Genetic safety in numbers for the duck-billed platypus

Another of Australia’s animal icons, the platypus, has joined the koala and the Tasmanian devil in facing the risk of being wiped out by disease.

The danger for the platypus, at least for the moment, is confined to the animals on two small islands around the south-east coast of Australia although it remains under threat from habitat destruction elsewhere.

Two recent studies of genetic variations in platypus populations, undertaken separately by researchers at Melbourne and Sydney universities, reached the same conclusion: the duck-billed monotremes on King Island in Bass Strait and Kangaroo Island off the South Australian coast have become inbred and could both be at high risk of becoming extinct.

Each of the two research groups has had the results of their work published this year in international science journals – the Sydney team in the Journal of Heredity and the Melbourne group in Ecology and Evolution.

The lead authors of both papers were young PhD students – Mette Lillie in Sydney’s faculty of veterinary science and Elise Furlan from Melbourne’s genetics department who has now graduated with her doctorate.

Lillie says platypuses on the Australian mainland and in Tasmania are in good genetic health but those on the two islands have limited diversity in key immune genes and may have lost their ability to fight off new diseases.

She says the research results have important implications not only for managing the platypus populations on the islands but also for other species with reduced genetic variation, including the koala and the Tasmanian devil.

The genetic study Lillie did was on DNA collected by other researchers from 70 platypuses caught during population surveys across eastern Australia and the two small islands. Tiny pieces of skin were taken from the animals’ toe webbing (the skin grows back quickly) and, by extracting DNA from the samples, Lillie carried out studies of their immune genes.

The focus of her platypus research was on a cluster of genes known as the major histocompatibility complex, or MHC, which plays a key role in the animal’s immune response to pathogens such as bacteria, fungi and parasites.

In Melbourne, however, Furlan used different genetic tools called microsatellite markers that are useful in inferring population histories.

Lillie says she found the platypuses on the mainland and in Tasmania had high genetic diversity in the immune regions and therefore high ‘immunological fitness’. This means appropriate resistance genes could exist in some of the animals, which would enable them to cope with unusual pathogens.

But although genetic diversity among the Tasmanian platypuses is comparable to those on the mainland, she says that on King Island there is no diversity at all. There the platypus has been isolated for more than 10,000 years, since the end of the last ice age when the seas rose and first cut Tasmania from the mainland and then King Island off from Tasmania.

“That left an isolated population on an island with limited carrying capacity and over time, inbreeding and an evolutionary process called genetic drift occurs. Genetic drift is the random change in the genetic variation in a population from generation to generation. It has been shown in many other studies to decay the original genetic diversity over long time periods.”

Lillie says that during the thousands of years, the diversity in the immune genes of the King Island platypus has decayed to a single variant now present in every one of the animals. On Kangaroo Island, the platypuses also have low diversity in their immune genes.

“But that’s likely because they were introduced in small numbers between 1928 and 1946 from Victoria and Tasmania,” she says. “The variation would be limited to how much was introduced at this time and it’s possible the population has become inbred.”

She says work by her colleagues in the veterinary science faculty had earlier shown that low immune gene diversity allowed a contagious cancer to decimate the Tasmanian devil population.

The discovery of the shallow gene pool among Kangaroo and King Island platypuses raises concerns about their long-term survival as well and these populations need to be carefully managed and monitored for signs of disease outbreaks because an introduced disease could reach epidemic proportions.