Neochrome evolution in ferns

Photo by Fay-Wei Li, via phys.org

Some very exciting new research on fern evolution was just published in the Proceedings of the National Academy of Sciences! Researchers have known for years that ferns possess a chimeric photoreceptor, called neochrome, that allows them to use both red and blue light for photosynthesis. This is a significant advantage in forest understories where blue light has often been filtered out by overhanging leaves of taller plants. In the new paper published yesterday, a team of researchers led by Fay-Wei Li at Duke has discovered that neochrome in ferns is actually most closely related to neochromes in hornworts, and that it likely passed from hornworts to ferns via horizontal gene transfer about 179 million years ago. The timing of this event suggests that this transfer, and the low-light photosynthetic ability it conferred on ferns, allowed them to subsequently diversify "in the shadow" of angiosperms, as the latter exploded in diversity starting around 120 million years ago. This is a really cool finding, and expands our understanding of fern evolution and diversification tremendously!

Check out more coverage of this story in the Economist:
Time and change: The survival of ferns to the present depended on an ancient accident

And at Phys.org:
Ferns borrowed genes to flourish in low light

And read the abstract of the paper here (subscription to PNAS required for full article):
Horizontal transfer of an adaptive chimeric photoreceptor from bryophytes to ferns

Australia ferns: part I

You may have noticed a distinct lack of ferns in the last few, Australia posts... that's because I've been saving the best for last! We did indeed see lots of ferns Down Under, though unfortunately I know very few of their names. All of these photos were taken in the various eucalypt forests we drove through, and there will be three of these fern-centric posts to round out my series on this summer's Australian adventures.

This is a tree-fern stump that was burned in the recent, awful fires that consumed thousands of acres around Marysville in January 2009. This fern didn't survive, but the vascular structure that remains inside the trunk is beautiful.

Polystichum sp.:

Blechnum sp., sterile leaf first, then fertile, then both together:

Lovely tree ferns adorn a brook:

Dryopteris spores

Hello! I apologize for the two-month hiatus, it was unintentional. Classes and research slowly hijacked my every waking moment this semester, and as the cold and snow settled in here in Wisconsin, there was less and less to talk about in terms of living plants. However, I did get to work on a really cool project for a plant anatomy course I took this fall, which I wanted to share. Along with one of my labmates and another grad student in our department, I undertook a study of the spores of Dryopteris (my research group), to satisfy the class's project requirement. We got to learn how to use the scanning electron microscope that lives in the botany building, and got some beautiful results. Above is a scattering of spores from Dryopteris marginalis, and below is a figure from the paper we wrote:

A = D. arguta, B = D. carthusiana, C = D. celsa, D = D. cristata, E = D. expansa, F = D. filix-mas, G = D. fragrans, H = D. goldiana, I = D. intermedia, and J = D. marginalis. The scale bar is 20 micrometers... really small! It was a fun project, and the results may prove useful to me down the road in my research. I'll try to post more often over the winter break, if I can find anything living to photograph! Happy Holidays!

Sori, I didn't see you there

A long overdue post on these beautiful, useful, and often-overlooked (though not by professional fern fanciers) structures found on the lower (abaxial) surface of many fern fronds: the sori. A sorus (singular) is a cluster of sporangia, which are very small spore-bearing sacs. So: each of the colored "dots" in some of the above photos is actually made up of many smaller "dots" which actually contain the plant's spores. The individual sporangia are actually visible in the first image, which is a Costa Rican Campyloneurum species. 

Sori are often round (as in the first, third, and last images, of Campyloneurum, Niphidium, and Tectaria, respectively), but may also be linear (second and fourth, Adiantum and Pteris), and some groups of ferns have more complex cup-shaped or valved sori. Another important structure when considering sori is the indusium, a piece of tissue that may sit on top of or surrounds the sorus. To complicate matters further, some ferns, like the Adiantum pictured in image #2 at top right, have what is known as a false indusium; their sori are surrounded by an indusium-like flap, but it is not derived from the same source as a true indusium. 

The presence and shape of the sori, and the presence or absence of indusia, are some of the most important characters to consider when trying to identify a fern, as they are very often specific to families or genera, or even to a certain species. For example, all members of the genus Adiantum have a false indusium that is simply the edge of the leaf rolled over the sori, and this single diagnostic feature is extremely helpful in positively identifying a fern as a member of this genus. 

Well-meaning but uninformed folk sometimes mistake sori for fungal or insect infections on leaves, and scrape them off thinking that they're doing the plant a favor. Big mistake! Doing so could potentially destroy a fern's entire reproductive output if the scraping were thorough enough. And besides, then you couldn't enjoy looking at them anymore. 

Petiole Vasculature

One fern feature that is useful in identifying a specimen is the pattern of its vascular tissue at the base of the petiole. We talked a lot about this on the fern course in Costa Rica, because it is particularly useful in determining which major group a fern belongs to, which will narrow down the possible families it could be in. 

The petiole of a fern is the narrow, stem-looking tissue (that's not actually the stem-the stem is the rhizome) to which the leafy blade is attached. If you clip this stalk right at the base, and look at it through a hand lens or with the naked eye if the fern is large enough, you should be able to see a pattern of lines or circles. These are the vascular bundles, composed of xylem and phloem, which conduct water and sugar through the plant. Certain patterns of the bundles are characteristic of the group of fern you are looking at; the omega shape in the first three photos above can be found in Eupolypods I and a few other groups, while the last image is of Angiopteris, a fern in the more basal family Marattiaceae.

Fiddleheads

Although the sun is shining brightly at the moment here in Madison, my computer tells me it's 5 degrees outside. I think many people are starting to feel a hankering for spring and some semblance of warmth in the outside world, and for me one of the most welcome signs of spring (besides the crocuses) will be new ferns. When new fern leaves emerge, they are typically rolled up in these familiar fiddleheads, also called croziers, which slowly unfurl to reveal the delicate architecture of the fronds.

This unfurling process of the tightly-curled leaf is called circinate vernation. Circinate refers to the circular or ring-shape of the fiddlehead, and vernation comes from 'vernal' (meaning 'spring'), when new leaves tend to emerge. By remaining tightly rolled up, the growing tip of the leaf is protected during emergence, and this protection is often supplemented by thick hairs or scales that cover the fiddlehead, as you can see in some of these photos. You can also see that the lateral leaves are in fiddlehead-form too, just as the tip is.