Character displacement among bat-pollinated flowers of the genusBurmeistera: analysis of mechanism, process and pattern

Abstract

Coexisting plants that share pollinators can compete through interspecific pollen transfer. A long-standing idea holds that divergence in floral morphology may reduce this competition by placing pollen on different regions of the pollinator's bodies. However, surprisingly little empirical support for this idea exists. Burmeistera is a diverse neotropical genus that exhibits wide interspecific variation in the degree to which the reproductive parts are exserted outside the corolla. Coexisting Burmeistera share bats as their primary pollinators, and the degree of exsertion determines the site of pollen deposition on the bats' heads. Here we study the mechanism, process and pattern of floral character displacement for assemblages of coexisting Burmeistera. Flight cage experiments with bats and pairs of Burmeistera species demonstrate that the greater the divergence in exsertion length, the less pollen transferred interspecifically. Null model analyses of exsertion lengths for 19 species of Burmeistera across 18 sites (each containing two to four species) demonstrate that observed assemblage structure is significantly overdispersed relative to what would be expected by chance. Local evolution, rather than ecological sorting, appears to be the primary process driving this pattern of overdispersion because local adaptation of the nine widespread species accounts for a large portion of the observed pattern. Taken together, results of this study provide strong support for the idea that competition through interspecific pollen transfer can drive character displacement in plants.

Going to great lengths: selection for long corolla tubes in an extremely specialized bat–flower mutualism

“…Although Burmeistera flowers are an important part of the diet of other species of nectar bats (Muchhala 2008), A. fistulata do not visit them. Thus, by evolving a longer corolla and specializing on A. fistulata, C. nigricans would reduce pollen loss to Burmeistera stigmas and possible stigma blockage by Burmeistera pollen (see Muchhala & Potts 2007). Since other nectar bats lack glossal tubes, they could not respond by evolving longer tongues.…”

Evolution of reproductive isolation in plants

“…It may thus help to explain the observation that plant groups characterized by strong and often complete postzygotic isolation have also evolved a considerable amount of prezygotic isolation. Few studies have found convincing evidence for reproductive character displacement between sympatric, congeneric plant species (Armbruster et al, 1994;Muchhala and Potts, 2007), and to the best of our knowledge, none of these studies have related reproductive character displacement to the strength of postzygotic reproductive isolation. Thus, much remains to be learned about ecological interactions among premating and postmating reproductive barriers, and the order of their evolution.…”

Recurrent modification of floral morphology in heterantherousSolanumreveals a parallel shift in reproductive strategy

“…Indeed, the transition from large to small flowers in S. grayi closely matches the zone of sympatry with its sister species S. lumholtzianum [35]. Differences in flower morphology, and more specifically in the relative size and position of pollinating and feeding anthers within a flower, could serve as pre-zygotic barriers by placing and collecting pollen from different areas of the pollinator's body, or by exploiting different pollinators [35,65]. Because hybrids between species in Solanum section Androceras often fail to develop [33,35], selection should favour reproductive character displacement as divergence in floral morphology in sympatric populations could reduce pollen and ovule wastage [37,66].…”