Specialists to Generalists in Tropical EcosystemsDNA barcoding allowed scientists to challenge the importance of the 30-year old Mating Rendezvous Hypothesis in the specialization of flower parasites on host plants.
Laura Bizzarri sampling mites on the beak of a hummingbird with the overlayed illustration highlighting the hummingbird-ﬂower-mite study system. PHOTO CREDIT: Mario Alberto Salazar Araya. IMAGE CREDIT: Erin K. Kuprewicz
Commentary on: Bizzarri, L., C. S. Baer, and C. Garcia-Robledo. 2021. DNA barcoding reveals generalization and host overlap in hummingbird flower mites: implications for the Mating Rendezvous Hypothesis” The American Naturalist https://doi.org/10.1086/718474.
Testing hypotheses on how nature works is at the heart of the biological sciences and determines our rate of progress in understanding the complexity of nature and the natural world. We know that the diversity of life on Earth is concentrated in the tropics: the gradient of increasing species diversity with decreasing latitude from polar regions to the equatorial tropics and decreasing elevation from montane summits to lowland forests has been recognized since the explorations of Alexander von Humboldt and other scientists in South America in the early 1800s.
There are many hypotheses as to why the tropics are so diverse and speciose. Most of them are dependent on accurate and reliable identifications of species. One such hypothesis is that ecological specialization of interactions among tropical species can lead to reproductive isolation and speciation, hence greater biotic diversity1.
As a tropical biologist focused on the interactions of species of plants and their pollinators, I first encountered floral mites while I was mist-netting hummingbirds at La Selva Biological Station in Costa Rica as part of my ongoing investigations of the pollination biology of the genus Heliconia. What were these funny little “insects” that I observed running up and down the bills of the hummingbirds and hiding in the hummers’ nostrils as they were transported from heliconia flower to flower, stealing nectar and having sex along the way?
I learned from papers by Rob Colwell and his colleagues2,3 that these “insects” were specialized flower mites that robbed nectar from the flowers where they mated and were then carried between plants by the hummingbirds. I was fascinated and began my own observations of these interactions. In fact, I have a flower mite named after me, Tropicoseius kressii Naskrecki & Colwell, which Rob and his collaborator discovered in a preserved flower of Heliconia that I had collected in Costa Rica.
From his observations in the field at La Selva, Colwell4 suggested that a process analogous to sexual selection may result in rapid ecological shifts of mites to specific host flowers, which in turn would lead to speciation in the mites. Doug Futuyma and Gabriel Moreno1 later labelled this as “Mating Rendezvous” and recognized it as “an appealing hypothesis” for host specialization in flower mites. However, they admitted that evidence was slim for mate location as a source of “selection for specialization.” Colwell’s observations on flower host specificity by the mites were based on morphological identification of the tiny floral parasites, which was admittedly difficult.
Nearly twenty years later along comes the idea of “DNA barcoding” and the implementation of DNA sequence data as a much more accurate means of species identification across the tree of life5. A short 500-bp stretch of DNA of the cytochrome c oxidase I mitochondrial gene in many cases provides a much more precise and repeatable identification than morphology. It didn’t take long for Laura Bizzarri, Christina Baer, and Carlos Garcia-Robledo at the University of Connecticut to apply this new genetic tool to identify Colwell’s flower mites and test the Mating Rendezvous Hypothesis. Not unexpectedly, after collecting 10,654 mites from 489 flowers at La Selva and generating CO1 DNA barcodes for 1,928 of the mites, they report in the latest issue of The American Naturalist6 that generalist flower mites were much more common than predicted among 14 species of plants pollinated by hummingbirds. Moreover, they discovered that multiple species of mites, often in the same genus, could be found in the flowers of a single host plant. They did determine that eight species of mites were specialized on a single host plant, but these specialists often shared their host with other generalists. The hypothesis on the importance of “Mating Rendezvous” in the specialization of parasites on hosts now appears to be on wobblier ground than originally suggested by Futuyma and Moreno1.
A mite riding the beak of a long-billed hermit (Phaethornis longirostris) as the hummingbird is visiting a Heliconia imbricata plant at La Selva.
PHOTO CREDIT: Laura Bizzarri
Colwell’s data supporting the Mating Rendezvous Hypothesis stood the test of time for over thirty years. However, as is common and necessary for science to progress, a new tool, DNA barcoding, which is now easily available to ecologists and evolutionary biologists7, allowed scientists to challenge that hypothesis.
How many other hypotheses on the origins of tropical specialization and high species diversity can be tested using DNA barcoding and other new tools? Understanding and explaining tropical diversity remains a significant challenge for biologists. The investigation by Bizzarri, Baer, and Garcia-Robledo is just one example of how we advance our understanding of the complexity of nature and the natural world.
W. John Kress
Distinguished Scientist and Curator Emeritus
National Museum of Natural History, Smithsonian Institution
1. Futuyma, D. J., and G. Moreno. 1988. The evolution of ecological specialization. Annual Review of Ecology and Systematics 19:207–233.
2. Colwell, R. K. 1973. Competition and coexistence in a simple tropical community. The American Naturalist 107:737–760.
3. Colwell, R. K., B. J. Betts, P. Bunnell, F. L. Carpenter, and P. Feinsinger. 1974. Competition for the Nectar of Centropogon valerii by the Hummingbird Colibri thalassinus and the Flower- Piercer Diglossa plumbea, and Its Evolutionary Implications. The Condor 76:447–452.
4. Colwell, R. K. 1986. Population structure and sexual selection for host fidelity in the speciation of hummingbird flower mites. Pp. 475–495 in S. Karlin & E. Nevo (eds.), Evolutionary Processes and Theory. Academic Press, New York.
5. Hebert PDN, Cywinska A, Ball SL, deWaard JR. 2003. Biological identifications through DNA barcodes. Proceedings of the Royal Society B: Biological Sciences 270: 313–321.
6. Bizzarri, L., C. S. Baer and C. Garcia-Robledo. 2021. DNA barcoding reveals generalization and host overlap in hummingbird flower mites: implications for the Mating Rendezvous Hypothesis. The American Naturalist https://doi.org/10.1086/718474.
7. Kress, W. J., C. García-Robledo, M. Uriarte, and D. L. Erickson. 2014. DNA barcodes for ecology, evolution, and conservation. Trends in Ecology and Evolution 30: 25-35.
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