Spur Variation in Jewelweed

For over a century, pollination systems have provided a classic example of coevolution and adaptive specialization between flowers and their pollinators. Some researchers have recently challenged this view, by suggesting that plant-pollinator systems are generalized, and that the morphology of flowers is only weakly under selection by pollinators. This paradox can be resolved if 1) generalized systems are actually specialized, with pollinators varying predictably in space and time, and 2) researchers have not correctly identified the floral traits associated with pollination by different animal groups.

In collaboration with honors student Irvin Pan [‘99], I investigated these possibilities through studies of a previously overlooked floral trait: the nectar spur of jewelweed, Impatiens capensis. Jewelweed flowers exhibit considerable variation in the angle of the floral spur both within and between populations, with two prominent morphs having spur angles of 0o (curved) and 90o. Greenhouse experiments with Dr. Steve Travers, an Amherst College Copeland Fellow, indicate that spur angle has a heritable basis.

Through laboratory experiments we determined that curved, male-phase flowers deposit significantly more pollen on the bills of hummingbirds than straight, male-phased flowers. This difference in pollen deposition is a result of curved spurs causing more oscillations in the horizontal plane than straight spurs, creating more contact between bill and pollen. See video.

In field observations, bumble bees (Bombus vagrans) exhibited significant preferences for male-phase flowers in legitimate pollination visits, and preferred to pierce, and rob nectar from, curve-spurred flowers. Subsequent observations and experiments revealed that both naturally and experimentally pierced flowers spent less time in male phase than unpierced flowers. Nectar robbery significantly reduced the standing crop of nectar in male-phase flowers relative to unpierced male-phase flowers, but had no significant effect on nectar volume of female-phased pierced flowers relative to unpierced flowers. As a consequence of their lower standing crop of nectar, pollen removal was significantly reduced in male phase, pierced flowers, relative to unpierced, male phase flowers. In contrast, pierced, female-phase flowers did not have significantly less pollen on their stigmas than unpierced female-phase flowers, suggesting that nectar robbery had no effect on female reproductive success. These differences in pollen removal suggest that male-phase jewelweed flowers may change their sex to female upon being pierced by bees as a strategy to optimize their reproductive success.

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