Jay Wile
[Originally published as Lichen Hid This Secret From Scientists for More Than 140 Years!]
For more than 140 years, scientists have taught that lichens are the result of a relationship between a fungus and an alga (singular of algae). The fungus gives the lichen most of its visible characteristics and provides a protected place for the alga to grow. In exchange, the alga does photosynthesis and shares what it makes with the fungus. In other words, the fungus provides housing for the alga, and the alga provides food for the fungus. This is a form of symbiosis, in which organisms of different species exist in a long-term relationship. Since both organisms benefit in this symbiosis, it is called a mutualistic symbiosis, one of the most fascinating aspects of the biological world (see here, here, here, here, here, here, and here, for example).
Despite the fact that lichens have been studied for more than 140 years, there has always been one nagging mystery: The relationship cannot be recreated in a lab. Lichens can be found in all sorts of ecosystems, but no matter what you do with the fungus and the alga, you cannot get them to form the same relationship in a laboratory setting. A recent study might explain why. The authors of the study analyzed two different species of lichen, Bryoria fremontii and Bryoria tortuosa. They are easily distinguished from each other, since the first is dark brown, while the second is yellow. However, recent studies have indicated that the fungus and alga in each are the same. How is it that two lichens can be so different when their fungus and alga are the same? That’s what the authors wanted to find out.
They decided to look at the specific genes that were actually being used by the two species. After all, even if both lichens have the same fungus DNA and the same alga DNA, it’s possible that one lichen uses one set of genes more than the other lichen, and perhaps that could explain the differences between them. However, their initial analysis indicated that both lichens used essentially the same set of genes. That’s when they decided to think “outside the box.”
When doing a study like this, you have to decide what gene products you are looking for. They had limited themselves to the genes found in the fungus and the alga that were known to exist in the lichens. They decided to change their analysis to include all known fungus genes. When they did that, they found that genes from an entirely different fungus were also being used by the lichens! That fungus is a type of yeast (specifically from genus Cyphobasidium), which is very different from the fungus that was already known. The authors did some very difficult microscope work and confirmed the presence of the yeast in the lichen. In addition, they did the same genetic tests on many different species of lichen, and they found the yeast genes in the vast majority of the lichens that they studied. As a result, the authors suggest that the vast majority of lichens are made up of at least three different species. Here is how they conclude their paper:
The assumption that stratified lichens are constructed by a single fungus with differentiated cell types is so central to the definition of the lichen symbiosis that it has been codified into lichen nomenclature. This definition has brought order to the field but may also have constrained it by forcing untested assumptions about the true nature of the symbiosis. We suggest that the discovery of Cyphobasidium yeasts should change expectations about the potential diversity and ubiquity of organisms involved in one of the oldest known and most recognizable symbioses in science.
While this discovery in and of itself is remarkable, it is also an excellent illustration of how assumptions can put blinders on science. Why haven’t these yeasts been discovered in more than 140 years of lichen study? Partly, because they are well-hidden. To confirm the presence of the yeast in the lichen required some rather detailed microscopic analysis. In addition, when you are doing genetic analysis, you have to decide what to search for, which means your results will be limited by that decision. However, here’s the main reason: No one was looking for them. Since the assumption that lichens are mutualistic symbioses between two different species was so ingrained in biological thought, no one ever considered looking for a third, until these authors decided to “think outside the box.”
I wonder how many more scientific discoveries are waiting on other scientists who are willing question old paradigms and look for things that no one else has been looking for!
