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Pollinator efficiency in Blakea gracilis (Melastomataceae)

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Pollinator efficiency in Blakea gracilis (Melastomataceae)
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Eficiencia de los polinizadores en Blakea gracilis (Melastomataceae) ( )
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English
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Peraino, Sullivan
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Pollination by bees--Costa Rica--Puntarenas--Monteverde Zone   ( lcsh )
Flowers--Morphology   ( lcsh )
Polinización por abejas--Costa Rica--Puntarenas--Zona de Monteverde
Flores--Morfología
Tropical Ecology 2008
Ecología Tropical 2008
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Reports   ( lcsh )
Reports

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Abstract:
Blakea gracilis (Melastomataceae) is one of 20,000 species of flowering plants whose flowers have poricidal anthers. This morphology is adapted for “buzz-pollination,” where bee-induced vibrations result in the expulsion of pollen through the anther pores. The frequency of vibrations significantly affects pollen release, with peak anther discharge occurring at 500 Hz. Bees, however, are only able to buzz at about 300 Hz, and rarely reach frequencies higher than 400 Hz. This gap leaves the potential for a “most efficient” pollinator, a particular bee able to buzz at the highest frequency, thereby procuring the most pollen per visit and moving more pollen from flower to flower. In this study, I aimed to determine whether a “most efficient” pollinator exists for Blakea gracilis. I focused on large-bodied bees and small-bodied bees, predicting that larger bees would be capable of reaching higher frequencies and therefore would be better pollinators than their smaller counterparts. I found that large- and small-bodied bees open the same number of anthers per visit (t = 0.553, df = 36.277, p = 0.584), even though small bees spend significantly more time buzzing each flower (t = 3.753, df = 22.5, p = 0.001). Large-bodied bees achieved the same success as their smaller counterparts in less time by opening more anthers per second (t = -3.266, df = 36.3, p = 0.002). Despite their length of visitation, the amount of time each size-class spent buzzing a flower did not affect how many anthers they opened (large-bodied: R2 = 0.061, p = 0.080, t = 1.786; small-bodied: R2 = 0.169, p = 0.057, t = 2.019). I found one exception in Bombus sp., which was able to open more anthers the longer it vibrated the flower (Spearman’s Rho = 0.576, p = 0.041, N = 10). I found that there is no “most efficient” pollinator of Blakea gracilis, instead the results are consistent with those of past studies which concluded that buzz-pollination is generalized to all bee visitors.
Abstract:
En este estudio, yo tenía como objetivo determinar si existe un polinizador más eficiente para Blakea gracilis. Me enfoqué en abejas grandes y pequeñas, prediciendo que las abejas grandes serían capaces de alcanzar frecuencias más altas y por lo tanto serían mejores polinizadores que sus contrapartes más pequeñas.
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Born Digital

