‘Ferns have not bowed to the ages’

When I started my undergrad, botany was something of an afterthought – I just needed a third course to go along with chemistry and zoology, which had been my mainstays in A Levels. But once I sat down in Dr Duncan’s class, I was enthralled – while chemistry and zoology largely rehashed what I had learned in A Levels, botany took me into an entirely new world. As we partook in an old-fashioned march across the plant kingdom, I was amazed. Lectures were interesting, but it was the labs that truly drew me in.

By the time we got to ferns, we had covered a variety of algae and simpler plants. Much of what I was taught slipped past me, adrift without a proper foundation. The difference between isotomous and dichotomous branching in the thallus. Steles, and steles within steles. But one thing that stuck with me was a quote from Frederick Orphen Bower: ferns, he said, had not bowed to the ages. While other ancient groups like the liverworts and horsetails had retained only a shadow of their former diversity, ferns were not only diverse, much of their diversity reflected a (relatively) recent diversification that had occurred ‘in the shadow of angiosperms‘.

The simple fact that ferns diversified after the rise of the group that is generally seen as replacing and displacing them is in itself remarkable. It clashes, not with evolutionary theory, but rather, with our (mistaken) perception of an orderly progression of evolution. If we can put aside our evolutionary misconceptions, it becomes less disconcerting. Or it did, until recently.

A recent paper by Fay-Wei Li and colleagues uncovers the mechanism by which ferns were able to diversify in the shadows: a gene from a hornwort. In is usual way, Ed Yong does an great job of explaining the research, as does Carl Zimmer. While the discovery is a great story, what really intrigues me is the mechanism of gene transfer. Horizontal gene transfer (i.e., transfer from one species to another) is no big deal in bacteria. Migration of genes from the mitochondrial and chromosome genome to the nuclear genome is strange, but it’s still the kind of thing for which you can envision a reasonable mechanism. Gene flow between species via introgression (hybridisation followed by extensive backcrossing to one parental species) is intuitive (or can be, if you don’t think about it too much). But transfer of a gene from a hornwort to a fern. How is that supposed to happen?

As odd as it sounds, I’m sure there’s a reasonable way for genes to make that journey. I’ve heard about people coaxing plants to take up RNA molecules. Viruses could also have unwittingly played a part. I suppose the most important message in this story may be the realisation that transgenic higher plants may not be as unusual as we tend to think they are.

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