Monday, April 30, 2012

Essence of a term paper (deconstructed).

In honor of final exams week.  

Disclaimer: Hypotheses in this essay are mostly conjecture and not vetted by the scientific community. Please use the original sources (in parenthetical citations) in your own research and contact the blog author with further question.

On a recent visit to the North Carolina Aquarium’s nature trail at Pine Knoll Shores on the outer banks, I fell under the thrall of an astonishingly vibrant lichen and after snapping an embarrassing number of pictures immediately went in search of the name of this beauty. It did not take much research to learn that this lichen was Pyrenula cruenta, since, as Harris (1987)puts it, “Homo lichenologicus, like much of the animal kingdom, is attracted to bright colors.” However, despite the ease with which I discovered its identity, there was not much more to be found about the habits and relations of this rather conspicuous lichen.

A lichen is a fungus that grows symbiotically with an algae or cyanobacteria (see previous post), but most people know of them as “that grey thing growing on that tree over there… wait, maybe that’s moss”. Those who paid attention in high school biology might remember the crucial role lichens played in the evolution of dark-colored moths in Europe during the industrial revolution (link to Heidi’s Blog). But, for the most part, lichens are not exactly well known- even the red ones.

Pyrenula cruenta and Pyrenula cruentata are bright red because they produce anthraquinone pigments (Brodo et al. 2001). Although these are the only two red Pyrenula lichens, lichens in this group often go by such appealing names as ‘rash lichens’ or ‘pox lichens’ because they grow inside the top layer of a tree’s bark, causing it to be slightly or vividly discolored with brown or black pimples.

Aptroot (2012) has recently published a worldwide key that can be used to identify Pyrenula lichens- all you need is a microscope, a few chemicals, and years of training as a lichenologist. After sorting through 745 potential names, he pared the group down to 169 species, most of which are found in the tropics. There are however, a handful of Pyrenula lichens that seem to avoid tropical areas; for example, Pyrenula pseudobufonia lives in eastern North America and East Asia and Pyrenula nitida lives in Europe. My question was, why are most Pyrenula lichens spread across the tropics, while some are only found in temperate areas? I didn’t find an answer, but I did find a hypothesis.

The clues to a hypothesis come from who’s related to whom and where these relatives live. Unfortunately, no one has worked out the Pyrenula family tree using genetic data, but those scientists who have studied lichens intensively are able to propose which species may be closely related based on physical characteristics. For example, Harris (1989) believes that Pyrenula cruenta and Pyrenula cruetata are each other’s closest relative. Not only are they both red, but the only other character that differentiates them is how the cells in their spores are arranged (it is tempting to conjecture that this trait is under simple genetic control, i.e. easily mutated). Furthermore, Pyrenula cruentata only lives on the southern tip of Florida and in the northwest Caribbean, a small subset of Pyrenula cruenta’s range, meaning that it may have recently evolved from a Pyrenula cruenta living in this area.

But, back to the main question- where did the temperate Pyrenula lichens come from? Both common and rare species of Pyrenula tend to be widely distributed across the tropics (Aptroot 2012) and in North America, the number of Pyrenula species is highest in south Florida and declines northward (Harris 1989). This suggests that Pyrenula originated in the tropics, perhaps around the Tethys Ocean, which wrapped around the equator during the Cretaceous, as has been suggested for other lichens distributed across the tropics (Galloway 1991). However, Harris (1989) has hypothesized that the Pyrenulaceae family of lichens, of which the Pyrenula are members, first evolved in the northern hemisphere because some of the oldest members of this family occur in the north, rather than the tropics.   

So, here’s the hypothesis: Pyrenula lichens first evolved and diversified during the Cretaceous when the world was tropical and the extensive coastlines around the Tethys Ocean created widely dispersed maritime environments that these lichens seem to prefer. When the climate later cooled, only a few of these lichens were able to adapt and remain in the northern temperate regions.

One way to test this hypothesis would be to use genetic data to construct a phylogenetic tree of who’s more closely related to whom. A tropical origin for Pyrenula would be supported if the Pyrenula lichens that live in temperate areas are younger species that have recently shared a common ancestor with a tropical species.

The relatives of Pyrenula psuedobufonia provide some evidence that this hypothesis might be true. Harris (1987) believes that Pyrneula pseudobufonia is closely related to Pyrenula fetivica, Pyrenula nitidula[1], and Pyrenula cocoes, all of which have pantropical distributions (i.e. live all across the tropics). The existence of many other tropical Pyrenula lichens that are more distantly related means that this group probably has a tropical origin and that Pyrenula pseudobufonia has recently evolved to be able to live in the temperate zone. Pyrenula pseudobufonia’s closest relative is probably Pyrenula occidentalis which is found in the Pacific Northwest of the United States. But, these two species may have diverged from each other fairly recently since the only difference between them (other than where they live) is that Pyrenula occidentalis no longer produces lichexanthone, a chemical that fluoresces yellow when UV light is shined on it (in fact, this is how lichenologists tell the two species apart).

The movement of ancestral species from the tropics to the temperate zone and vice versa is a topic that biogeographers and evolutionary biologists have been getting pretty excited about in recent years. Many scientists believe that the explanation for why some parts of the world have many species whereas others have few is substantially dependent on where lineages of species originated and how they subsequently dispersed. A large tropical group like Pyrenula that includes several temperate species can be an intriguing system in which to study such patterns and how they contribute to the geography of diversity.

Works Cited

Aptroot, A. 2012. A world key to the species of Anthracothecium and Pyrenula. The Lichenologist 44:5–53. doi: 10.1017/S0024282911000624.

Brodo, M. I. M., M. S. D. Sharnoff, and S. Sharnoff. 2001. Lichens of North America1st Printing. Yale University Press.

Galloway, D. J. 1991. Biogeographical relationships of Pacific tropical lichen floras. in D. J. Galloway, editor. Tropical Lichens: their Systematics, Conservation, and Ecology. Clarendon Press, Oxford.

Harris, R. C. 1987. Some distinctive tropical pyrenolichens in the southeastern United States. Evansia 4:28–30.

Harris, R. C. 1989. A sketch of the family Pyrenulaceae (Melanommatales) in eastern North America. Memoirs of the New York Botanical Garden 49:74–107.

[1] Harris (1987) used the names Pyrenula citriformis and Pyrenula plittii, which Aptroot (2012) synonymized with Pyrenula fetivica and Pyrenula nitidula.