Friday, August 8, 2014

For a wider range, pick a perfect partner


Clearing out the poison ivy from my backyard last weekend, I alternated between swatting mosquitos swarming around my temptingly bare calves and worrying that the subsequent transfer of ivy juices would lead to unfortunate consequences in the not-so-distant future. Times like these I wish that mosquitos would just make like a giant panda.

For that matter, why do some species live almost anywhere, while others struggle to hold on in a little piece of habitat? Sometimes a species is distributed far and wide simply because it’s a hardy traveler that can get swept up by winds, water or waterfowl and transported across the landscape. Other times it’s because a species is good at living just about anywhere- maybe it’s a vigorous grower that sucks up resources and just keeps on multiplying, or maybe it’s just plain hardy, able to cope with a wide range of environments.

Lichens aren’t exactly known for their prodigious growth, but they can survive in many environments and pump out tiny spores that float long-distances on air currents. Yet, they’ve also got another advantage that arises from the fact that a ‘lichen’ is not one species, but many. Most of a lichen’s ‘body’ is formed by one fungal species and inside this structure live many kinds of bacteria, algae, and other fungi. The algae photosynthesizes to provide food for the lichen, but scientists aren’t yet sure what exactly the other parts do. Such mutualisms, where two (or more) species have to swap resources or services in order to survive, are often thought of as fragile. Not only do conditions have to be good for one species, they have to work for all the species that participate in the mutualism. New research on lichens by Silke Werth and Victoria Sork, published last month in the American Journal of Botany, suggests that with a little bit of flexibility mutualism could actually be the key to thriving across a wide range of environments.

Lace lichen (Ramalina menziessi) is like a filmy mesh scarf that can be found draped from tree branches up and down the coast of western North America, from the hot dry desert of Baja California to the wet rainforests of Southeast Alaska. How can it cope with such different environments? Werth and Sork uncovered a couple key clues by looking into the genetics of the alga and fungus from lace lichens living throughout this range. It turns out that both the alga and fungus have different genetic signatures depending on the region they come from, which could be because they have adapted to the different environmental conditions found in the regions, or it could just be because the lichens are separated by a wide distance and rarely get to exchange genes. By investigating a bit further, the researchers uncovered a remarkable pattern: differences among individual lichens in the genetic signatures of the algae within the lichens are related to local climate variation and even more so to the tree species that the lichen grows on. That is, the algae are more similar to one another when they come from lichens living on the same tree species in similar environments. But, this wasn’t the case for the fungus, even though the alga and fungus inhabit the same lichen body!

These genetic patterns suggest that different descendants on the alga’s family tree have adapted to different climates and to the environmental conditions found on different tree species. The fungus, on the other hand, is just along for the ride and partners with whichever alga can be found on the tree that the fungal spore lands on. Thus, specialization on the part of the alga, and flexibility on the part of the fungus, has made lace lichen a successful inhabitant of trees up and down the western coast of North America. For lace lichen, a little flexibility makes mutualism an advantage, rather than a liability.

Flexibility may be the key to conquering a broad geographic area for many organisms, not just lichens or other mutualists. For example, plants that can change how they grow based on the environmental conditions they encounter (say, growing more leaves in low light to capture more of said light) can often be found across a broader range of habitats (check out Sonia Sultan’s cool case study with ‘plasticity’ in Polygonum). Perhaps giant panda’s need a lesson in dietary flexibility, but in this case I suspect the root of the panda’s distributional woes lie in our own species’ amazing adaptability.

Your can find this article at:

Werth S. (2014). Ecological specialization in Trebouxia (Trebouxiophyceae) photobionts of Ramalina menziesii (Ramalinaceae) across six range-covering ecoregions of western North America, American Journal of Botany, 101 (7) 1127-1140. DOI: http://dx.doi.org/10.3732/ajb.1400025

And for a previous post on one of the first papers to show environmental preference in lichen-forming algae, go here.