A Frond Fared Well: Genes hint that ferns proliferated in shade of flowering plants

Analyses of genetic material from a multitude of fern species suggest that much of that plant group branched out millions of years after flowering plants appeared, a notion that contradicts many scientists’ views of plant evolution.

Botanists estimate that ferns include more than 10,000 living species. Most modern ferns belong to a set of species called the leptosporangiates, which emerged at least 250 million years ago, says Kathleen M. Pryer of Duke University in Durham, N.C. Ferns dominated many ancient landscapes for more than 150 million years.

The fossil record suggests that the diversity and abundance of ferns sharply declined in the Cretaceous period, which began about 145 million years ago. The fossil decline appears to coincide with the emergence and dramatic rise of angiosperms, or flowering plants, which today account for up to 300,000 species, says Pryer. Botanists often have linked the two trends, suggesting that flowering plants claimed the ferns’ resources and relegated ferns to an evolutionary backwater.

New analyses of fern DNA, however, hint at a different scenario, in which ferns reversed their decline. In 45 species of ferns, Pryer and her colleagues looked at two genes taken from chloroplasts, the structures in plant cells that convert sunlight into energy. They also scrutinized two chloroplast genes and one other gene taken from 84 species of angiosperms. The surprising results suggest that polypod ferns—a group within leptosporangiates that includes more than 80 percent of living fern species—experienced an explosion of diversification between 20 million and 50 million years after the angiosperms appeared on the botanical scene. The scientists report their findings in the April 1 Nature.

Previous studies that focused on the fossil record didn’t show such a pattern, says Pryer. However, she notes that many species of polypod ferns are epiphytes, which grow on the surfaces of other plants rather than on the ground. For this reason, epiphytes may not have been readily preserved in the fossil record, Pryer suggests. Also, fossils of fern spores don’t often include the distinctive outer walls, so researchers have had difficulty discerning fern diversity during ancient eras.

What spurred the newly recognized spurt of fern evolution? Pryer and her colleagues suggest that ferns, rather than being crowded out by the upstart angiosperms, took advantage of the shady ecological niches provided by the flowering plants’ spreading canopies.

Indeed, many types of large plants that dominated ecosystems before angiosperms evolved typically produced open forests, says Kathleen B. Pigg, a paleobotanist at Arizona State University in Tempe.

A protein recently discovered in one variety of polypod fern might have been key to such ferns flourishing in the low-light conditions of forest floors, says Pryer. The substance, dubbed PHY3, absorbs both red and blue wavelengths of light.