Bird-pollinated flowers in an evolutionary and molecular context

Abstract

Evolutionary shifts to bird pollination (ornithophily) have occurred independently in many lineages of flowering plants. This shift affects many floral features, particularly those responsible for the attraction of birds, deterrence of illegitimate flower visitors (particularly bees), protection from vigorous foraging by birds, and accurate placement of pollen on bird's bodies. Red coloration appears to play a major role in both bee-deterrence and bird-attraction. Other mechanisms of bird-attraction include the production of abundant dilute nectar and the provision of secondary perches (for non-hovering birds). As a result of selection for similar phenotypic traits in unrelated bird-pollinated species, a floral syndrome of ornithophily can be recognized, and this review surveys the component floral traits. The strong convergent evolution evident in bird-pollinated flowers raises a question about the nature of the genetic mechanisms underlying such transitions and whether the same gene systems are involved in most cases. As yet there is too little information to answer this question. However, some promising model systems have been developed that include closely related bee and bird-pollinated flowers, such as Ipomoea, Mimulus, and Lotus. Recent studies of floral developmental genetics have identified numerous genes important in the development of the floral phenotype, which are also potential candidates for involvement in shifts between bee-pollination and bird pollination. As more whole-genome information becomes available, progress should be rapid.

The genetic basis of speciation in the Giliopsis lineage of Ipomopsis (Polemoniaceae)

“…Hence, flower width may have evolved under weak directional selection or even neutrally. This inference is consistent with the view that, while traits such as flower color, flower length and nectar volume are directly selected by pollinators, flower width tends to be less important for pollination efficiency (Cronk and Ojeda, 2008). However, in an elegant examination of phenotypic selection by hummingbirds on I. aggregata floral traits, which are similar, via convergence, to those of I. tenuifolia, tube width was positively correlated with pollen export (Campbell et al, 1991).…”

Secretion and composition of nectar and the structure of perigonal nectaries in Fritillaria meleagris L. (Liliaceae)

“…Indeed, during 3 years of observations, we noted only about 0.2 visits/flower/h (Zych and Stpiczyńska 2012). However, the presence of nectaries alone most probably does not affect attractiveness of the flowers as they are green and invisible to pollinators from outside of the flower, contrary to the nectaries of ornithophilous Fritillaria species (Cronk and Ojeda 2008).…”

“…However, the nodding of flowers of F. meleagris limits access to the reward to a relatively small group visitors, predominantly large Hymenoptera (Zych and Stpiczyńska 2012). On the other hand, the pendant flowers of F. imperialis L. are pollinated by passerine birds in its native range (Búrquez 1989;Peters et al 1995;Cronk and Ojeda 2008). A high rate of nectar secretion of very low solute concentration (4-10% w/w), the absence of sucrose from the nectar, and the low amino acid concentration are all indicative of passerine bird pollination.…”

Parallel evolution of angiosperm colour signals: common evolutionary pressures linked to hymenopteran vision

“…Interestingly, for bird-pollinated plants, there is some evidence of both flower colours evolving spectral signals to maximize discrimination by birds [84], and/or birds evolving different spectral sensitivities to enhance discrimination of certain flower colours [14]. This would be an interesting topic to explore considering plant flowers that are either exclusively bird, or insect-pollinated.…”