Manipulation of Female Architecture in Flowers Reveals a Narrow Optimum for Pollen Deposition

The invariability of animal-pollinated flowers relative to vegetative organs, such as leaves, suggests that these flowers may have been finely tuned for an optimum pollination performance. The evolutionary mechanism that could optimize and conserve a species' flower architecture is stabilizing selection, which occurs when intermediate phenotypes attain the highest fitness. Evidence for the operation of such a mechanism is rare in this context, however. Here, I report an experiment that used oil-seed rape (Brassica napus cv. Westar) as the basis of a model system in which the spatial disposition of flowers' sexual organs was manipulated in order to characterize the relationship between form and function. Specifically, individual flowers were manipulated by replacing a section of the style with a length of fine wire in order to produce flowers with pistils of various lengths. The pollination performance of these virgin, model flowers was then assessed by measuring the amount of pollen removed and deposited during a single bumblebee visit. There was no evidence that pollen removal was affected by pistil length. Variation in pistil length, however, was associated with a pronounced optimum in pollen deposition, which was attained by only a narrow range of pistil variants of intermediate length. These results suggest that stabilizing selection may maintain the architectural invariability of animal-pollinated flowers.