Extinction Countdown

Extinction Countdown

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Platypus Populations on Small Australian Islands Show Lack of Genetic Diversity, High Risk of Disease


Last year we learned that climate change could soon make Australia too hot for the cold-loving, iconic platypus (Ornithorhynchus anatinus). Now we have word of a new threat to these unique, egg-laying mammals: inbreeding, which has put the platypuses living on two small Australian islands at enhanced risk of disease.

According to research published March 28 in Ecology and Evolution and May 4 in the Journal of Heredity, the platypus populations on mainland Australia and its island state Tasmania have perfectly normal levels of genetic diversity. But populations on two nearby islands aren't in as good shape. University of Sydney doctoral student Mette Lillie, lead author of the Journal of Heredity paper, says platypuses on the 1,100-square-kilometer King Island—located off the coast of Tasmania and separated genetically from the mainland by the last ice age 14,000 years ago—now show no diversity in their major histocompatibility complex (MHC) gene, putting them at high risk of an epidemic disease. "If you have lots of variation in the MHC gene, it means the population is better able to resist disease and pathogens," she told the Australian Broadcasting Corp (ABC). Lillie was one of the researchers who sequenced the platypus genome in 2010.

The Ecology and Evolution paper, by researchers from the University of Melbourne and other organizations, said the King Island population shows one of the lowest levels of genetic diversity ever found in any vertebrate population.

Platypuses on the 4,400-square-kilometer Kangaroo Island, located off the state of South Australia, aren't doing much better. But that population was introduced there by humans in the 1930s and '40s, and Lillie says the lack of genetic diversity is to be expected from such a small population with no influx of new breeding stock.

Lillie told ABC Radio's PM news program that genetic diversity problems can often be solved by introducing animals from a different population, but that would be too risky on King Island because any newcomers to the island could bring a new disease for which the existing population would be unprepared.

The most serious disease afflicting platypuses today is ulcerative mucormycosis, a fungal disease affecting the species on Tasmania. The disease—much like the devil facial tumor disease that affects Tasmanian devils—causes nasty, ulcerated lesions on animals, which can prove fatal when the wounds become infected. Mucormycosis is caused by Mucor amphibiorum, a fungus native to mainland Australia, where it does not affect the resident platypuses. The disease was first observed on Tasmania in 1982. Experts suspect it arrived on Tasmania via illegally transported green tree frogs.

Platypuses—one of five egg-laying mammal species now living in the world—were overhunted by fur traders until the early 20th century but are now protected under Australian law. They are also the sole living species in their genus and one of world's only venomous mammals.

Meanwhile, life on small islands may actually benefit another Australian species, the eastern quoll (Dasyurus viverrinus), which disappeared from mainland Australia decades ago. The marsupials can now only be found on Tasmania—where their populations have declined 50 percent in the past 10 years—and Tasmania's nearby Bruny Island (362 square kilometers), where University of Tasmania researcher Bronwyn Fancourt has found that they are flourishing. It's too early to say why Bruny has provided a safe haven, but the decline on Tasmania has been linked to feral cats. Ironically, eastern quolls are also known as "eastern native cats" even though they are not felines. The genetic diversity of the eastern quoll has not yet been studied.

Photos: Platypus by Cha222 via Flickr. Eastern quoll by Matthias Siegel via Flickr. Used under Creative Commons license

The views expressed are those of the author and are not necessarily those of Scientific American.

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