Homework: Fungi
Published: April 16, 1993
They may seem awfully vegetative in their habits, and the university researchers who study them
may often be counted as members of the botany department, but fungi are turning out to be far
more closely related to animals than to plants, scientists say.
In a new analysis of genetic relationships among organisms with complex cells, including
sponges, protozoa, algae, plants and animals, researchers have concluded that animals and fungi
share a common evolutionary history and that their limb of the genealogical tree branched away
from plants perhaps 1.1 billion years ago. Fungi and animals then went their own way some
undetermined time after that.
The new findings, which appear today in the journal Science, suggest that the common ancestor
of animals and fungi was a so-called protist, a single-celled creature that very likely possessed
both animal and fungal characteristics -- perhaps spending part of its early life cycle in a
membranous and mobile form resembling a human sperm, and at a different stage growing a stiff
cell wall similar to that seen in today's fungi. Evolution Study by Genes.
The new report did not look at fossil data or physical traits of organisms, as more traditional
taxonomic studies have done, but rather took the currently popular approach of studying
evolution by examining genes. Through analyzing the same genes in many different species and
tracking how many mutational changes have occurred in the genes from one organism to the next,
scientists are able to calculate kinships based on complex mathematical models rather than on an
eyeball appraisal of how species look.
In this case, the reckoning overturned previous evolutionary trees that for any number of
anthropocentric reasons, had placed the kingdoms of fungi and animals very far apart.
The discovery was greeted with enthusiasm by many mycologists, the specialists who study fungi
and who long have felt their field has been ignored in favor of animal science. New Status for
Mycology
"I think this is very interesting, and it's quite gratifying," said Dr. John W. Taylor of the
University of California at Berkeley. "Perhaps it's time for us to move out of the botany
department and into the zoology department."
Or better yet, he said, to promote mycology to a status worthy of its own department. Dr. Taylor
is a mycologist, but his title is professor of plant biology. And he admits that convincing people
that they have anything in common with a mushroom, a packet of baker's yeast, or a bloom of
mold on an fetid piece of cheese will take some doing.
Researchers said, however, that the evolutionary affinity between animals and fungi could explain
why fungal diseases in humans are so difficult to treat. "A lot of the metabolism is so similar that
you can't target a fungus sufficiently without gravely affecting the human host as well," said Dr.
Mitchell L. Sogin of the Center for Molecular Evolution at the Marine Biological Laboratory in
Woods Hole, Mass., the main author of the new report.
Fungal diseases are a particularly severe problem in those with suppressed immune systems,
including AIDS patients and people who have had organ transplants.
Dr. Sogin also suggested that the new results will buttress long-standing arguments among yeast
geneticists that fungal cells offer a wonderfully tractable way of looking at essential biological
problems of relevance to humans.
"If you're looking for precursors to the nervous system, you might consider looking for them in
fungi," he said.
Citing another example of how fungal cells can yield benefits for human studies, Dr. Gerald Fink,
director of the Whitehead Institute for Biomedical Research in Cambridge, Mass., and a yeast
geneticist, pointed out that there is a mutation in yeast cells much like the one that gives rise to
Lou Gehrig's disease, a fatal neurodegenerative disorder. The new discovery of kinship, he said,
"just shows once again that yeast is a perfect model for man," adding, "I feel vindicated."
Agreement and Discord
Another report by scientists at Indiana University in Bloomington, now under consideration for
publication by a major scientific journal, also reaches the conclusion that fungi and animals are
closely related.
But molecular geneticists working in the laboratory of the late Allan Wilson of the University of
California at Berkeley, who before his death in 1991 was a celebrated proponent of looking at
genes for clues to evolution, said their analysis of the molecular data contradicts the latest report.
They said their study places animals and plants together in a group, with fungi having branched
off from the tree earlier.
"Mitch Sogin's results aren't significant and he's over-interpreting his data," said Arend Sidow, a
graduate student working at Berkeley on molecular evolution. But Mr. Sidow's results have yet to
be published and thus scientists said it was impossible to judge the merits of his claim.
In the new study, scientists considered the genes that produce so-called ribosomal RNA, a
component of the protein-making factories found in all living cells. They compared the genes
from dozens of species of eukaryotes, a group of organisms with complex cellular structure that
includes everything from yeast to plants to mammals. Many of the eukaryotes the scientists
considered were obscure creatures like choanoflagellates, microscopic beings surrounded by
tentacle structures, but they did include in their analysis such familiar species as frogs and
jellyfish.
Through comparing changes that occurred in the ribosomal genes, the scientists were able to
piece together an evolutionary tree for the vast group of eukaryotic organisms.
Questions
1. What was the common ancestor of animals and fungi?
2. Why do you think researchers did not use fossil data?
3. Why are fungal diseases so difficult to treat in humans?
4. Who is most susceptible to fungal diseases in humans?
5. Cite the example of how yeast are beneficial in human studies.
6. What is the basis for establishing the evolutionary relationships between fungi
and animals?