9629427 Donoghue Homobasidiomycetes are a diverse group of fungi that includes mushrooms, polypores, puffballs, stinkhorns, coral fungi, and others. Many species decay dead organic matter and are important in nutrient cycling. Others include plant pathogens, ectomycorrhizae (symbiotic associates of tree roots), lichen-forming fungi, and mutualist partners with arthropods such as the leaf-cutter ants of the neotropics. Despite their ecological and economic importance, the evolutionary or phylogenetic relationships of homobasidiomycetes are not well understood. One reason for this is that fungi are anatomically simple, and hence they display few characters that could be useful for discriminating among them and inferring taxonomic relationships. Another confounding factor is the poor fossil record for fungi in general, and for fleshy homobasidiomycetes in particular. Research by Drs. Michael Donoghue and David Hibbett at Harvard University is directed toward the goal of determining evolutionary relationships among the homobasidiomycetes using DNA sequences. The genes to be sequenced, called rDNA, encode the nucleic acids that make up ribosomes (organelles involved in protein synthesis). The rDNAs are highly conserved in sequence, which makes them useful for inferring very ancient evolutionary divergences. Both nuclear and mitochondrial rDNAs are being sequenced, from the same set of exemplar taxa. The sequences are aligned to each other and analyzed using computer algorithms that estimate relationships among genes based on the pattern of variation (mutational substitutions) in their DNA sequences. The resulting evolutionary hypothesis is represented as a phylogenetic tree. Two main groups of homobasidiomycetes are the focus of this research, the Polyporaceae and the Corticiaceae. The Polyporaceae includes fungi with poroid fruiting bodies such as many "bracket fungi", whereas the Corticiaceae includes fungi that have simple, resupinate (smooth, crust-like) fruiting bodies. Both families are de limited primarily on the basis of a few macromorphological features, but anatomical evidence suggests that they are composed of species from many different evolutionary lineages. One major question being addressed is: are the morphologically simple corticioid fungi a plesiomorphic (primitive) group from which other more complex forms have been derived, or have the corticioid forms evolved by reduction and simplification from more elaborate forms?
