Dating the evolutionary radiations of the true fungi

In this paper we construct a relative time scale for the origin and radiation of major lineages of the true fungi, using the 18S ribosomal RNA gene sequence data of 37 fungal species, and then calibrate the time scale using fossil evidence. Of the sequences, 28 were from the literature or data banks and the remaining 9 are new. To estimate the order of origin of fungal lineages we reconstructed the phylogeny of the fungi using aligned sequence data. To compensate for the differences in nucleotide substitution rates among various fungal lineages, we normalized the pairwise substitution data before estimating the relative timing of fungal divergences. We divided the fungi into nine groups. We then calculated the average percent substitution for each group, and also the average for all the groups, for the time period beginning when the fungi diverged from a common ancestor and ending at the present. We used the ratios of group-specific percent substitutions to the average percent substitution to normalize our pairwise substitution data matrix. To infer the relative timing of the origin of lineages we superimposed the normalized percentages of nucleotide substitutions onto the parsimony-based phylogeny. Calibrating the rate of sequence change involved relating the normalized percent substitution associated with phylogenetic events to fungal fossils, the ages of fungus hosts, and ages of symbionts. These calibration points were consistent with a substitution rate of 1% per lineage per 100 Ma. Based on phylogeny and calibrated percent substitution, the terrestrial fungi diverged from the chytrids approximately 550 Ma ago. After plants invaded the land approximately 400 Ma ago, ascomycetes split from basidiomycetes. Mushrooms, many ascomycetous yeasts, and common molds in the genera Penicillium and Aspergillus may have evolved after the origin of angiosperm plants and in the last 200 Ma. Key words: fungus evolution, molecular clock, ascomycete phylogeny, basidiomycete phylogeny, 18S rRNA.