PrepTest 45, Section 4, Question 18

A lichen consists of a fungus living in symbiosis (i.e., a mutually beneficial relationship) with an alga. Although most branches of the complex evolutionary family tree of fungi have been well established, the evolutionary origins of lichen-forming fungi have been a mystery. But a new DNA study has revealed the relationship of lichen-forming fungi to several previously known branches of the fungus family tree. The study reveals that, far from being oddities, lichen-forming fungi are close relatives of such common fungi as brewer's yeast, morel mushrooms, and the fungus that causes Dutch elm disease. This accounts for the visible similarity of certain lichens to more recognizable fungi such as mushrooms.

In general, fungi present complications for the researcher. Fungi are usually parasitic or symbiotic, and researchers are often unsure whether they are examining fungal DNA or that of the associated organism. But lichen-forming fungi are especially difficult to study. They have few distinguishing characteristics of shape or structure, and they are unusually difficult to isolate from their partner algae, with which they have a particularly delicate symbiosis. In some cases the alga is wedged between layers of fungal tissue; in others, the fungus grows through the alga's cell walls in order to take nourishment, and the tissues of the two organisms are entirely enmeshed and inseparable. As a result, lichen-forming fungi have long been difficult to classify definitively within the fungus family. By default they were thus considered a separate grouping of fungi with an unknown evolutionary origin. But, using new analytical tools that allow them to isolate the DNA of fungi in parasitic or symbiotic relationships, researchers were able to establish the DNA sequence in a certain gene found in 75 species of fungi, including 10 species of lichen-forming fungi. Based on these analyses, the researchers found 5 branches on the fungus family tree to which varieties of lichen-forming fungi belong. Furthermore, the researchers stress that it is likely that as more types of lichen-forming fungi are analyzed, they will be found to belong to still more branches of the fungus family tree.

One implication of the new research is that it provides evidence to help overturn the long-standing evolutionary assumption that parasitic interactions inevitably evolve over time to a greater benignity and eventually to symbiosis so that the parasites will not destroy their hosts. The addition of lichen-forming fungi to positions along branches of the fungus family tree indicates that this assumption does not hold for fungi. Fungi both harmful and benign can now be found both early and late in fungus evolutionary history. Given the new layout of the fungus family tree resulting from the lichen study, it appears that fungi can evolve toward mutualism and then just as easily turn back again toward parasitism.

A lichen consists of a fungus living in symbiosis (i.e., a mutually beneficial relationship) with an alga. Although most branches of the complex evolutionary family tree of fungi have been well established, the evolutionary origins of lichen-forming fungi have been a mystery. But a new DNA study has revealed the relationship of lichen-forming fungi to several previously known branches of the fungus family tree. The study reveals that, far from being oddities, lichen-forming fungi are close relatives of such common fungi as brewer's yeast, morel mushrooms, and the fungus that causes Dutch elm disease. This accounts for the visible similarity of certain lichens to more recognizable fungi such as mushrooms.

In general, fungi present complications for the researcher. Fungi are usually parasitic or symbiotic, and researchers are often unsure whether they are examining fungal DNA or that of the associated organism. But lichen-forming fungi are especially difficult to study. They have few distinguishing characteristics of shape or structure, and they are unusually difficult to isolate from their partner algae, with which they have a particularly delicate symbiosis. In some cases the alga is wedged between layers of fungal tissue; in others, the fungus grows through the alga's cell walls in order to take nourishment, and the tissues of the two organisms are entirely enmeshed and inseparable. As a result, lichen-forming fungi have long been difficult to classify definitively within the fungus family. By default they were thus considered a separate grouping of fungi with an unknown evolutionary origin. But, using new analytical tools that allow them to isolate the DNA of fungi in parasitic or symbiotic relationships, researchers were able to establish the DNA sequence in a certain gene found in 75 species of fungi, including 10 species of lichen-forming fungi. Based on these analyses, the researchers found 5 branches on the fungus family tree to which varieties of lichen-forming fungi belong. Furthermore, the researchers stress that it is likely that as more types of lichen-forming fungi are analyzed, they will be found to belong to still more branches of the fungus family tree.

One implication of the new research is that it provides evidence to help overturn the long-standing evolutionary assumption that parasitic interactions inevitably evolve over time to a greater benignity and eventually to symbiosis so that the parasites will not destroy their hosts. The addition of lichen-forming fungi to positions along branches of the fungus family tree indicates that this assumption does not hold for fungi. Fungi both harmful and benign can now be found both early and late in fungus evolutionary history. Given the new layout of the fungus family tree resulting from the lichen study, it appears that fungi can evolve toward mutualism and then just as easily turn back again toward parasitism.