Bilaterally symmetrical (zygomorphic) flowers are considered to have evolved from primitive, radial symmetrical (actinomorphic) flowers under selection favoring pollinator specificity. The hypothesis has been supported by several facts that zygomorphical flowers are often visited by limited pollinator groups, and that zygomorphic flowers have less flower-size variation than actinomorphic flowers. we compare pollinator specificity, pollination efficiency and pollen-ovule (P/O) ratio between zygomorphic and actinomorphic flowers at a community level. The hypothesis predicts that high pollination efficiency by specialized pollinators favors low P/O ratios in zygomorphic flowers.

We find that (1) zygomorphic flowers were specialized to bee-pollination, that (2) zygomorphic flowers visited less frequently than actinomorphic flowers, that (3) pollination efficiency was not different between actinomorphic and zygomorphic flowers, and that (4) P/O ratio did not differ between symmetrical types, but significantly increased with the visitation frequency and the degree of bee-dependence.

Thus, our results suggest that zygomorphic flowers have evolved under selection favoring bee-pollination. We did not find lower P/O ratios in zygomorphic flowers, perhaps because bee-pollination requires more pollen grains as rewards than other pollination system.