- 1We assessed the effects of population density and the spatial arrangement of genetically compatible mates on the seed set and pollen flow of a heterostylous, bumblebee-pollinated perennial, Primula sieboldii E. Morren (Primulaceae), by using an experimental population under natural pollination conditions.
- 2We also examined the intermorph differences in the pollen dispersal distance and the frequency of self- and intramorph fertilization.
- 3Seed set was significantly correlated with the number of opposite-morph flowers within 2 m in short-styled genets, and within 3 m in long-styled genets.
- 4Mean pollen dispersal distance within the experimental population was 5.4 m, which was slightly shorter than that in a low-density population measured in a previous study (7.2 m).
- 5The proportions of seedlings sired by the same morph or selfed were 10.2% and 3.0%, respectively, in long-styled mothers, compared with 1.9% and 0.0% among short-styled mothers. However, these differences were not statistically significant and no marked differences in pollen dispersal distances were observed between the two morphs.
- 6Our results suggest that the presence of opposite-morph genets within several metres is essential for success of seed reproduction in P. sieboldii, irrespective of population density, and that stochastic deviation of the morph ratio in small populations is therefore likely to cause considerable reduction in seed set.