Arlington, Va. -- Belying the popular notion of ferns as delicate, lacy relics surmounted by the evolution of flowering plants, biologists have presented evidence for a much different scenario. Their studies indicate that when flowering plants, or angiosperms, evolved some 144 million years ago, ferns took advantage of ecological niches in the new angiosperm forests to diversify into a far richer array of species.
The study offers a new insight into the critical period in evolution when the rise of flowering plants sparked a dramatic increase in species diversity that eventually fostered the rise of birds, bees and mammals, including humans.
As a result of such dynamic ecological opportunism, ferns now comprise more than 10,000 living species, making them the second largest group of vascular plants, following flowering plants.
Duke University researchers Harald Schneider, Eric Schuettpelz and Kathleen Pryer will publish their findings in the April 1, 2004, issue of the journal Nature. The team also includes Raymond Cranfill, Susana Magallσn and Richard Lupia.
The National Science Foundation (NSF), the independent federal agency that supports fundamental research and education across all fields of science and engineering, funded the study.
"Pryer and her colleagues have radically revised the textbook story of the rise of the angiosperms by suggesting a more interesting dynamic about ecological interactions molding concurrent patterns of diversification in these two groups," said James Rodman, program director in NSF's division of environmental biology.
The researchers based their findings on concurrent analyses of the fossil record of plant species as well as genetic studies of existing species. They analyzed the DNA sequences of telltale genes in ferns and angiosperms and found family trees charting the relationships among ferns and angiosperms, which revealed their evolutionary history.
The team used insights from the genetic comparisons to reevaluate the fossil record, and to assign fern and angiosperm fossils to currently existing lineages.
"The integration of fossils with molecular phylogenetic data is an extremely powerful tool for identifying historical trends and events across the tree of life," Pryer said. "This approach permits the most comprehensive understanding of the past by combining information from the incomplete fossil record with information in the genetic blueprint of living organisms."
The researchers theorize that the key to ferns' ability to diversify—literally in the shade of angiosperm forests—was the evolution of a low-light photoreceptor in the ferns, which responded to a wider range of light wavelengths, giving ferns an evolutionary advantage in the low-light conditions in angiosperm forests.
"Angiosperm forests are ecologically and structurally more complex than are the more uniform forests dominated by the conifers or cycads present in more ancient forests," said Pryer. "So, these forests offered a diverse array of microclimates and habitats that likely promoted speciation in many lineages of plants and animals. Ferns may have been able to exploit this increased complexity through the evolutionary acquisition of this novel, more versatile photoreceptor," she said.
Relic descendants of the ancient ferns still remain, the scientists pointed out, especially in tropical environments. But those ancient ferns have been joined by the opportunistic species that evolved since the rise of angiosperms.
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