Important pathogens and cures belong to little-known group of fungi

Field Museum research points way to discovering other agents

CHICAGO – Researchers studying medicinal, pharmacological, antibiotic, carcinogenic and food-production agents would do well to study an often-overlooked group of fungi that once had – but then lost – the ability to form lichen symbioses, according to a study to be published in the journal Nature June 21, 2001.

Using DNA sequence data, the scientists have determined a more accurate fungus family tree and reconstructed the evolution of the lichen symbiosis. As a result, this little-understood group of lichen-forming fungi is now recognized as being more important to humans than previously thought.

“Phylogenies coupled with statistical methods that reconstruct ancestral states can help identify additional species with possible benefits and provide a better understanding of fungi that are detrimental to humans,” says François Lutzoni, PhD, assistant curator of botany at the Field Museum and lead author of the study. “This is one of many reasons why reconstructing the complete tree of life is so compelling and should be one of the main scientific endeavors of the new millennium.”

This study demonstrates that lichen-forming fungi are especially important to humans by determining for the first time that several groups of non-lichen-forming fungi known to include species with beneficial and detrimental properties to humans are derived from lichen ancestors. For example, the study determines that Penicillium, from which penicillin was derived, belongs to one of these non-lichen-forming fungal lineages (Eurotiales) that originated following the loss of lichenization.

Another example is Aspergillus flavus and A. parasiticus, also members of the Eurotiales. They produce aflatoxins on various nuts and grains (including peanuts and corn) that are among the most potent carcinogenic compounds known.

Other members of the Eurotiales are used to ripen bleu cheese and are pathogens of citrus fruits. Still others are infective agents of animal and human diseases, such as aspergilloses caused by the secondarily derived Aspergillus.

The Herpotrichiellaceae (Chaetothyriales) is another example of a major fungal lineage derived from a lichenized ancestor that gave rise to opportunistic pathogens to humans. Fonsecaea pedrosoi is one of the causative agents of chromoblastomycosis (a skin disease), and certain species of Exophiala are pathogens of fish, including salmon.

Clues for further study

Better insight into animal and human diseases caused by fungi may be derived from studying the distinction between fungi that never formed lichen symbioses during their evolution and those that had but later lost the ability to form lichens.

The authors go on to speculate why other important fungal species might be found within this secondarily derived non-lichen-forming category of fungi. “When a fungus loses its ability to form a lichen symbiosis, some of the genes involved in that process may be diverted to new functions that inadvertently offer possible benefits or detriments to humans,” says Mark Pagel, PhD, co-author and professor at the School of Animal and Microbial Sciences of the University of Reading (UK).

Lichens are obligate mutualistic symbioses between fungi (mycobiont) and green algae or cyanobacteria, or with both types of photobionts. They grow on many different substrates such as rocks, soils or trees, and in dry to aquatic habitats from the tropics to the poles. About one-fifth of all known fungi currently in existence form lichens.

Years ago, scientists thought that lichen-forming fungi were a sideshow, a small closely knit fringe group. This study demonstrates that lichen-forming fungi originated much earlier than previously thought. Therefore, the lichen symbiosis has played a larger than expected role in the evolution of fungi. One of the consequences of this early origin is that several major lineages of non-lichenized Ascomycota species unexpectedly turn out to be derived from lichen-forming ancestors.

The research concludes that losses of the ability to form a lichen outstripped gains by three to two, at least for Ascomycota species, which include 98% of known lichenized fungal species.

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