Researchers studying medicinal, pharmacological, antibiotic, carcinogenic and food-production agents would do well to look at an often-overlooked group of fungi that once had -- but then lost -- the ability to form lichen symbioses, according to a National Science Foundation (NSF)-funded study published last week in the journal Nature.
"The research challenges the common notion that mutualistic symbioses are endpoints of evolution," says James Rodman, program director in NSF's division of environmental biology.
Using DNA sequence data, the scientists have determined a more accurate fungus family tree and reconstructed the evolution of lichen symbiosis. As a result, a little-understood group of lichen-forming fungi is now recognized as more important to humans than previously thought. 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 did but later lost the ability.
Lichens 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 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, lichen symbiosis has played a larger-than-expected role in the evolution of fungi.
"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, assistant curator of botany at The Field Museum in Chicago 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. It determined for the first time that several groups of non-lichen-forming fungi, including species with both beneficial and detrimental properties to humans, are derived from lichen ancestors.
The authors speculate about 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, co-author of the paper. Pagel is affiliated with the School of Animal and Microbial Sciences of the University of Reading (UK).
The research concludes that loss of the ability to form a lichen outstripped gains by three to two, at least for the genus Ascomycota, which includes 98 percent of known lichenized fungal species.
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