From Plant Press, Vol. 19, No. 3, July 2016.
By Gary A. Krupnick
Pollination biology was the focus of the 14th Smithsonian Botanical Symposium, held 20 May 2016 at the National Museum of Natural History (NMNH) and the United States Botanic Garden (USBG) in Washington, DC. Titled, “Bats, Bees, Birds, Butterflies and Bouquets: New Research in Pollination Biology,” the meeting featured seven invited speakers, a poster session, and an evening reception. The conference brought together over 340 biologists, ecologists, government officials, horticulturalists, and master gardeners to celebrate the progress of pollination biology and to address issues of pollinator loss and conservation.
The Symposium began with opening remarks by Maureen Kearney (Associate Director for Science, NMNH) and Laurence Dorr (Chair of Botany, NMNH). Kearney praised the partnership between NMNH and USBG, and mentioned how the symposium adds to the intellectual environment of the museum which often exercises an interdisciplinary research approach with a deep organismal focus in systematics, ecology, evolutionary biology, and behavior.
After the opening remarks, Kenneth Wurdack (Curator and Cuatrecasas Committee Chair, NMNH) presented the 14th José Cuatrecasas Medal in Tropical Botany to Kamaljit S. Bawa. This prestigious award is presented annually to a scholar who has contributed significantly to advancing the field of tropical botany. Bawa, a Professor of Biology from the University of Massachusetts, was commended for his extensive contributions to tropical biology, international conservation, and pollination biology. In his acceptance speech Bawa expressed his appreciation and gratitude to the Smithsonian Institution and the selection committee. He was delighted to see so many friends and colleagues and gave thanks to all who have inspired him.
Both Laurence Dorr and Seán Brady (Curator of Bees, NMNH) served as conveners.
The first speaker was Sam Droege (USGS Patuxent Wildlife Research Center) who spoke about “Patterns in pollen and plant specialization among native bees in eastern North America.” Droege’s macro photographs of bee portraits from his online reference catalog provided the backdrop for his talk about pollination from the bee’s point of view. First he spoke about the high diversity of bee species in the United States – about 4,000 species. He pointed out that there are nearly as many bee species in Prince George’s County, Maryland (249 species) as there are in the United Kingdom (250 species). He explained that 250 million years of evolution has led to complex and varied floral bee designs, nutritious pollen, complex secondary compounds, and complex bee communities that reflect plant diversity.
Droege said that while there are few plant species that are pollinator specialists, there are many bee species that are pollen specialists. Specialist bee larvae often fail to thrive on the pollen of other plants and are often restricted to target plant blooming periods. Population sizes tend to be low for specialists which might lead to conservation concerns. Droege’s mid-Atlantic bee collection shows about 20 percent as pollen specialists, with the most common being Andrena erigeniae (a Spring Beauty specialist), Ptilothrix bombiformis (Hibiscus/Mallow specialist), and Andrena violae (Violet specialist). The plant families with the most specialists include Asteraceae, Ericaceae, and Salicaceae. Specialist bees tend to be the most vulnerable due to habitat loss, fragmentation, invasive species, and herbicide usage. Non-native plant species mostly favor generalist bee species, and thus Droege encourages the fostering of native habitat.
David Roubik (Smithsonian Tropical Research Institute) next asked the question, “Whose bees are these? The pollen taxonomy of bee nests and its story in recent decades.” Roubik explained that pollen grains are nearly indestructible, taxonomically informative, and open to creativity and curiosity. His research on Barro Colorado Island, a 1,560-hectare island in the Panama Canal, makes use of botanical knowledge, reference collections, photographs, and field and laboratory work. Among his studies are orchid bees, honey bees, Africanized honey bees, Centris bees, and megachilids.
Roubik’s pollen research has also included studies of honey bee competition with native bees. A 17-year study from the Yucatan Peninsula in Mexico shows the impact of invasive Africanized honey bees on solitary megachilid bees. He found honey bees may have forced the native bees to switch their food plants. Pollen from plants in the families Euphorbiaceae and Anacardiaceae were present in lower amounts in megachilid nests after honey bee invasions, while Sapotaceae, Fabaceae, and Rubiaceae pollen were present in great amounts in the same nests.
Tatyana Livshultz (Drexel University) continued the focus on pollen with her talk, “Putting milkweeds in context: The evolution and function of pollen aggregation in Apocynaceae.” Livshultz studies the structural and functional diversity of pollen. The most common type of pollen aggregation are tetrads, 4 pollen grains dispersed as a unit. Orchidaceae and Apocynaceae have pollinaria, two or more pollinia (entire pollen content of an anther sac fused into a mass) attached at a translator. Livshultz showed that aggregated pollen and translators evolved three times independently within Apocynaceae.
Livshultz demonstrated how aggregation affects pollen performance through measures of pollen transfer efficiency (PTE), the fraction of exported pollen grains that reach conspecific stigmas. She discussed how milkweed pollinarium evolved in an ancestor that already had functionally aggregated pollen and high PTE. The evolution of the pollinarium further elevated PTE. She explained that current evidence does not indicate that evolution of the gynostegium nor tetrads and translators in Apocynum and Periplocoideae elevated PTE. Finally, Livshultz put milkweeds in a paleoecological context and explained that drought-induced mortality of plants and pollinators during early Oligocene aridification of Africa may have created the selective pressure for evolution of increased PTE and pollinaria in milkweeds.
