From Plant Press, Vol. 11, No. 3 from July 2008.
By Gary A. Krupnick
The Seventh Annual Smithsonian Botanical Symposium, held on 26 April at the National Museum of Natural History in Washington, DC, gave 175 participants the opportunity to discover how natural selection has shaped animals and plants together through their ancient and persistent dependencies with one another. The theme, “Partners in Evolution: Interactions, Adaptations, and Speciation,” was explored by a distinguished group of specialist speakers, and was highlighted with a reception in a pavilion of live plants and butterflies.
The meeting, held in collaboration with the United States Botanic Garden and the National Tropical Botanical Garden, was the first Smithsonian Botanical Symposium to be co-hosted by three departments from the museum—the Departments of Botany, Entomology and Paleobiology. The symposium addressed the various ecological interactions, evolutionary adaptations, and co-radiations of plants and animals in habitats across the planet and explored the processes of coevolution.
Giving the introductory greeting, Warren Wagner, symposium convener, welcomed the audience to Baird Auditorium. Hans-Dieter Sues, Associate Director for Research and Collections, then had the opportunity to say a few words about the museum’s role in hosting symposia, before he trotted off to a concurrent symposium elsewhere in the museum.
Laurence Dorr took the stage to award the Seventh José Cuatrecasas Medal for Excellence in Tropical Botany to Mireya D. Correa A. from the University of Panama and the Smithsonian Tropical Research Institute (STRI). An expert in the systematics of vascular plants with special emphasis on the Flora of Panama, Correa expressed gratitude and was highly honored. She recalled having wonderful conversations in the past with Cuatrecasas during her first visit to the U.S. National Herbarium. Very humble, Correa stated that she had “done what I have to do in a country very rich in species of plants.” Correa thanked her large group of students, many who have worked with her in the field and in the herbarium. Of her students, Correa said, “[they] do what I cannot do – they climb, and I can think and teach.” She also thanked STRI for presenting her with opportunities and support to do research that she would not have been able to accomplish at the university.
The first lecture of the day, moderated by the morning session chair Terry Erwin (Chair of Entomology, Smithsonian Institution), was presented by Judith Bronstein, Professor at the University of Arizona and current Program Director at the National Science Foundation, who spoke about “Mutualism, Diversity, and Diversification.” Bronstein, who chose to “paint issues with a broad brush,” spoke about the many benefits of mutualism: protection, nutrition, and transportation (e.g., pollination). She explained how mutualisms are maintained. Biologists have often assumed that mutualisms are evolutionarily fragile, but new evidence (both models and phylogenetic studies) points to the remarkable robustness of mutualism in the face of ecological cheating.
Bronstein also discussed how anthropogenic change threatens biological diversity worldwide. Conservation strategies typically focus on individual charismatic species, but management approaches should include mutualists as well. Current threats that disrupt mutualisms include (1) invasive species that outcompete mutualists, (2) habitat fragmentation that preclude mutualists from moving between patches, and (3) other disturbances, such as fire and hurricanes. The ultimate threat is coextinction, when one partner is lost due to the loss of the other partner; but Bronstein explained that there is very little evidence of coextinction, most likely because they are very difficult to detect.
Conrad Labandeira, Research Scientist, Curator and Chair of the Department of Paleobiology, Smithsonian Institution, asked the symposium participants “What Can We Learn from the Fossil Record of Plant-Insect Associations?” The fossil record of plants, insects, and their associations is evident in fossilized plant damage, coprolites (fecal remains that include plant parts), gut contents, and insect mouthparts.
Labandeira explained that there were four main spatiotemporal phases in the evolution of herbivory during the past 420 million years. The first phase (Late Silurian to Late Devonian) shows three distinct feeding groups: piercing, sucking, and boring damage. Phase 2 (Late Mississippian to end-Permian) is represented by six feeding groups, including the addition of galling, seed predation, and non-feeding oviposition. Leaf mining is the seventh feeding group added during Phase 3 (Middle Triassic to Recent). The most recent and intensively studied phase of arthropod herbivory on vascular plants is Phase 4 (mid Early Cretaceous to Recent), represented by a greater diversity of functional feeding groups.
Olle Pellmyr, Associate Professor at the University of Idaho, finished the morning session with the lecture “Coevolution and Obligate Mutualism: What We are Learning from Yuccas and Yucca Moths.” Pellmyr covered (1) the biology of the interaction, (2) phylogeny and codiversification, and (3) the role of coevolution in yucca-yucca moth interactions. He then presented a case study of a two-morph population of Joshua trees (Yucca brevifolia) pollinated by two different moth species (Tegeticula spp.). His data show that (1) moth ovipositors and ovary shape are closely correlated, which can lead to strict sense coevolution; (2) there was no abiotic niche divergence; and (3) there were different phylogenetic histories between the plants and the two moth species (e.g., moth morphology differences appeared when they first became associated with Joshua trees). Finally, Pellmyr noted how global climate change may cause a range shift of moth species (future populations will move north and to higher elevations), which may directly affect pollination of the Joshua tree populations.
