olpidium's unicellular thallus may have been derived from ancestral hyphae. Early in their evolution, terrestrial hyphal fungi may have reproduced with zoospores.Fungi in modern ecosystems are able to cause plant diseases, serve as mycorrhizal partners to plants, or decompose litter and woody debris using the tubular hyphae (filaments of walled cells) that make up fungal bodies. Hyphae use hydrostatic pressure to penetrate tough substrates such as soil and plant tissue, secreting enzymes across their chitinous cell walls to break down complex organic compounds into simple, diffusible molecules that are absorbed to nourish growth. An increasing body of phylogenetic evidence indicates that fungi, animals, and protists, such as nucleariid amoebae and Ichthyosporea, all share a close common ancestor [1-3]. This pattern implies that the original fungus-like organisms were not terrestrial and hyphal in their assimilative phase but were instead aquatic, flagellated and unicellular.Fungi th