Early inbreeding depression and pollen competition in Calluna vulgaris (L.) Hull.

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.