Active Pollinator Choice by Heliconia ‘fits the Bill’
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Spotlights Trends in Plant Science July 2015, Vol. 20, No. 7 the authors state, however, regardless of the exact 3 Crepet, W.L. and Niklas, K.J. (2009) Darwin’s second ‘‘abominable mystery’’: Why are there so many angiosperm species? Am. J. Bot. 96, mechanisms involved, the slower evolution of reproductive 366–381 isolation in non-flowering land plants compared to flowering 4 Smith, A.R. (1972) Comparison of fern and flowering plant distributions plants may account for the relative paucity of species, in with some evolutionary interpretations for ferns. Biotropica 4, 4–9 part, by enforcing a ‘low birth rate of new species.’ The 5 Ackerman, J.D. (2000) Abiotic pollen and pollination: ecological, functional, and evolutionary perspectives. Plant Syst. Evol. 222, relatively high rates of gene flow that would follow from 167–185 wind-mediated spore dispersal would reduce the rate of 6 Traveset, A. et al. (2014) The ecology of seed dispersal. 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Bot. 101, 737–753 Active pollinator choice by Heliconia ‘fits the bill’ Judith L. Bronstein and Sarah K. Richman Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721 USA A new study documents that a tropical plant only repro- er, researchers have focused on how evolved traits, such as duces when pollen has been deposited by a visitor corolla length and nectar chemistry, allow plants to at- capable of extracting nectar from its deep flowers. Large, tract and reward the best pollinators. Such mechanisms long-billed hummingbirds generally carry greater quan- are far from perfect, though. Plants lose much of their tities of, and more genetically diverse, pollen. Thus, nectar to visitors that deposit low-quality pollen (or none plants can exert more active partner choice than previ- at all). ously considered possible. However, a new study by Matthew Betts, Adam Hadley, and John Kress[3] suggests that plants may be more able to A hummingbird inserting its long bill into a red, tubular selectively associate with good pollinators than has previ- flower is an iconic image of pollination. Although this ously been thought possible. image leads one to think about trait-matching and pair- Heliconia tortuosa Griggs (Heliconiaceae) possesses wise coevolution, as a rule each pollinator species visits long, tubular, curved, yellow flowers which are held more than one plant species, which in turn is visited by within bright red bracts. In Costa Rica, they are visited more than one pollinator [1]. On each side of the mutual- by a suite of hummingbird species that vary considerably ism, some partners confer higher benefits than others. in traits likely to affect their quality as pollinators. Betts How can plants and animals filter out the worst partners, et al. started their study of H. tortuosa with a standard and consistently associate with the best? Pollinators com- experiment in pollination biology: they compared pollen- monly learn to discriminate among plants, choosing those tube formation between flowers exposed to humming- likely to offer the most nectar [2]. By contrast, plants have birds in nature, and flowers protected from humming- been thought incapable of assessing pollinator quality and birds but pollinated by hand. Perplexingly, flowers given then acting instantaneously upon this information. Rath- an excess of hand-deposited pollen initiated fewer pollen tubes, not more. This effect, however, disappeared when Corresponding author: Bronstein, J.L. ([email protected]) Keywords: coevolution; hummingbird; partner choice; pollination; reproduction; the researchers also drained the nectar during the hand- mutualism. pollination treatment. Betts et al. formulated a clever 1360-1385/ explanation for these odd results. They conjectured that ß 2015 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tplants.2015.04.010 the plant will only invest in reproduction if some type of 403 Spotlights Trends in Plant Science July 2015, Vol. 20, No. 7 signal is sent by the flower visitors, e.g., the pollen was somewhat surprising from the standpoint of mutualism deposited by a visitor capable of extracting the deep theory. The net effect of any mutualist is the benefit of its nectar. By doing so, the plant would increase the likeli- effects minus the costs involved in attracting and reward- hood that its seeds will be fathered by the best mates. ing it. Hence, selection should favor partners whose ben- Large, long-billed species often fly long distances among efits can be obtained at the lowest cost [8]. However, in this flowers of a single species (‘traplining’); by contrast, case, the best mutualists are intrinsically the most ener- smaller short-billed hummingbirds establish territories, getically demanding ones (because they are large and fly restricting their foraging to fewer plants. Trapliners long distances). That is, to receive a high benefit, H. generally carry more genetically diverse pollen from tortuosa may be forced to make a high investment. The numerous parents and for this reason are considered present study serves to emphasize that costs and benefits superior pollinators. can act as constraints upon each other. To test their hypothesis, six hummingbird and one Questions remain about the physiological mechanism butterfly species were allowed to make single visits to by which H. tortuosa can discriminate among floral visi- flowers on whose stigmas the researchers had placed pol- tors. Betts et al. argue that nectar removal, in conjunction len; pollen tube numbers were then compared to flowers with pollen deposition, is all that is needed to stimulate that had been hand-pollinated only. Two large traplining pollen tube formation. These results do not indicate how hummingbirds with long, curved bills, the violet sablewing this happens – for example, whether plants are able to and green hermit, removed significantly more nectar than identify high quality pollen on the stigmatic surface, or the other visitors. Their visits also led to significantly more whether low quality pollen is selectively inhibited from pollen tubes. The authors concluded that nectar removal germinating. This distinction is particularly important, provides a cue for H. tortuosa to recognize floral visitors because, in nature, individual H. tortuosa flowers likely and, in the authors’ words, to ‘turn on reproductively.’ receive multiple visits, possibly by more than one species of Betts et al.’s study demonstrates clearly that appar- visitor. Quantifying pollen discrimination in response to ently generalized interactions can hide cryptic speciali- pollen deposition by multiple visitors would seem to be a zation. The authors use their results to suggest that critical follow-up study. pairwise coevolution may be more prevalent in multispe- Finally, we note that this intriguing work rests solidly cies associations than many researchers had assumed on a strong foundation of understanding the natural earlier. history of the system. It would have been easy for Betts We would like to stress two additional broad implica- and his colleagues to have dismissed their initial, puzzling tions of this study. First, plants apparently possess much results as an experimental artifact. Instead, building on a more active abilities to discriminate among partners than deep understanding of pollination biology, hummingbird previously believed. (We note that the researchers have behavior, and tropical ecology and exhibiting a keen eye not yet demonstrated that the pollen deposited by the for a clever and revealing experiment, they have offered violet sablewing and green hermit is indeed of higher us new insight into an unsuspected aspect of plant bio- quality, although it is a reasonable conjecture.) Recent logy. So many equally exciting discoveries remain to be research has uncovered diverse but cryptic adaptations discovered. that allow plants to deliver rewards only to the subset of visitor species that, on average, are good mutualists, not References only in pollination mutualisms [4] but in seed dispersal 1 Va´zquez, D. et al. (2012) The strength of plant-pollinator interactions. [5] and ant protection [6] mutualisms as well. In the Ecology 93, 719–725 2 Goulson, D.