After lunch, the talks, moderated by afternoon session chair Conrad Labandeira, began with John Kress, Research Scientist and Curator in the Department of Botany, Smithsonian Institution, who spoke about “From Generalization to Specialization…and Back Again in the Coevolutionary Mosaic of a Heliconia-Hummingbird Pollination System in the Eastern Caribbean.” Kress’ talk examined the close ecological relationship between the shape and size of flowers and bird bills, and the co-radiation of Heliconia and hummingbirds. The study focused on hummingbird pollination of Heliconia bihai and H. caribaea on the islands of St. Lucia and Dominica, where male and female birds of the same hummingbird species, the Purple-throated Carib, pollinate different species at different rates depending upon a variety of factors (e.g., island, elevation, and season).
According to Kress, the data support John Thompson’s Geographic Mosaic Theory of Coevolution. In the northern range, H. caribaea is the dominate species; in the southern range, H. bihai dominates; and in the middle, both co-dominate with variation. Further, in H. bihai, specialist pollinators are found in the north, while generalist pollinators are found in the south.
Next, Scott Hodges, Professor at the University of California at Santa Barbara, presented the talk “Understanding Speciation and Adaptation to Pollinators: From Field to Genomic Studies of Aquilegia.” Nectar spur length varies greatly among the 23 taxa of Aquilegia in North America, making this genus a model system for examining the natural selection of floral traits. The length of nectar spurs, Hodges argues, has developed in response to pollinators. Fertile hybrids of Aquilegia can also be made in any North American cross, which allows Hodges the ability to conduct genetic studies.
Hodges presented a case study examining how Aquilegia species are reproductively isolated. Flower orientation, spur length, and flower color each have different effects on visitation rates and pollen removal by pollinating bumblebees, hummingbirds, and hawkmoths. Field studies show that the pollinators are distinguishing among flowers for the most part: bumblebees visit flowers with shorter spurs; hawkmoths with longer spurs; and hummingbirds in the middle. Paternity analysis shows that the distance between plants and differences in floral morphology reduces the probability of mating. Thus reproductive isolation is due to strong assortative mating by flower morphology.
The final invited talk of the afternoon was presented by Ted Schultz, Research Entomologist in the Department of Entomology, Smithsonian Institution, who spoke about “The Evolution of Agriculture in Ants: 50 Million Years of Symbiosis between Ants, Fungi, and Bacteria.” With over 230 species of fungus-growing ants in the New World tropics, leaf-cutting ants are the dominant herbivores. Schultz described a complex system in which the mutualism goes beyond just two species: attine ants, fungal cultivars, filamentous bacteria that the ants culture, a fungal crop disease, and an antibiotic produced by a bacterial symbiont. The mutualism is so tight that higher attine cultivars are incapable of a free-living existence. This system presents a clear adaptation for “many-to-one” coevolution.
Invited guest speaker John N. Thompson, Professor at the University of California at Santa Cruz, concluded the symposium with a summary and perspective: “Coevolving Networks of Species.” Thompson began his summary with a quote from Charles Darwin, who wrote the first statement on coevolution in a description of bee pollination. Thompson continued with the observation that if honey bees were “taken out of the equation,” local extinction of plants would ensue. According to Thompson, “dismantling the web of life is such a dangerous thing to do.”
Thompson described coevolution not as a species property but as a population-level process. Using a coevolutionary case study (saxifrage plants and their pollinating seed parasite Greya politella), Thompson showed that within a short distance (Washington, Idaho, and Oregon), one can find mutualistic, antagonistic, and commensalistic interactions. Selection is pushing different populations to different places. He ended with the big question, “Why should we care?” Coevolution, according to Thompson, is the process of how species stay in the evolutionary game. Understanding coevolution, for example, can help reduce money spent on herbivore resistance in agriculture, or determine the best control for the outbreak of an invasive species.
After the formal lectures, the participants were given the chance to interact and further discuss the process of coevolution during a reception and dinner. The highlight of the evening events was a private tour of the museum’s newest exhibit: “Partners in Evolution: Butterflies + Plants.” The exhibit gave the visitors the opportunity to walk among living tropical and temperate butterflies and plants, and learn how they have co-evolved over millions of years generating the biodiversity we see today. The reception was followed by a dinner in the museum’s rotunda.
Next year, the Smithsonian Botanical Symposium will consider the study of genomics, with a focus on maize. The date of the Symposium is tentatively set for 27-28 March 2009. All are invited to attend.
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