The so-called rusts are an important group of obligately parasitic plant pathogenic fungi. Wheat rusts are probably the best known because of their serious effects on crop yields. Although host plant specificity is a hallmark of the rusts, little is known about the evolutionary patterns of host tracking and host shifting. This lack of understanding is caused by our rudimentary knowledge of the evolutionary relationships, or phylogeny, of the rusts, both among the rust species and genera and with related basidiomycetes. Dr. Thomas Bruns of the University of California- Berkeley will use molecular genomic techniques to investigate evolutionary relationships at several taxonomic levels among the rust fungi. At the broadest level, DNA nucleotide sequences of the ribosomal genes will be analyzed and mutational differences will be compared among the major groups of rusts and their putative fungal relatives. At lower taxonomic levels, evolutionary relationships among species of two genera, Cronartium and Melampsora, that are important pathogens on pine trees will be investigated by DNA sequencing methods. Patterns of relationship among the rust fungi will be compared with patterns of relationship among pine species to detect lineages where hostplant shifts have occurred. Results from this work should provide a better understanding of host tracking and host shifting in obligate plant pathogens, and will also develop the database and methodology needed to facilitate rapid identification of species within the two selected genera.