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PAGE 1

Pollinator efficiency in Blakea gracilis (Melastomataceae) Sullivan Peraino Department of Agronomy, Purdue University ABSTRACT Blakea gracilis (Melastomataceae) is one of 20,000 species of flow ering plants whose flowers have poricidal anthers. This morphology is adapted for “buzz-poll ination,” where bee-induced vibrations result in th e expulsion of pollen through the anther pores. The frequency of vibrations significantly affects pollen release, wi th peak anther discharge occurring at 500 Hz. Bees, however, are only able to buzz at about 300 Hz, and rarely reach frequencies higher than 400 Hz. This gap leaves the potential for a “most efficient” pollinator, a particular bee able to buzz at the highest frequency, thereby procuring the most p ollen per visit and moving more pollen from flower to flower. In this study, I aimed to determine whether a “most ef ficient” pollinator exists for Blakea gracilis I focused on largebodied bees and small-bodied bees, predicting that larger bees would be capable of reaching higher fre quencies and therefore would be better pollinators than their sm aller counterparts. I found that largeand smallbodied bees open the same number of anthers per visit (t = 0.553, df = 36.277, p = 0.584), even though small bees spend significantly more time buzzing each flower (t = 3.753, df = 22. 5, p = 0.001). Large-bodied bees achieved the same success as their smaller counterparts in less time by opening more anthers per second (t = -3.266, df = 36.3, p = 0.002). Despite their length of visitation, the amount of t ime each size-class spent buzzing a flower did not affect how many anthers they opened (large-bodied: R2 = 0.061, p = 0.080, t = 1.786; small-bodied: R2 = 0.169, p = 0.057, t = 2.019). I found one exception in Bombus sp. which was able to open more anthers the longer it vibrated the flower (Spearman’s Rho = 0.576, p = 0.041, N = 10). I fou nd that there is no “most efficient” pollinator of Blakea gracilis instead the results are consistent with those of pa st studies which concluded that buzz-pollination is generalized to all bee visitors. RESUMEN Blakea gracilis (Melastomataceae) es una de las 20,000 especies de plantas con flores que poseen anteras porosas. Esta morfologa es especializada para la “polinizac in por zumbido,” donde vibraciones inducidas por a bejas resultan en la expulsin del polen a travs de los poros de las anteras. La frecuencia de las vibraci ones afecta significativamente la liberacin de polen, con un p ico de descarga que ocurre a los 500 Hz. Las abeja s, sin embargo, son capaces de zumbar solamente a 300 Hz, y raramen te estas frecuencias superan los 400 Hz. Este vac o crea un espacio para polinizadores “ms eficientes,” una ab eja en particular capaz de vibrar a mayores frecuen cias, as procurando el mayor polen por visita y moviendo ms polen de flor a flor. En este estudio, yo busco d eterminar si existe un polinizador ms eficiente para B. gracilis Enfocndome en abejas grandes y pequeas, predic iendo que abejas ms grandes sern capaces de alcanzar frecue ncias mayores, y as ser mejores polinizadores que sus contraparte ms pequeas. Encontr que abejas gran des y pequeas abren el mismo nmero de anteras por visita (t=0.553, df=36.277, p=0.548), an cuando las abeja s pequeas pasan ms tiempo zumbando en cada flor ( t=3.753, df=22.5, p=0.001). Abejas grandes alcanzan el mism o xito que las pequeas en un menor tiempo, abrien do ms anteras por segunto (t=-3.226, df=36.3, p=0.002). Ms all del largo de la visita, la cantidad de tie mpo de cada clase de tamao usado zumbando en una flor, no afecta cua ntas anteras ellas abren (grandes: R2 =0.061, p=0.08, t=1.786; pequeas: R2 = 0.169, p= 0.057, t= 2.019). Encontr una excepci n en Bombus sp. el cual es capaz de abrir ms anteras durante un mayor tiempo de zumbido en la fl or (Spearmans Rho = 0.576, p= 0.041, N= 10). Enco ntr que no existe un polinizador “ms eficiente” para B. gracilis en cambio los resultados son consistentes con est udios anteriores que concluyen que la polinizacin por zu mbido es generalizada para todas las abejas visitan tes.

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INTRODUCTION The angiosperm family Melastomataceae is one of 72 families world-wide whose flowers have poricidal anthers. In this flower morphology, each anther is separated into thecae, or anther halves, which contain two pollen sacs. The thecae open by dehiscing along a longitudinal slit, the stomium, which runs the entire length of the th ecae (Fig. 1). The 20,000 species with such poricidal anthers are presumed to be “buzz-pollinat ed,” where bee-induced vibrations result in the expulsion of pollen through the anther pores (B uchmann, 1983; Knudsen & Olsen, 1993). The anthers come together in a cone-shape, which fo rces the bee to assume a position where the pollen is placed in locations on the bee’s body where it cannot be removed during in -flight grooming. These flowers are called “solenoid,” whe re morphology localizes pollinator-flower contact to e nhance pollen transport (Renner, 1989). Old World members and New World members of all 72 families share this morphology and nearly every fam ily within Apoidea use this pollen-collecting behavior. The i ndependent evolution of these solenoid flowers in unrelated fa milies represents millions of years of coevolution between these plants and their buzz-pollinating bees, resulting in an “a daptive peak” for buzz-pollinated plants (Renner, 1989; Harder & Barclay, 1994). This evolutionary achievement is supported by Macior (1971), who observed that buzz pollination is so su ccessful that it is rarely succeeded by any other mechanism, even in speciesrich groups. Bees (and one species of Syrphid fly) are the only natural pollinators able to effectively buzz open poricidal anthers. The frequency of vibration is the trigger for anther de hiscence, and pollen collection is possible artificially by touch ing a tuning fork to the anthers (Buchmann, 1983). It has also been found that the vibration frequency significantly affects pollen re moval, with peak anther sensitivity at 500 Hz. Large-bodied be es are able to buzz at a frequency of ~300 Hz, rarely buzzing any higher than 400 Hz. Due to the difference between peak sensitivity and bee ability, Harder & Barclay (1994) concluded that the anthers of buzz-pollinated plants are “tuned out” of the frequencies used by their pollinators. This allows the plant to promote pollen transport on many different pollinators, thereby en hancing reproductive fitness (Harder & Barclay, 1994). Figure 1. The intact poricidal anther. Each pore opens at the apex of the individual theca (anther h alves containing the male sporangia). The thecae are pre ssed tightly together prior to buzz-pollination, af ter which they splay open. Buzzing opens the pores, and poll en is released explosively. Figure from Buchmann, 1983.

