In terms of species numbers the rust fungi (Pucciniales) are an incredibly successful lineage. Together, the more than 7000 described species form the largest known monophyletic group of plant pathogens. All are obligate parasites of vascular plants including agricultural, forest and ornamental crops resulting in billions of dollars of damage worldwide each year. An intriguing aspect of rust biology is that species display an alternation of generations, with many species also alternating hosts during development, termed heteroecious. Additionally, most rusts require five different developmental stages to complete their life cycle. Whether these unique characteristics are ancestral or derived within the rusts has never been satisfactorily resolved. Most classical treatments of rust phylogeny and classification were based on the hypothesis that “primitive” hosts (e.g., ferns) harbored “primitive” rusts (e.g., Uredinopsis, Hyalopsora) that alternate on members of the Pinaceae. However, alternative hypotheses of rust evolution have proposed various short-cycled primarily tropical rusts as ancestral, with the defining characteristic of heteroecism thus being derived within the group. Molecular studies based on rDNA genes have since disproved the fern rust hypothesis, but the second hypothesis has not been previously tested. This study analyzes loci from multiple genes and taxa selected from all known families to resolve the base of the rust fungi and infer ancestral characters including the origins of heteroecism, alternation of generations, and the five stage developmental cycle for the order. Implications for rust diversification are discussed.
M. Catherine Aime, Professor of Mycology and Director of the Arthur and Kriebel Herbaria, Department of Botany & Plant Pathology, Purdue University.
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