|BERRES MARK E - UNIVERSITY OF MINNESOTA
|MCLAUGHLIN DAVID - UNIVERSITY OF MINNESOTA
Submitted to: Mycologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/3/1995
Publication Date: N/A
Interpretive Summary: Rust fungi cause some of the most destructive diseases of small grain crops. Rust fungi are notorious for their ability to overcome resistant cereal varieties by evolving new pathogenic races. Understanding their genetics is one of the keys to learning how to better combat these shifty enemies. Unfortunately, the unique biology of rust fungi makes them much more difficult than most fungi to study genetically. Knowledge gained from research on other fungi can accelerate progress in genetic studies of rust fungi if those fungi are closely related through common ancestry. We analyzed these evolutionary relationships among a diverse group of fungi including cereal rust and smut fungi by comparing structural similarities of their genetic material. Our results show that the rusts are only distantly related to smut fungi or to some of the common wood decay fungi for which much genetic knowledge is available. The results indicate that rusts are part of a very old lineage of fungi that includes pathogens of both insects and plants. Scientists will use this knowledge to find related fungus species for which genetic studies are not difficult and will advance research on genetics of economically important rusts fungi.
Technical Abstract: The nucleotide sequences of the 5' end of the nuclear large subunit ribosomal RNA (nlsrDNA) were obtained for 15 basidiomycetes representing the orders Agaricostilbales, Atractiellales, Auriculariales sensu lato, Boletales, Septobasidiales, Tilletiales, Uredinales and Ustilaginales. These sequences were analyzed by methods of maximum parsimony, minimum- evolution, neighbor-joining and maximum likelihood. Using Saccharomyces cerevisiae as an outgroup, the basidiomycetes analyzed formed six distinct clades: the monocot smuts, a complex-septate group, two separate groups of auriculariaceous simple-septate species, the rusts and a unique branch comprising a single taxon Agaricostilbum pulcherrimum. All algorithmic methods produced trees with nearly identical topologies except in the placement of Agaricostilbum pulcherrimum. To statistically differentiate among the topologies, each tree with branch lengths estimated under models of maximum parsimony and maximum likelihood was subjected to a likelihood ratio test. Under assumptions of maximum parsimony, the test rejected the minimum-evolution and neighbor-joining trees. With branch lengths estimated under a maximum likelihood model, all topologies except the bootstrapped parsimony were rejected and the maximum likelihood topology was the highest likelihood tree. Of these two topologies, only the relative position of Agaricostilbum pulcherrimum varied. These results suggest that the 5' end of the nuclear large subunit ribosomal DNA is useful for intergeneric comparisons of basidiomycetous fungi. The sequence data clarified the relationships among the genera of the Auriculariales sensu lato and support the hypothesis that the Uredinales are derived from simple-septate taxa in this order.