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Owing to the wide range of frequencies recognized by poricidal anthers (300-500 Hz), it is reasonable to question if a specific bee is able to reach a higher, more anther-responsive frequency, thereby procuring more pollen per visit. If a bee is able to extract more pollen per visit, they would effectively move more pollen from flower to flower, thereby acting as a more efficient pollinator than those bees restricted to lower frequencies. In this study, I aimed to determine whether there exists a “most efficient” p ollinator of the buzz-pollinated, poricidal Blakea gracilis (Melastomataceae). I focused on two broad categor ies of pollinators: largebodied bees (two species in the family Anthophorida e, S.F. Anthophorinae, and one species of the genus Bombus Apidae) and small-bodied bees ( Apis mellifera Apidae, one species of Euglossinae, and a small member of Anthophoridae). I predicted that the large-bodied bees would be able to vibrate at a higher frequency than the small-bodied bees, thereby eliciting more pollen per visit. MATERIALS AND METHODS I observed two individuals of B. gracilis in Monteverde, Costa Rica for a period of three we eks. One individual was a large bush and the other was a n epiphytic vine. I observed each individual in the early morning, beginning at sunrise (0600 hr s) when pollinator visitation was at its highest. To rank pollinator success, I used the separation o f the anthers’ thecae as a proxy for apical dehiscence. In newly opened flowers that have not been visited, the anthers are stuck together in a semi-circular cone, and each anther’s thecae are tightly “glued” together. Once visited by a bee, the thecae are separated, and it is clearly vi sible which anthers have dehisced and which are still unopened. Upon arriving at each study site, I surveyed all open (and reachable) flowers. I marked those which had been buzzed by clipping the tip of one sepal. After observing a bee visit a flower, I recorded the type of bee, the amount of time the bee spent buzzing the flower, total number of anthers, and the number of anthers opened after the visit. The pollinator’s success was measured as the proportion of anthers opened du ring the visit (all but two observed flowers had 12 anthers). RESULTS I recorded 73 visits by a total of six different po llinators. I observed visits by three species in t he family Anthophoridae (S.F. Anthophorinae), one spec ies of the genus Bombus (Apidae), Apis mellifera (Apidae), and one species of Euglossinae. I recorded 51 visits from large-bodied bees (two s pecies in the family Anthophoridae, S.F. Anthophorinae, and one species of the genus Bombus Apidae) and 22 visits from smallbodied bees ( Apis mellifera (Apidae), one species of Euglossinae, and a small member of Anthophoridae). I found that largeand small-bodi ed bees opened the same proportion of anthers per visit (t = 0.553, df = 36.277, p = 0.58 4) (Fig. 2), even though small-bodied bees spent significantly more time buzzing each flower (t = 3. 753, df = 22.5, p = 0.001). Large-bodied bees achieved the same success as their smaller counterp arts in less time by opening more anthers per second (t = -3.266, df = 36.3, p = 0.002). Despite their length of visitation, the amount of time each size-class spent buzzing a flower did not pred ict how many anthers they opened (largebodied: R2 = 0.061, p = 0.080, t = 1.786; small-bodied: R2 = 0.169, p = 0.057, t = 2.019).

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0.00 0.20 0.40 0.60 0.80 1.00 1.20 051015202530Time Spent Buzzing Flower (sec)Proportion of Anthers Opened 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 010203040506070 Time Spent Buzzing Flower (sec)Proportion of Anthers Opened Figure 2. The proportion of anthers opened as a fu nction of the time spent buzzing the flower. The amount of time large-bodied bees (A) and small-bodi ed bees (B) spent buzzing a flower did not affect the proportion of anthers that were opened ( R2 A= 0.06, pA = 0.08, nA = 51; R2 B = 0.17, pB = 0.06, nB = 22) A. B.

