By John P. Roche
For flowering plants to reproduce sexually, they need to disperse pollen so that the male gametes in the pollen can reach and fertilize the egg in the plant embryo sac. Plants have evolved a variety of mechanisms to disperse pollen. Among them, orchids have evolved a mechanism using pollination structures called pollinaria. Pollinaria include parts called pollinia that hold pollen sacs with the male gametes, and adhesive pads that adhere to insects and other pollinators. Some present-day beetles use orchids for nectar, and these beetles also disperse orchid pollinaria. But no fossil evidence has ever been found showing beetles in the evolutionary past pollinating orchids — until now.
A new study by George Poinar, Jr. in American Entomologist identified the first fossil beetles dispersing pollinaria from orchids. Poinar discovered two beetles dispersing orchid pollen that were discovered in fossilized tree sap called amber. One was a hidden-snout beetle (subfamily Cryptorhynchinae) found in amber from the Dominican Republic. This Dominican specimen had pollinaria from an orchid described as Cylindrocites browni attached to its thorax. The other specimen was a toe-winged beetle (family Ptilodactylidae) that was found in amber from Mexico. This toe-winged beetle had pollinaria from an orchid described as Annulites mexicana attached to its mouthparts.
The beetle in Dominican amber was in strata estimated to be from 20 to 45 million years old, and the beetle in Mexican amber was in strata estimated to be from 22 to 26 million years old. Figure 1 (above) shows the beetle from Dominican amber. Pollinia are attached to this beetle with an adhesive pad called a viscidium, which is visible in the photo. Figure 2 (below) shows the beetle from the Mexican amber. The black arrow indicates pollinia attached to the beetle’s mouthparts.
When asked about the significance of his study, Poinar said, “My paper points out that beetles may play a more important role in pollinating orchids than originally thought, and that they have been doing so for some 20 million years.”
With more than 350,000 species described, beetles are the most diverse order on Earth; 40 percent of insects and 25 percent of all animals are beetles. Because of their diversity and abundance, they offer rich potential for plants as pollinators. Beetles of several families have been observed with pollinaria from orchids, including leaf beetles (family Chrysomelidae), dermestid beetles (family Dermestidae), and click beetles (family Elateridae). But no current-day hidden-snout beetles (subfamily Cryptorhynchinae) have been seen visiting orchid plants, and no current-day toe-winged beetles (family Ptilodactylidae) have been seen with pollinaria attached.
Why is that? The answer might be in their behavior. Poinar points out in his paper that one reason that could explain the limited current-day evidence of beetles pollinating orchids is that beetles are secretive in behavior, and thus are difficult species from which to collect data.
Poinar is an entomologist at Oregon State University, and he has led the world in the field of discovering fossil insects in amber. He has written extensively on his research, including the books The Quest for Life in Amber and The Amber Forest: A Reconstruction of a Vanished World, both by George and Roberta Poinar.
When asked about the best next steps in the investigation of beetle-orchid associations, Poinar said, “While no present-day cryptorhynchid weevils or ptilodactyline beetles are known to carry pollinaria, past and future collections of these and other beetles should be examined to search for attached pollinaria. Orchids may have evolved beneficial associations with a much wider range of beetles and other insects than we thought possible.”
Read More at:
John P. Roche is a science writer and author with a PhD and a postdoctoral fellowship in the biological sciences. He has served as editor-in-chief of periodicals at Indiana University and Boston College, as a senior scientist at Boston College, and as a science writer at Indiana University and the University of Massachusetts Medical School. He has published more than 150 articles, and has written and taught extensively about science. Dr. Roche also directs Science View Productions™, which provides writing and editing services for clients in academia and business. For more information about Dr. Roche’s writing, visit http://authorjohnproche.com.