We investigated whether partial self-sterility in Calluna vulgaris results
from abortion of selfed offspring owing to inbreeding depression or a late-
acting self-incompatibility mechanism, and whether self-pollen interferes w
ith normal functioning of cross-pollen. Self-pollination resulted in 75% le
ss seed set than cross-pollination. Self-pollen tubes reached ovaries and p
enetrated ovules as often as those of cross-pollen. Following self-pollinat
ion, examination of the size of undeveloped seeds showed that at least 70%
resulted from ovule fertilization and arrest of development occurred at var
ious stages. All self-pollinated plants produced seeds and self-fertility v
aried among plants. These results indicate that the reduced seed set observ
ed in self-pollination is more likely the result of inbreeding depression r
ather than a late-acting self-incompatibility system. The fecundity compone
nt of inbreeding depression was high (0.762). Seed set was reduced by an av
erage of 40% when self-pollen was mixed with cross-pollen, compared to pure
cross-pollination. Using genetic markers, we found about 20% of seeds resu
lted from self-pollination in mixed pollinated fruits. C. vulgaris is likel
y to experience self-pollination in nature and our data suggest this will r
educe the number of ovules that might otherwise mature after cross-pollinat
ion. (C) 1999 Annals of Botany Company.