Peridermium harknessii, cause of western gall rust of pines, comprises two populations of multilocus electrophoretic types (zymodemes) in the western United States. When stained with a DNA-specific fluorochrome, mature, ungerminated aeciospores from zymodeme I were found to be predominantly binucleate (70%), as were those of the related macrocyclic species, Cronartium quercuum (74%), whereas aeciospores from zymodeme II were predominantly uninucleate (93%). Within each zymodeme, aeciospores with two nuclei had significantly (P=0·01) more DNA than spores with one nucleus, and numbers of nuclei in germlings increased arithmetically over time. These data suggest that aeciospore nuclei in both zymodemes I and II divide mitotically, not meiotically, as is consistent with an asexual life cycle.

Photometric measurements also indicated that the amount of DNA in one nucleus of a uninucleate zymodeme II aeciospore was similar to the total amount of DNA in a binucleate zymodeme I aeciospore. These data, coupled with recent isozyme studies, suggest either that zymodeme II evolved after karyogamy of zymodeme I and an unidentified zymodeme, or that zymodeme I evolved after haploidization of zymodeme II.