Pollen Transfer by Hummingbirds and Bumblebees, and the Divergence of Pollination Modes in Penstemon

Abstract

We compared pollen removal and deposition by hummingbirds and bumblebees visiting bird-syndrome Penstemon barbatus and bee-syndrome P. strictus flowers. One model for evolutionary shifts from bee pollination to bird pollination has assumed that, mostly due to grooming, pollen on bee bodies quickly becomes unavailable for transfer to stigmas, whereas pollen on hummingbirds has greater carryover. Comparing bumblebees and hummingbirds seeking nectar in P. strictus, we confirmed that bees had a steeper pollen carryover curve than birds but, surprisingly, bees and birds removed similar amounts of pollen and had similar per-visit pollen transfer efficiencies. Comparing P. barbatus and P. strictus visited by hummingbirds, the bird-syndrome flowers had more pollen removed, more pollen deposited, and a higher transfer efficiency than the bee-syndrome flowers. In addition, P. barbatus flowers have evolved such that their anthers and stigmas would not easily come into contact with bumblebees if they were to forage on them. We discuss the role that differences in pollination efficiency between bees and hummingbirds may have played in the repeated evolution of hummingbird pollination in Penstemon.

Sequential stamen maturation and movement in a protandrous herb: mechanisms increasing pollination efficiency and reducing sexual interference

“…This could be the main reason for the decline in the pollen transfer efficiency of halictid bees in the manipulated treatment. In some other studies, researchers have also indicated that the pollen available for pollination would rapidly decline and that pollination efficiency would decrease if plants were visited by pollen-collecting pollinators (Castellanos et al 2003; Young et al 2007). Notably, under such circumstances, sequential stamen maturation (staggered pollen presentation) could be a mechanism to increase the pollen transfer efficiency of pollinators in order to alleviate the effect of diminishing returns on pollen transfer.…”

Why Are So Many Bird Flowers Red?

“…One flower type, the generalist flower, is blue and is efficiently pollinated by bees and birds alike. The blue Rocky Mountain penstemon, Penstemon strictus, provides a good example (Castellanos et al 2003). The other flower type, or bird flower, is yellow and is efficiently pollinated by birds, but not by bees—the red beardlip penstemon P.…”

“…The other flower type, or bird flower, is yellow and is efficiently pollinated by birds, but not by bees—the red beardlip penstemon P. barbatus provides an example of this type (Castellanos et al 2003). If bee visits were costly for the ancestral bird flowers, they would experience a selective pressure to become red.…”

Functional Diversity of Nectary Structure and Nectar Composition in the Genus Fritillaria (Liliaceae)

“…This is, especially, evident in F. olgae , a species that produces copious, but rather dilute, sucrose-rich nectar (Castellanos et al, 2003). …”