Peridermium harknessii, cause of western gall rust of
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
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
unidentified zymodeme, or that zymodeme I
evolved after haploidization of zymodeme II.