Isozyme diversity among hard pine stem rust fungi in the western United States

Abstract

Isozyme analysis was used to determine species relationships and diversity among stem rust fungi (Cronartium and Peridermium spp.) infecting hard pines (subg. Pinus) in the western United States. Electrophoresis was carried out on aeciospore extracts of 270 hard pine stem rust isolates representing nine species and forms. Consistent banding patterns (electromorphs interpreted as phenotypes) were obtained for all isolates at 11 enzyme loci. Within species groups, multilocus phenotypes were diverse at several loci. Nevertheless, each species exhibited a unique set of phenotypes that distinguished it from all others. For diagnostic purposes, banding patterns from just two loci were sufficient to distinguish all species and most forms or races examined. Two taxa that were thought to be forms of existing species may be new, undescribed species. For comparison with western hard pine stem rusts, multilocus phenotypes were determined for 13 additional isolates of four related pine stem rust species. Phenotypes for all rust species were analyzed in all possible combinations using pairwise distance analysis (neighbor-joining). When Cronartium ribicola is treated as the outgroup, the resulting phenogram groups all 72 multilocus phenotypes into distinct species clades. Origins of autoecious, short-cycled Peridermium spp. may be inferred from their placements relative to macrocyclic, heteroecious Cronartium spp. Keywords: Cronartium, Peridermium, species diversity, species relationships.