By Seth Price of Binghamton University
Formally described by none other than the famous British naturalist Charles Darwin, Darwin’s Fungus (Cyttaria darwinii) is a real-life illustration of the complex ecological forces that can shape a species. In Spanish, the fungi is also referred to as Pan del Indio (Indian’s Bread), largely because of the robust, circular shape of its fruiting body -the structure that can be seen by the naked eye- resembling the shape of a loaf of bread. When fully mature, the fungus can often be spotted quite easily due to its bright orange color, its large cluster pattern of growth, and its golf ball-like shape, size, and texture (Figure 1).
The habitat of Darwin’s Fungus also contributes to the ease at which it can be spotted, as the fungus grows exclusively on tree species within the genus Nothofagus, or southern beech. However, to understand the geographic distribution of Darwin’s Fungus, a connection to the geographic range of the southern beech forest habitat must be taken with a grain of salt. Nothofagus forests primarily exist in two different regions of the world: the Patagonian region of Chile and Argentina, and Australasia. Yet, Darwin’s Fungus only exists in the southern beech forests of Patagonia (Figure 1), as recent studies have proven that Darwin Fungus look-alikes in Australasia are of different species of fungus. With strikingly similar genetics, it is thought that the two different fungi split from the same species when the two continents separated from one another 180 million years ago. As a result, Darwin’s Fungus contains one of the most fascinating biogeographical backgrounds, and is intensely studied to this day.
Often confused with the process of photosynthesis in plants, Darwin’s Fungus and all fungus exhibit rather unique “diets”. As mentioned above, Cyttaria darwinii’s colonization of Nothofagus tree species is obligatory to a large majority of the fungus’s life functions, including nutrient acquisition. All fungi are heterotrophic, meaning they cannot self-produce the nutrients required for life (autotrophic), but alternatively they must derive these nutrients from other sources. Darwin’s Fungus in specific is biotrophic, meaning it obtains nutrients through the parasitism of a host, but requires the host to be alive in order for it to survive as well. Therefore, Darwin’s Fungus weakly parasitizes Nothofagus trees and “steals” the autotrophic tree’s nutrients. To do so, rather than absorbing nutrients from a compound called lignin found in the woody cells that provide the structure of trees- a very common form of biotrophic behavior in fungi- Darwin’s Fungus actually absorbs sugary solutions produced by photosynthesis. To combat nutrient “hijacking” by the fungus, the Nothofagus trees often develop big warts surrounding Cyttaria darwinii to prevent the fungus from accessing the channels where these solutions flow. Such is the reason why trees parasitized by Darwin’s Fungus often look like they are exploding, and the fungus itself is the flames!
Going along with its unique feeding behavior, the reproduction method of Darwin’s Fungus is also a more unusual feat of evolution. In general, as a member of the fungus kingdom, Darwin’s Fungi produces sexual spores that serve as the basic unit of reproduction. Going further, Darwin’s Fungus is located within a phylum of the fungus kingdom known as the ascomycetes. As an ascomycete, Darwin’s Fungus produces a rather unique structure that produces sexual spores. For Darwin’s Fungus, this structure, called the hymenium, is represented by the great number of holes or tubes that penetrate the outside wall of the fruiting body. Now why is the spore-producing layer on the outside of the fungus? Well, when the individual reaches sexual maturity, this layer produces tiny sacs, called asci, which contain the reproductive spores, called ascospores. To spread these spores, the fungi rely on wind to discharge the sacs from the hymenium and carry them to a new location for the spores to then develop into another individual. For this reason, we can now understand why it’s beneficial for Darwin’s Fungi to be located on the branch of a tree. Due to its higher elevation, it provides a great launching spot for spores to fall on the branches of other surrounding trees.
Hidden among the Nothofagus forests so popularly known and associated with Patagonia, Cyttaria darwinii is a true symbol of the ecological beauty of one the wildest regions on earth. For as long as humans have occupied southern Chile and Argentina, we as a species have lived in close proximity and relationship to the tiny fungus. Prized as a natural edible, Darwin’s Fungus was a principal element in the cuisine of the native tribes of Patagonia. Today, Darwin’s Fungus still remains in strong abundance, as sightings were very common on our trips into Nothofagus forests with Round River Conservation Studies. However, the intimate relationship between these forests and Cyttaria darwinii could pose a threat to the species, as a loss of its food source would surely mean the loss of the parasite. With threats to southern beech forests through logging in recent decades, we must work to ensure the conservation of this tiny, yet incredibly complex piece of natural beauty.