PHYLOGENETIC-RELATIONSHIPS OF SELECTED CEREAL AND GRASS RUSTS BASED ON RDNA SEQUENCE-ANALYSIS

To determine phylogenetic relatedness of strains and formae speciales of Puccinia coronata, P. graminis, P. recondita and other cereal and grass rusts and related species, two regions of the ribosomal gene repeat were amplified by the polymerase chain reaction and sequenced. The internal transcribed spacer region provided sufficient variability for phylogenetic analysis, whereas the 5' end of the region coding for the large-subunit rRNA had little variation. Analysis of the internal transcribed spacer sequence data by distance and parsimony methods revealed that P. graminis strains and P. coronata strains from various hosts each composed a distinct cluster, but strains of the P. recondita species complex from various hosts did not. In comparisons among formae speciales of P. graminis and P. coronata, formae speciales that had identical sequences in the internal transcribed spacer region were those that are known to be closely related by crossing, isozyme, and host range studies. However, relationships among formae speciales based on sequence data in some cases contradicted recent taxonomy of the cereal rusts. The clustering of correlated species (microcyclic P. mesnieriana with macrocyclic P. coronata, and microcyclic Uromyces scillarum with macrocyclic P. hordei) demonstrated that internal transcribed spacer sequence data can identify closely related species pairs and is the first confirmation of the relatedness between microcyclic and macrocyclic species based on molecular characters.