Vogler, D. R., Kinloch, B. B., Jr., Cobb, F. W., Jr., and Popenuck, T. L. 1991.
Department of Plant Pathology, University of California,
Berkeley, CA 94720, and
U.S.D.A., Forest Service, Institute of Forest Genetics, Berkeley, CA 94701.
Can. J. Bot. 69:2434-2441. (1991)
We conducted a population genetic study of the western gall rust fungus (WGR, Peridermium harknessii) using isozymes as genetic markers. Electrophoresis of 341 single- gall aeciospore isolates collected from several pine species revealed that WGR is comprised of two distinct zymodemes (multilocus electrophoretic types) in the western United States. Within zymodemes, all 15 loci studied were monomorphic (0.95 criterion), although variants were found at low frequency (< 0.03) at three loci. Zymodeme I was characterized by single bands, indicating homozygosity at all loci; it was comprised of isolates from all pine species and environments studied, including the Pacific Coast and Cascade Ranges and the Sierra Nevada and Rocky Mountains. Zymodeme II, which was absent from coastal forests, was characterized by double or triple bands at six of 15 loci. The additional bands were interpreted as products of alternative alleles in heterozygous condition; isozyme phenotypes at the other nine loci were identical to those of zymodeme I. The presumed heterozygotes were fixed within zymodeme II, and homozygotes of alleles unique to this zymodeme were not found. Generally, all isolates within a forest stand were in the same zymodeme and, when isolates from both zymodemes were found in the same location, recombinant genotypes between zymodemes were not observed. Such extreme disequilibrium is inconsistent with sexual reproduction, indicating that P. harknessii is asexual.