Some of Sydney’s koalas are facing an uncompromising dilemma.
The koalas living in one corner of Australia’s largest city are perilously inbred, researchers report February 26 in Conservation Genetics. But the solution — interbreeding with neighboring koala populations — risks introducing the koalas to a deadly sexually transmitted disease.
In 2021 and 2022, University of Sydney conservation biologist Carolyn Hogg and her colleagues determined that koalas in the southwestern Sydney metro area had the lowest genetic diversity anywhere in the state of New South Wales. To investigate further, the researchers caught 111 wild koalas (Phascolarctos cinereus) from seven sites across the forested region south of Sydney and collected ear tissue before releasing them.
DNA analysis revealed that some neighborhoods — including Campbelltown, Heathcote and Liverpool — had koala populations with high genetic similarity, on average resembling that of half-siblings. Other nearby populations showed interrelatedness akin to first cousins or first cousins once removed.
The heavy inbreeding may stem from the isolation of these koalas, Hogg says, as their forested habitat is bound by urban areas to the north, east and west.
That same isolation may be why these sites are also among the few neighborhoods in New South Wales without reported infections of the bacteria Chlamydia. The sexually transmitted pathogen has decimated koala populations across Australia, leading to infertility, blindness and death. In at least one Queensland population, chlamydia accounted for 18 percent of koala deaths between 2013 and 2017, and some populations have a chlamydia prevalence of around 100 percent.
Southwestern Sydney is one of the few chlamydia-free areas in New South Wales. But its isolation also makes its koalas evolutionarily vulnerable, with less variation in their genetic tool kit for adapting to new threats, including bacteria like chlamydia. And the marsupials’ resilience may be tested sooner rather than later. Hogg and her colleagues’ analyses revealed substantial crossbreeding between koalas farther south and southwest in Wollondilly and the chlamydia-free site of Campbelltown.
“The likelihood that the disease will eventually arrive in the disease-free area is quite high,” Hogg says.
If the koalas are mating, they’re swapping not only genes but also bacteria. Such crossbreeding might increase genetic diversity, but it could cause a potentially catastrophic outbreak of chlamydia.
“Usually the quickest way to inject new diversity is by bringing in individuals from another population or structuring environments in a way to ensure that different populations are connected and don’t become geographically isolated — like building corridors,” says Chloé Schmidt, an evolutionary ecologist at Dalhousie University in Halifax, Canada. “But here, corridors wouldn’t be a great idea!”
Rebecca Taylor, a conservation genomicist with Environment and Climate Change Canada in Ottawa, says an alternative solution would be selectively moving uninfected animals from other populations into the chlamydia-free areas to safely boost genetic diversity.
“However, this approach would be very intensive and costly and would need to be repeated every couple of years,” Taylor says.
The study researchers see this as something of an intractable catch-22.
“The population is large, so introducing new genetic diversity and having it taken up into the population is complex and difficult to achieve,” says Hogg, adding that being pinched between urban isolation and disease threats is not unique to koalas. “This conundrum is found all over the world where urban areas are growing.”
The researchers suggest that the findings show how important it is to keep koala populations interconnected from the get-go. Habitat loss and human-made barriers can fragment populations into inbred enclaves that are far more susceptible to disease and other threats.
Hogg and her colleagues plan to continue monitoring the genetics of these and other koala populations to understand disease patterns and their relationship to koala immune system genes.
“We need to understand the genetic drivers that make individuals susceptible to disease so we can manage the species in the long term and ensure they have enough genetic diversity to be able to adapt to a changing world.”