Ten years ago this week, I went on my first scientific field excursion, and the experience hooked me on biology, and especially on peat mosses. With my new blog starting up, I thought it would be an opportunity to revisit that field trip. You can read the other parts here: Part I.
Part II: Ruining socks for science
Even among botanists, it’s pretty common for people to express surprise when they learn there is more than one species of peat moss. Not only are there probably 250+ species worldwide, but in an individual peatland, there can be up to 40 species, many of them growing intimately intermixed. I now know that this interspecies coexistence is helped by an amazing ability of sub-specialization within Sphagnum: they occupy very small niches, such as one species that grows aquatically, and another that grows into tall hummocks. Thanks in part to my PhD work, we now know that these preferences are phylogenetically conserved; related species tend to have related micro-niches.
Ten years ago, my first experience with this was noticing that when Sphagnum macrophyllum and Sphagnum cuspidatum occurred together, the former species would “push” the latter up onto the edges of puddles and roadside ditches. I have no idea if this is a common occurrence: micro-niches haven’t been studied in Sphagnum in the Southeast US. Still, given what I knew at the time about speciation, it was fascinating to find two species from the same genus growing side by side, presumably without hybridizing too much But relatively speaking, Sphagnum cuspidatum and Sphagnum macrophyllum are fairly far apart on the peat moss family tree.
The same was not true of Sphagnum cribrosum, a species that didn’t “officially” exist yet when we were out on our field trip in 2005. Various botanists had argued for years whether Sphagnum cribrosum was a true species, or just a variety of Sphagnum macrophyllum. The species look identical, until you get them under a microscope, where the main character distinguishing the species are the rows of tiny pores within the cells of the leaves– one row, or two rows. As far as we can tell (mostly from collecting plants on that field trip ten years ago), the two species occupy the same habitat, in the same geographic region (though Sphagnum macrophyllum has a larger overall range). Ping, the grad student whose grant was funding the trip, later published clear evidence that plants distinguished by this character are reciprocally monophyletic– all of the “two-rows-of-pores” plants are more closely related to each other than any of them is to a “one-row-of-pores” plant.
Since nearly all that I knew in biology came from a textbook of some sort, this realization that such a character could be the difference between two species changed the way I considered evolution. These were not two species of flowering plants whose flower colors attract different pollinators, or fruit flies that distinguish potential mates with complex dances. The arrangement of pores within the cells wasn’t for something, and didn’t have to be. We don’t have to invent some story to tell about the purpose of different numbers of pores within the cell, or go searching for minute ecological differences that might show that two rows of pores gives Sphagnum cribrosum an advantage. The pores were a signal of something; presumably some set of genes controls the development of pores, and differences between Sphagnum macrophyllum and Sphagnum cribrosum have developed over the years. But because the two groups don’t mate, there’s no mixing of genes, and therefore no mixing of pore types.
No place is this separation of species made more clear than the site where they are the least separated. In the coastal plain of eastern Georgia, just north of the town of Ludowici, nestled next to “Tattoos By Drew” and not far from the county prison, is a wet roadside ditch. We first came upon this site on our sixth day of the trip (June 27, 2005), and by this point we were getting pretty good at identifying Sphagnum macrophyllum habitat. Unlike the pine savannahs I described in the previous post, this species liked to inhabit wet depressions in the landscape where pines were absent, instead replaced by a denser canopy of cypress trees (Taxodium ascendens) and and even denser understory usually full of shrubs like Hypericum and vines like Smilax (the species in the southeast US has spines about 2 inches long, and the local common name is “blasphemy vine”). Local ecologists describe this mosquito breeding ground as a “Cypress dome.” Combine that with the standing water that was usually the color and pH of cola, and tromping through these field sites was no picnic.
Despite this scene, the morale couldn’t have been higher after we exited that roadside ditch. We had found sporophytes! This meant that the plants were reproductively active at this site, and Ping could hope to study their mating patterns more intensely. It wasn’t until later that night, in a hotel somewhere in South Carolina, when the real discovery was made: not only were there sporophytes, but both Sphagnum cribrosum and Sphagnum macrophyllum were present! This was extra exciting because it meant that this site was a candidate to study not only reproduction but the possibility of hybridization between the two species, which were so closely related. Though we knew the ranges of the species overlapped, here they were growing side by side, and clearly still distinct species. How is that possible???
And so you have the glamour of field work. We had the roachy motels, the soggy pants, the mosquito bites, the torn shirts, the ruined brown socks. But through all of that we couldn’t be happier because we had made a discovery, something no one had known before. Sure, no one is giving out Nobel Prizes for discovering two Sphagnum species breeding near each other, but that’s not really what drives me as a scientist. Given a gift (as a human) to explore and understand the world around us, I just hope for more moments where I just stop in awe. And, since I’ve spent the last few years considering plants only as sequences of A, C, T, and G, I secretly hope that more of them occur while shin-deep in cola water.