Wednesday, August 08, 2007

Genetics sheds light on frog fungus.

Genes shine light on mystery frog fungus

By ABC Science Online's Dani Cooper

Posted Tue Aug 7, 2007 6:31pm AEST

Scientists have uncovered genetic markers of a deadly fungus that is wiping out frog populations worldwide.

Researchers will now use them to pinpoint where on the globe the killer micro-organism originated.

Lead researcher Dr Jess Morgan, an Australian scientist from the Queensland Department of Primary Industries, says evidence has emerged that the frog-killing fungus Batrachochytrium dendrobatidis reproduces sexually and may be creating resistant spores, which can survive for a decade.

The international research findings, published in the journal Proceedings of the National Academy of Sciences, suggests the pathogen will be harder to eliminate.

Dr Morgan, who was a post-doctoral fellow at the University of California, Berkeley at the time of the study, says little is known about the fungus.

She says it was only identified in 1998 after a wave of frog population extinctions worldwide from chytridiomycosis, a disease caused by the fungus.

Scientists believe the fungus kills by attacking the frog's ability to absorb water through its skin, causing it to dehydrate to death.

But they still do not know exactly how the pathogen has spread around the globe.

In the paper, Dr Morgan says the team used genetic analysis of a well-studied population of mountain yellow-leg frogs in California's Sierra Nevada to determine whether the fungus was endemic or had been recently introduced.

Dr Morgan says of six sites studied, four were dominated by a single genetic make-up or genotype, suggesting the fungus had been recently introduced and spread through clonal reproduction.

But she says at two sites evidence of recombination was found with multiple genotypes present.

This indicates for the first time that the fungus reproduces sexually and may be producing resistant spores.

Dr Morgan says the presence of resistant spores helps explain the global spread of the disease and means the fungus can survive for long periods in areas where the frog population has been vastly reduced.

But it also means any attempts to reintroduce frog populations at sites of local extinction are likely to fail as the spores will re-infect the frogs.


Dr Morgan says of 10 attempts at reintroducing frogs in the Sierra Nevada during the past four years, seven have failed and three are ongoing.

She says resistant spores help spread the fungus as they are easily transported in dirt on tyres and shoes, and can hitchhike on birds and other wildlife.

Dr Morgan says during the study researchers isolated 15 marker genes for the fungus, which will now be used in a worldwide hunt to track the geographic origin of the killer fungus.

"The next thing in terms of genetics is to find out where this is coming from," she said.

"The area which is most likely the origin will not be suffering a decline in frog population. We are looking for a healthy population of frogs.

"If we can look at the frogs and find out how they are living with the disease then maybe we can [help] our frogs."

Dr Morgan says the study also found some frogs within one species are resistant to the disease and could survive a mass mortality.

"It could be the frogs and the fungus are evolving to be able to live together," she said.

But she says more research is needed on the factors, either physical or environmental, behind this phenomenon.

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