UMaine mycologist's research essential in the global race
to stem amphibian die-offs
About the Photo:
Nearly a decade ago, University of Maine mycologist Joyce Longcore
identified a particular species of chytrid, B. dendrobatidis, left,
that has been implicated in massive amphibian die-offs around the
world. The aquatic fungus was first thought to kill all amphibians
it infected. However, Longcore's study of frogs and toads throughout
the Northeast, and especially in Maine, found the "frog chytrid"
statewide in healthy amphibians. Why the chytrid kills some and not
others remains a mystery that researchers worldwide are racing to
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From cool, swirling streams in California's
High Sierra to foggy mountaintops in western Peru, frogs have begun to
disappear, wiped out by a mysterious fungus that can cause entire
populations — even entire species — to vanish.
The pandemic threat has helped to drive more than
100 species to extinction in the past 25 years, leaving researchers and
conservationists to wonder how frogs can be saved.
Some of the answers may well be found in a
refrigerator in a biological sciences laboratory at the University of
Dressed in faded jeans and a "Deep Hyphae" tee
shirt, UMaine mycologist and associate research professor Joyce Longcore
explained the fine points of fungal phylogeny with her good-natured and
characteristically direct approach. A respected researcher in a highly
field, Longcore's techniques have become required
reading for anyone wishing to study one of the world's deadliest fungi,
Patient and unassuming, Longcore has the dedication
and experience to guide the operation of a tiny, two-room lab in Deering
Hall, which could easily be described as command central for the global
battle to stop the fungus that threatens the world's frogs.
Longcore has become one of the world's leading
authorities on an unusual group of fungi casually known as the chytrids.
That fact alone might have left her to work in relative anonymity
outside of mycology circles, had it not been for the discovery of a
particular species of chytrid, B. dendrobatidis, that has been
implicated in massive amphibian die-offs around the world.
The first to isolate a pure culture of B.
dendrobatidis nearly 10 years ago, Longcore has made numerous trips to
remote locales to teach other researchers her methods for culturing the
problematic pathogen. Her laboratory refrigerator has become the
repository of numerous strains of the so-called "frog chytrid" from
around the world.
"The chytrids are different from most other types of
fungi. You have to use very particular methods to be successful because
they just don't grow well in pure culture," says Longcore, pointing out
a tiny white smear growing inside a test tube. "B. dendrobatidis is
peculiarly adapted to amphibian skin."
The affinity of the fungus for frog skin is
key to its growing lethality. From California to Australia, entire
populations of frogs, toads and other amphibians have been wiped out by
the chytrid, leaving researchers scrambling to determine how it kills
and, perhaps more importantly, how it spreads among populations.
Longcore maintains more than 100 different strains
of the frog fungus and distributes them to scientists for research
projects ranging from small field studies to full-blown genome projects.
Despite considerable attention in recent years, the enigmatic organism
has led to more questions than answers.
"We know that it grows in the skin, causing it to
thicken, and we suspect that it interferes with the movement of air and
water through the skin, but no one is exactly sure how it kills the
host. We know that it does not need an amphibian host to survive, but we
have never seen it in a resting stage," says Longcore. "There's still a
lot that we just don't know."
One of the most perplexing questions surrounding B.
dendrobatidis infections has to do with the degree to which the pathogen
affects the host. Working with her husband Jerry Longcore, a field
biologist with the U.S. Geological Survey, now retired, Longcore
determined that the same organism that had caused near 100 percent
mortality in amphibian populations elsewhere was present in frogs
throughout Maine, causing only a mild, sublethal infection.
"We looked at road-killed animals and were surprised
to see frogs that had been healthy with the frog chytrid. Until then, we
had always associated an infection with death," says Longcore. "We now
know that most species of anuran amphibians in Maine are infected, but
as far as we know there have been no die-offs attributed to the
Longcore first isolated the deadly chytrid in 1997
in response to a die-off of exotic frogs in captivity. At that time, the
Smithsonian National Zoological Park in Washington, D.C., was raising
blue poison dart frogs. But during metamorphosis, the change from
tadpole to frog, the amphibians native to South American rainforests
were dying of unknown causes. The only clues were the spherical bodies
inside their skin cells.
The search for answers led to Longcore.
"As soon as I saw the photographs, I knew (the
culprit) was a chytrid," says Longcore. "I had spent the last 10 years
isolating chytrids and growing them in pure culture. The blue poison
dart frog was the first (amphibian) from which I isolated this
particular chytrid, then we showed that it is capable of causing disease
It turns out that around the same time, scientists
in Australia found the organism decimating populations of frogs in the
wild. In addition, a researcher doing frog surveys in Central America
returned to the rainforest for a consecutive year to find the wilds
eerily quiet, with frogs dead along the streams.
The big questions have to do with what the deaths
mean environmentally and ecologically. Are frogs that are dying of
chytrids harbingers of a yet unseen shift in the ecosystem, much like a
canary in a coal mine? Or is this an invasive disease that has spread
from a different continent?
Longcore is quick to point out that she is not a
frog biologist. Her focus is on the fungi. Her current projects focus on
the relationships between chytrid species and their place among other
fungi. She cooperatively published the first phylogeny of chytrid fungi
in 2000, and she and colleagues are currently making use of recent
advances in molecular classification methods to improve and expand on
previous work. By determining how the strange and highly mobile chytrids
are related, Longcore hopes to contribute to their reclassification and
make it possible to study their role in the ecosystem.
Constantly looking for ways to improve
chytrid culture and identification techniques, Longcore has become an
expert on the behavior of the fungi. As a result, one of her primary
roles in many research programs by other scientists is to locate and
identify chytrid species in soil and water samples. To do that, she goes
Using tiny bits of shrimp chitin, onion skin, pollen
and other baits, she attracts the zoospores of
carbohydrate-and-protein-loving chytrids, growing them so they can then
be isolated and identified. It's fishing on micrometer scale, and a
prize catch means a pure culture of a chytrid for someone's research
"A big part of what we do is hunt for different
chytrids in samples that are sent here," says Longcore "Right now, we
have samples sent from mountain tops on three different continents. With
the baiting technique, instead of looking for a needle in a haystack, we
can draw the chytrids in."
Once the individual chytrids grow on the baits,
Longcore and graduate student Rabern Simmons can begin the daunting task
of culturing the uncooperative fungi. When working with the frog chytrid,
they must maintain strict safety protocols to ensure that the cultures
don't escape into the environment.
Established cultures are kept in two refrigerators
in Longcore's lab; back-up specimens are frozen in liquid nitrogen in
case the active cultures fail. Establishing and rotating the
ever-growing collection is time-consuming work, but there is certainly
no shortage of researchers requiring samples and Longcore's expertise.
Recently, one of Longcore's cultures was used by
researchers at Duke University to isolate high-quality DNA, which was
sent to the Broad Institute, where scientists from MIT and Harvard
University are sequencing the B. dendrobatidis genome.
Longcore's frog chytrid cultures also are being used
by Cornell University researchers who are examining the role of repeats
in the organism's genetic code, and by U.S. Fish and Wildlife Service
technicians seeking to determine the risk of spreading the disease to
new bodies of water through fish stocking programs.
"We have a lot of different strains and there are a
lot of different projects. Some researchers will ask for one and some
will ask for multiple strains from different areas," Longcore says as
she checks some samples under the microscope.
"Right now we have isolates from Africa going out
for a study in population genetics. Just yesterday we sent an isolate to
Montana for some DNA research. It definitely keeps us busy."
By David Munson
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