The next two speakers spoke about unseen floral attractants—scent and ultraviolet pigmentation. First was Robert Raguso (Cornell University) who discussed “Floral scent: The dark matter of plant-pollinator interactions.” Raguso explained that in studies of the structure of plant-pollinator networks, floral scent is almost never mentioned, yet it can be just as important as visual and tactile patterns. He showed how scent can affect seed fitness and structure floral networks through attraction and repellence. In one study floral fragrances of Cirsium arvense and Achillea millefolium were swapped, and the links between insect visitors to the two plant species were restructured until the scents dissipated. In Penstemon digitalis, where the corolla limb is scentless and the nectar tube has volatiles, Raguso showed how floral scent emission and floral display were under selection, and flower size and color were not. This suggests that smelling stronger benefits reproductive success in this flowering species.
Raguso also explained how scent can help integrate complex floral phenotypes and mediate reproductive isolation through attraction, preference and constancy. Dracula lafleurii, an orchid species from Ecuador, is pollinated by flies that breed in mushrooms, and Dracula flower parts resemble mushrooms. In a study using 3d-printed silicon models of Dracula flowers, Raguso was able to tease apart visual, olfactory, and tactile responses in which a combination of the three gets the best response by flower visitors. A study examining visitation by Hyles lineata hawkmoths to scent-augmented and color-swapped Ipomopsis flowers demonstrated that moths require both scent and high visual contrast to feed on flowers at dusk. He concluded by stressing the importance of scent combined with visual traits in pollination studies.
Matthew Koski (University of Virginia) followed with a talk on “The evolutionary ecology of ultraviolet floral pigmentation at micro- and macroevolutionary scales.” Ultraviolet (UV) pigmentation leading to a bullseye pattern has been found in at least 23 of 62 angiosperm orders. Koski’s research focused on how UV bullseye mediates pollinator behavior and if pollinators or abiotic factors contribute to UV variation. He showed that insects are more attracted to UV bullseye in Potentilla anserina, that the UV pattern, not the reflection, increases attractiveness, and that the bullseye does not serve as a nectar guide.
The pollinator communities of Potentilla anserina change with altitude (bees predominantly at lower altitudes and flies at higher altitudes), as does the bullseye size (larger bullseye at higher altitudes). Koski found that by manipulating the size of the bullseye at low and high altitude sites, both bee and fly preference for bullseye shifted with altitude. Plants with larger bullseyes received more visits and pollen by both pollinators at higher altitudes, while plants with smaller ones received more pollen at lower sites. In terms of abiotic factors, he found that larger bullseyes protect pollen from UV-damage. Looking at macroevolutionary patterns of diversity in the genus Potentilla, Koski explained that the UV bullseye is larger in more alpine species which experience higher UV-irradiance and cooler temperatures. He concluded that while pollinators are important in driving variation among populations of Potentilla, it is the abiotic factors that drive diversity at both micro- and macroevolutionary levels.
Moving the discussion from insect to vertebrates, Nathan Muchhala (University of Missouri-St. Louis) discussed “Bats, birds, and bellflowers: The evolution of specialized pollination systems in the Neotropics.” To examine bat-flower coevolution, Muchhala examined the efficiency of bat tongue at extracting nectar and the ability of bats to transfer pollen. He showed how tongue length varies among three bat species, with Anoura caudifer and A. geoffroyi having short tongues (3.7 cm and 3.9 cm, respectively) while A. fistulata have long tongues (8.5 cm). The diet of Anoura fistulata includes many plant species, but not the short corolla Burmeistera. In pollen transfer experiments in the field, Muchhala showed how angle and depth are more important than duration and force –bats lift their heads for deeper feeding which led to greater transfer of pollen. He argued that Anoura fistulata appears to have evolved its incredibly long tongue through coevolutionary races with long-tubed flowers.
Muchhala also spoke on interspecific competition (plants competing for pollinators), floral character displacement, and evolution. He discussed his research on overdispersion of color in Iochroma, a hummingbird- and bee-pollinated genus of 33 species of shrubs and small trees throughout the Andes. Looking at color differences, phylogenetic distances, and differences in plant morphology (lengths of petiole, leaf, and pedicel), Muchhala asked whether coexisting communities of species differ more than expected by chance. He found that hummingbird and bee pollinators are driving the difference in color among the species, and that there is ongoing selective pressure among the plant species to be different in color as much as possible to reduce the costs of receiving pollen from close relatives.
The talks concluded with a presentation by Candace Galen (University of Missouri) who spoke about “Shifting baselines and changing partners: Ecological and evolutionary responses to climate change in alpine bumble bees and their host plants.” To examine the impacts of global climate change on plant-pollinator relationships, Galen conducted field studies at three sites in the Colorado Rocky Mountains where overnight lows are on average 2oC warmer and flower production has seen a 60 percent decrease since the 1970s. Galen examined whether Bombus species are broadening their food choices. She found that the foraging breath of bumble bees has more than doubled, and that flower shape of host plants has also broadened—many short-tubed flowers are now among those they visit.
Since bees with shorter tongue (proboscis) lengths have a wider breath of food choices, Galen hypothesized an evolutionary shortening of tongue length due to warmer summers and reduced host plant flower abundance. She showed that among museum specimens of Bombus balteatus and B. sylvicola, tongue length has decreased by 25 percent, demonstrating strong selection and rapid evolution within 40 generations of these species. Galen explained that the silver lining to global climate change is that these pollinators have been able to adapt rapidly to changing resources. However, reproductive success of host plants still need to be measured. As diets of these bumble bees have broadened, Galen asks how the change in vegetation will impact pollinator dynamics and whether pollen contamination will be a result of competing bee species visiting competing plant species.
The Symposium ended with a reception and poster session in the Conservatory of the United States Botanic Garden. A group of 14 presenters displayed their posters and spoke about their research ranging in topics from floral rewards and the diet of hummingbirds and bats to honey bee health and the conservation of endangered species.
Next year’s 15th Smithsonian Botanical Symposium will take place on Friday, May 21, 2017, with a theme yet to be determined. Be sure to check the symposium website at <https://botany.si.edu/sbs> for updates.