We are very grateful to have a guest entry from Dr. Carl Rothfels, a postdoctoral fern researcher at the University of British Columbia (and soon assistant professor at Berkeley). Dr. Rothfels and a team of researchers recently published the following: Rothfels, CJ et al.. 2015. Natural hybridization between parental lineages that diverged approximately 60 million years ago. American Naturalist 185:3(443-442). The first photograph is shared by another member of the research team, Harry Roskam, while the latter two photographs are courtesy of Carl. Thank you Carl and Harry, for the write-up and photographs!
Today’s plant featured on BPotD is an extraordinary fern from the Pyrenees mountains in France, which goes by the rolls-off-your-tongue name of ×Cystocarpium roskamianum (seen in the first photo, courtesy of Harry Roskam). This species was first noticed growing in a nursery in the UK, by a fern taxonomist by the name of Christopher Fraser-Jenkins. While it may not look that unusual to us, Fraser-Jenkins noted that this innocuous-appearing plant shared characteristics of two groups of ferns–the fragile ferns (Cystopteris, exemplified in the second photo with Cystopteris fragilis) and the oak ferns (Gymnocarpium, exemplified by Gymnocarpium appalachianum). These two genera are very different from each other (at least to people who study ferns!): Cystopteris species have elongate leaves, short compact stems, a unique hood-shaped covering protecting the sori (the “spore dots”), and tend to grow in cracks in cliff-faces and other inhospitable habitats, whereas Gymnocarpium species have triangular leaves elevated on tall stalks, long-creeping underground stems (rhizomes), unprotected sori, and grow in rich soil on forest floors. Until recently, many taxonomists didn’t think that they even belonged in the same plant family. How could two groups of ferns be more different? And as we all know, very different things are not able to successfully mate with each other…
Fraser-Jenkins, however, was not dissuaded–in pretty much every feature he examined, the plant from the nursery was intermediate between Cystopteris and Gymnocarpium and it didn’t produce viable spores, which is another indication that it could be a hybrid. In researching the plant more, Fraser-Jenkins discovered it had been collected in the Pyrenees by a Dutch horticulturalist, Harry Roskam, who had brought it into cultivation (it grows vigorously and although it is sterile, it can reproduce rapidly via its long creeping rhizome, rather like a strawberry would). To honour the collector, Fraser-Jenkins formally described this plant in a new hybrid genus, as ×Cystocarpium roskamianum (the rules of botanical nomenclature stipulate that the genus name for intergeneric hybrids must start with the “×” symbol and then portions of each of the putative parental genus names, here “Cysto” from Cystopteris and “carpium” from Gymnocarpium).
But is ×Cystocarpium roskamianum really a hybrid between Cystopteris and Gymnocarpium? To answer this question, a team including Fraser-Jenkins, Harry Roskam, and researchers from Leiden University in the Netherlands, Duke University in the U.S.A, and the University of British Columbia in Canada, turned to the fern’s DNA. If the fern was a hybrid it might be expected to have DNA sequences from both its parents, and that’s exactly what the researchers found (to the great surprise of at least some of them): at a given gene, half the ×Cystocarpium sequences matched those from Cystopteris, and the other half were from Gymnocarpium. The DNA data were even more specific than that–×Cystocarpium is a hybrid between the cosmopolitan species Gymnocarpium dryopteris (which was the mother in the cross) and a European member of the Cystopteris fragilis complex (the father).
More astoundingly, the researchers were able to determine that the hybridization event probably happened only once, and very recently (maybe within our lifetimes) and that the last common ancestor of ×Cystocarpium roskamianum’s parents lived approximately 60 million years ago. In other words, each of the parents had been evolving independently from the other for around 60 million years before the hybridization happened. Sixty million years is a very long time for two organisms to retain the ability to interbreed–typically that ability is lost within a few million years at most. To put this duration in perspective, the ancestors of humans diverged from those of chimpanzees a mere five or so million years ago; the hybridization event that formed ×Cystocarpium is roughly akin to a human producing a hybrid with a lemur or an elephant with a manatee. The fact that Cystopteris and Gymnocarpium retained some compatibility with each other after that amount of independent evolution raises interesting questions on how new species are formed, and how this process might differ in different groups of organisms. For example, ×Cystocarpium and the other reported cases of deep hybridizations tend to involve ferns and other plants that don’t use animals to assist with fertilization. If there is something about the ecology or genetic structure of these species that allows them to retain reproductive compatibility among populations for longer than other groups do, and thus to form new species more rarely, could this explain why there are only around 10000 species of ferns (and approximately 1000 gymnosperms, 1200 lycophytes, 12000 mosses, 9000 liverworts, and 100 hornworts) compared to the nearly 300000 species of flowering plants?
Daniel adds: Another account of this research is available via NPR: “Weird” Fern Shows The Power Of Interspecies Sex.