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I also focused on a specific set of pollinators to see if the size-class patterns were consistent at a smaller scale. I analyzed data fro m three pollinators, a large-bodied species of Anthophorinae (Anthophoridae), Bombus sp. (Apidae), and Apis mellifera (Apidae). I found most patterns at the size-class level carried throu gh to these three species. Anthophorinae, Bombus sp., and A. mellifera opened the same number of anthers per visit (Krusk all Wallis Test, X2 = 2.366, df = 2, p = 0.306, N = 59) (Fig. 3). 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 Mean Proportion of Anthers Opened Apis mellifera the smallest of the three bees, spent significant ly more time than both Anthophorinae and Bombus sp. buzzing each flower (Kruskal Wallis Test, X2 = 18.661, df = 2, p = 0.000, N = 59) (Fig. 4A.). Anthophorinae and Bombus sp. were able to open more anthers per second than A. mellifera (Kruskal Wallis Test, X2 = 12.452, df = 2, p = 0.002, multiple comparison showed that Anthophorinae and Bombus sp were not significantly different, Q = 2.98, Q0.005, 3 = 3.314) (Fig. 4B.). An interesting trend did sur face, however, when correlating the amount of time spent buzzing and the number of anthers opened. In the size-class comparison, the time spent buzzing did not affect t he total number of anthers opened. However in the tri-pollinator analysis, I found that Bombus sp was able to open more anthers the longer it buzzed the flower (Spearman’s Rho = 0.576, p = 0.04 1, N = 10). Anthophorinae and A. mellifera opened the same number of anthers regardless of ho w long they buzzed the flower (Anthophorinae: Spearman’s Rho = 0.171, p = 0.166, N = 34; A. mellifera : Spearman’s Rho = 0.003, p = 0.496, N = 15). Figure 3. The mean and standard error of anthers o pened by each pollinator. All three pollinators are equally successful at opening anthe rs (Kruskal Wallis Test, X2 = 2.366, df = 2, p = 0.306, N = 59) Apis mellifera Bombus sp. Anthophorinae

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0 5 10 15 20 25 30 35 40 Average Time Spent Buzzing (seconds) -0.05 0 0.05 0.1 0.15 0.2 0.25 Mean Proportion of Anthers Opened per Second Figure 4 A,B. A) The mean and standard error of th e amount of time each pollinator spent buzzing indi vidual flowers. Bombus sp. and Anthophorinae spent the same amount of time buz zing, while A. mellifera spent a significantly longer period at each flower. A. mellifera spent an average of 22.3 seconds buzzing each flow er. B) The proportion of anthers opened per second by each pollinator. Anth ophorinae and Bombus sp opened more anthers per second than A. mellifera (Kruskal Wallis Test, X2 = 12.452, df = 2, p = 0.002, multiple comparison s howed that Anthophorinae and Bombus sp were not significantly different, Q = 2.98, Q0.005, 3 = 3.314). A. B Apis mellifera Anthophorinae Apis mellifera Bombus sp. Anthophorinae Bombus sp.

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DISCUSSION I have determined that there is no “most efficient” pollinator of B. gracilis. My findings are consistent with those of past studies suggesting th at buzz-pollinated flowers are generalized to all pollen gathering bees in a given region, and that t he only requirement for pollen extraction is the ability to “buzz” (Larson et al. 1999). Blakea gracilis falls into the same category as other buzzpollinated species, all of which are presumed to ha ve generalized bee pollinators (Knudsen & Olsen, 1993). The ability of the large-bodied bees to open more anthers per second does give them the potential to visit more flowers and plants in a giv en day, which may enhance their performance as pollination vectors. In future studies, it woul d be valuable to keep a thorough record of how many flowers and plants individual bee species visi t, in order to determine whether expedited pollen removal allows for more efficient pollen tra nsfer. In the tri-pollinator analysis, the results were c onsistent with the size-class trends. The two larger bees, Anthophorinae and Bombus sp., spent less time buzzing and were able to open more anthers per second than A. mellifera. All three pollinators achieved the same success during their visits, supporting the idea that B. gracilis is generalized to all bee fauna. The ability of Bombus sp. to open more anthers the longer it buzzed an indiv idual flower was inconsistent with the broader comparison, however. Buchmann (19 83) observed that Bombus sp. had an added advantage, since they utilize energy gained f rom consuming honey while in the nest prior to foraging in the early morning. Bumblebees are a lso able to thermoregulate, which is in fact enhanced during the buzzing of flowers when the bee vibrates about twice as fast as it does during flight (Harder & Barclay, 1994; Thorp, 1979) Possibly the honey-derived energy and superior thermoregulation ability allow Bombus sp to exert more energy over a longer period of buzzing, thus achieving a higher frequency later in the visit, and making a longer visit more rewarding. Anthophorinae, due to its lack of non-p ollen sustenance, and A.mellifera due to its small size, are perhaps unable to maintain a favora ble frequency over a long visitation period, and therefore open the same number of anthers regar dless of how long they spend vibrating. Buzz-pollination of B. gracilis is generalized, but perhaps to the flower’s favor. The generalization of the flower to a wide range of fre quencies promotes more efficient pollination than if it were specialized to a narrow range. If one bee were able to gather all of the pollen, the probability of the pollen being deposited on anothe r plant’s stigma would decrease. By regulating pollen expulsion and promoting transport on multiple vectors, B. gracilis is increasing the probability of successful pollination, and furt hermore its reproductive fitness. The range of anther receptivity also increases the species’ resi lience in face of stochastic events that may alter the composition of the pollinator community. If on e species of buzzing bee is removed from the community, B. gracilis will still have a plethora of capable pollinators. AKNOWLEDGEMENTS I sincerely appreciate all of the help and advice I received from Karen Masters. My mornings would not have been half as enjoyable if I had not been able to spend them in her garden! Also, I would like to thank Patricia Maynard for giving me access to her property. Last but not least, Pablo Allen for his patience and skills in statisti cs and bee identification, and Mancho Caldern, for endless support and advice.

