Ecological and selective effects of stigma-anther separation in the self-incompatible tropical tree Ipomoea wolcottiana (Convolvulaceae)

Abstract

We evaluated the effect of stigma-anther separation (SAS) in flowers of the self-incompatible tropical tree Ipomoea wolcottiana (Convolvulaceae). We measured the deposition of self pollen on the stigma and the growth of pollen tubes. We also evaluated phenotypic selection on female fertility using univariate regressions of SAS with fruit production. SAS varied among individuals in two years, with a range of 0–6 mm. There was a negative correlation between SAS and the number of self pollen grains on the stigma; trees with SAS of 0–2 mm received averages of ca 80–130 self grains. The effect of loading the stigma with self-pollen was tested by allowing isolated flowers to self-pollinate, then cross-pollinating by hand: there was a negative correlation between the pollen load and the proportion of grains forming pollen tubes. The highest proportion of grains forming tubes was found with loads of 50 grains. The results suggest that more self pollen deposition leads to fewer tubes from outcross pollen. There was a significant effect of SAS on fruit production in both years. The analyses suggested stabilizing selection; very small SAS is disadvantageous due to the high load of self pollen, while very large SAS probably led to damage to the style during pollinator visits. We suggest that herkogamy may be a key character for determining the success of fruit production in self-incompatible hermaphroditic species.