Sometimes I wonder how I must look to casual hikers. There I was sprawled out next to the trail, focusing all of my attention on a nondescript patch of leaves poking up where the trail ended and the grass began. This wasn't just any sort of leaf though. The object of my attention was an ancient member of the fern lineage commonly referred to as an adder's tongue. I will gladly look like a weirdo if it means spending time in the presence of such a cool plant.
To be more specific, the species in question here is the southern adder's tongue (Ophioglossum pychnostichum). Though not overtly showy like its more derived cousins, this little fern is nonetheless quite the show stopper if you know what you're looking for. It is generally considered a grassland associate and is most often encountered growing alongside trails. I'm not sure if this has to do with some disturbance related factor or the fact that even modestly sized plants can overshadow it.
Regardless, I felt very fortunate to be in the presence of at least one reproductive individual. For much of its life, the southern adder's tongue exists as a gametophyte followed by an underground fleshy rhizome. It can exist in this state for years, being nourished solely by an obligate association with mycorrhizal fungi. When a certain energy threshold is reached, individuals will then produce a single, sterile leaf. This can go on for season after season as the fern slowly stores away nutrients. When enough energy has been stored, mature individuals can then produce a spore bearing structure called a "sporophyll."
Despite its common name, this particular species distributed throughout the Northern Hemisphere. It can be found growing in North America, Europe, and temperate Asia. Still, since it is such a nondescript little plant, it rarely gets the attention it deserves when it comes to conservation. It is of conservation concern in at least a handful of states. Because its lifecycle can be hard to predict, growing some years and not others, accurate estimates of population size and health can be difficult.
The family to which is belongs is quite interesting on a genetic level as well. Ophioglossaceae is known for having staggeringly large chromosome counts. One species in particular - Ophioglossum reticulatum - boasts a whopping set of 1260 chromosomes. To put that into perspective, we humans only have 46. I guess thats what can happen to a genome that has had millions upon millions of years of natural selection working upon it.
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)