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LITERATURE CITED Buchmann, S.L. 1983. Buzz pollination in angiospe rms. In C. E. Jones & J. R. Little (Eds.). Handbook of Experimental Pollination Biology, pp. 7 3-113. Van Nostrand Reinhold, New York. Harder, L.D. and R.M.R. Barclay. 1994. The Functio nal Significance of Poricidal Anthers and Buzz Pollination: Controlled Pollen Removal from Do decatheon. Functional Ecology. 8:24. 509-517. Knudsen, J. T. and J. M. Olesen. 1993. Buzz-Pollin ation and Patterns in Sexual Traits in North European Pyrolaceae. American Journal of Botany. 80:8. 900-913. Larson, B. M. and S. C. Barrett. 1999. The Pollinat ion Ecology of Buzz-Pollinated Rhexia virginica (Melastomataceae). American Journal of Botany 86:4. 502-511 Macior, L. W. 1971. Co-evolution of plants and ani mals – systematic insights from plant-insect interactions. Taxon. 20: 17-28. Renner, S. S. 1989. A survey of reproductive biol ogy in Neotropical Melastomataceae and Memecylaceae. Annals of the Missouri Botanical Gar den. 76: 496-518. Thorp, R. W. 1979. Structural, Behavioral, and Ph ysiological Adaptations of Bees (Apoidea) for Collecting Pollen. Annals of the Missouri Bota nical Garden. 66:4. 788-812.


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Blakea gracilis (Melastomataceae) is one of 20,000 species of flowering plants whose flowers have poricidal anthers. This morphology is adapted for buzz-pollination, where bee-induced vibrations result in the expulsion of pollen through the anther pores. The frequency of vibrations significantly affects pollen release, with peak anther discharge occurring at 500 Hz. Bees, however, are only able to buzz at about 300 Hz, and rarely reach frequencies higher than 400 Hz. This gap leaves the potential for a most efficient pollinator, a particular bee able to buzz at the highest frequency, thereby procuring the most pollen per visit and moving more pollen from flower to flower. In this study, I aimed to determine whether a most efficient pollinator exists for Blakea gracilis. I focused on large-bodied bees and small-bodied bees, predicting that larger bees would be capable of reaching higher frequencies and therefore would be better pollinators than their smaller counterparts. I found that large- and small-bodied bees open the same number of anthers per visit (t = 0.553, df = 36.277, p = 0.584), even though small bees spend significantly more time buzzing each flower (t = 3.753, df = 22.5, p = 0.001). Large-bodied bees achieved the same success as their smaller counterparts in less time by opening more anthers per second (t = -3.266, df = 36.3, p = 0.002). Despite their length of visitation, the amount of time each size-class spent buzzing a flower did not affect how many anthers they opened (large-bodied: R2 = 0.061, p = 0.080, t = 1.786; small-bodied: R2 = 0.169, p = 0.057, t = 2.019). I found one exception in Bombus sp., which was able to open more anthers the longer it vibrated the flower (Spearmans Rho = 0.576, p = 0.041, N = 10). I found that there is no most efficient pollinator of Blakea gracilis, instead the results are consistent with those of past studies which concluded that buzz-pollination is generalized to all bee visitors.
En este estudio, yo tena como objetivo determinar si existe un polinizador ms eficiente para Blakea gracilis. Me enfoqu en abejas grandes y pequeas, prediciendo que las abejas grandes seran capaces de alcanzar frecuencias ms altas y por lo tanto seran mejores polinizadores que sus contrapartes ms pequeas.
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Flores--Morfologa
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