A Fern With Flower Genes - An Odd Case of Horizontal Gene Transfer

When researchers at Harvard decided to take a look at the genome of the rattlesnake fern (Botrypus virginianum) they found something completely unexpected. Whereas one set of genes they looked at placed this species firmly in the family to which it belongs, Ophioglossaceae, two other genes placed it in the Loranthaceae, a completely unrelated family of flowering plants. What are flowering plant genes doing in a fern?

The rattlesnake fern is a ubiquitous species found throughout the northern hemisphere. It is believed to have evolved in Asia and then radiated outward using ancient land bridges that once connected the continents. At some point before this radiation occurred, the rattlesnake fern picked up some genes that were entirely foreign.

Horizontal gene transfer, the transfer of genes from one organism to another without reproduction, is nothing new in nature. Bacteria do it all the time. Even plants dabble in it every now and then. The surprising thing about this recently documented case is that it is the first discovery of horizontal gene transfer between an angiosperm and a fern. Up until this point, examples within the plant realm have been seen between ferns and hornworts as well as some parasitic plants and their hosts.

This is why the rattlesnake fern genome is so interesting. How did this occur? Though there is no way of telling for sure, researchers believe that one of two things could have happened. The first involves root parasitism. The family Loranthaceae is home to the mistletoes, a group of plants most famous for their parasitic nature. Although the majority of mistletoe species are stem parasites, at least three genera utilize root parasitism. It could be that an ancient species of mistletoe transferred some genes while parasitizing a rattlesnake fern.

This scenario seems to be the least likely of the two as no representatives of the root parasitic mistletoes currently exist in Asia, though it is entirely possible that some did at one time. The other possibility doesn't involve parasitism at all but rather fungi. Rattlesnake ferns are obligate mycotrophs and thus cannot live without certain species of mycorrhizal fungi. Perhaps the transfer of genes was achieved indirectly via a shared mycorrhizal network. This hypothesis is especially tantalizing because if it is found to be true, it would help explain many other examples of horizontal gene transfer that currently lack a mechanism. Only time and more research will tell.

Photo Credit: Aaron Carlson (http://bit.ly/1OAVhNZ)

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In my opinion at least, the smaller a plant is, the more character it has. Wall rue (Asplenium ruta-muraria) is a wonderful demonstration of this. The genus of ferns to which it belongs, "Asplenium," is rather large, containing somewhere along the lines of 700 species worldwide. 

Wall rue can be found growing both in North America and Europe. Its distribution is a reminder of the great land bridges that once connected the continents back when ocean levels were much lower than they are today. The specific epithet "ruta-muraria" roughly translates to "bitter herb of walls." Along with its common name, these seem to hint at where this tiny fern likes to grow. Indeed, at least in Europe, this is a fern of stone walls, growing among the myriad cracks and crevices where microclimates are favorable for its spores to germinate. 

In North America, however, wall rue seems to be a bit more picky. Wall rue is a calciphile meaning it can really only be found in abundance on natural limestone outcroppings. As a result, it is considered a threatened or endangered species throughout most of the continent. The aspect of its habitat I find most interesting is that the limestone it relies upon is the result of an ancient sea that covered parts of North America during the Silurian Period some 443.8–419.2 million years ago. Along with plenty other plants in these habitats, if it were not for the solidified remains of ancient marine organisms, these extant plant lineages would not be here, at least not in the way in which we know them. 

Another interesting aspect of wall rue biology is that this little fern is helping paleontologists in Europe discover potential glacial refugia - ice free areas where plants and animals were able to survive during the height of glaciation. Refugia were likely epicenters of biodiversity, which expanded to recolonize the continents once the ice sheets receded. 

Wall rue, as well as other rock ferns in the genus Asplenium occur in two forms in nature - a diploid form with two sets of chromosomes and a polyploid form containing multiple sets of chromosomes. Polyploids arise from mutated diploids and can be found growing over a wider range than their more restricted diploid parents. By studying the relatedness of different diploid populations, researchers are able to deduce where some glacial refugia may have been located. In this way, these tiny little ferns are offering a rare but clear window into the Earth's long gone past. 

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Growing Ferns

I am finally having some success intentionally growing ferns from spores. I collected and sowed spores from some interrupted ferns (Osmunda claytoniana) over the summer. They have been hanging out as gametophytes for months now and some are finally starting to grow sporophytes. Here is how it worked for me:

I kept my eye on a batch of adult plants this summer. Once their fertile fronds developed I would flick them every now and then to see if they were releasing spores. Once I saw that they were I shook the fronds over some paper to collect the spores. I then took some old potting soil and sterilized it with boiling distilled water. I use old takeout containers because they are small and have clear lids that form a seal which keeps the humidity high.

Once the soil was cool I sprinkled the spores over it and then placed it on a shelf where it gets a small amount of ambient light every day. The rest they did themselves. You just have to remember to check on them and keep the humidity quite high because they can dry out really fast. They seemed stuck as gametophytes for months. I just noticed the start of these sporophytes the other day.