Phylogenetic relationships among the pine stem rust fungi (Cronartium and Peridermium spp.)

Abstract

Species in Cronartium and its anamorphic genus Peridermium form a morphologically homogeneous group of rust fungi that infect stems, branches, and cones of Pinus species in North and Central America, Asia, and Europe. Several species are important forest pathogens, causing severe economic and aesthetic damage to pines in forests and parks throughout the northern hemisphere. Morphological characters have been used to distinguish among these rust fungi, but have proven inadequate for assessing phylogenetic relationships within the group. In this study, we extracted fungal DNA from spores and infected host tissue representing collections of all major North American and Eurasian Cronartium and Peridermium species and of two related rust genera, Chrysomyxa and Pucciniastrum. Using the polymerase chain reaction, we amplified and sequenced the internal transcribed spacer region from the nuclear ribosomal RNA genes, aligned sequences for all taxa, and generated phylogenetic trees using parsimony and neighbor-joining techniques. Parsimony analysis yielded a strict consensus of 24 trees that is congruent with the single tree generated by neighbor-joining analysis. Neighbor-joining was also used to compare trees derived from nucleotide sequences and from isozyme phenotype data from an earlier study; in general, the same species groups were supported in both data sets. Our analyses indicate that the pine stem rust fungi form distinct clades that correlate with telial host families (e.g., Fagaceae and Santalaceae). Close phylogenetic relationships between autoecious, short-cycled Peridermium species and putative heteroecious relatives were confirmed (e.g., P. harknessii with C. quercuum f. sp. banksianae, and P. bethelii with C. comandrae). Nucleotide sequences and isozyme phenotypes were used to identify four undescribed taxa, including: California oak rust, which may be a cone or gall rust that has become isolated from its aecial host; white-spored gall rust from the southwestern U.S., which was thought to be an albino form of orange-spored P. harknessii; a previously undescribed yellow-spored gall rust from California; and stalactiform limb rust of Jeffrey pine, which was thought to be a host-form of C. coleosporioides. In light of these results, a systematic reevaluation of Cronartium and Peridermium morphology and biology is needed.

Key Words:

• DNA sequence analysis
• isozyme analysis
• Urediniomycetes