Richard Musgrove
Sydney’s Koalas are facing a unique existential dilemma. New research from the University of Sydney reveals an isolated, chlamydia-free population, but attempts to reverse the extreme inbreeding, and limited genetic diversity, could introduce the disease and mean their demise.
South-western Sydney’s koalas are among the very few populations in New South Wales (NSW) still free of chlamydia, a highly contagious bacterial disease causing infertility and mortality across much of the marsupial’s range.
Koala chlamydia is sexually-transmitted, caught from faeces or passed between mother and young at birth or during feeding.
The disease joins another disease, koala retrovirus, as well as habitat destruction, traffic strike and dog attacks as a major cause of Koala mortality. All of which have contributed to listing of the species as ‘Endangered’ under the EPBC Act, in Queensland, NSW and Australian Capital Territory.
In a strange twist, these SW Sydney koalas are also highly inbred, with low genetic diversity, and may be less likely to adapt to the disease should it arrive, say Dr Elspeth McLennan and Professor Carolyn Hogg at the University’s School of Environmental and Life Sciences.
Isolation has offered protection, but inbreeding has weakened the population, say the researchers.
Ear tissue samples were collected from 111 koalas by NSW Government staff at 7 sites in the south-western Sydney suburbs of Liverpool, Campbelltown, Heathcote and
Wollondilly, and at Wingecarribee in the Southern Highlands.
Genetic analysis showed a high level of interrelatedness, inbreeding and a worryingly low genetic diversity across Sydney koalas, says McLennan.
“On average, koalas in the Sydney populations have cousin or half-sibling relationships,” says McLennan.
Low genetic diversity means populations can’t always adapt to change, which makes them vulnerable to environmental threats and disease outbreaks.
McLennan says koalas from neighbouring Wollondilly Shire, where chlamydia is present, may find their way to the disease-free Sydney populations.
“It’s a classic Catch-22 situation,” says McLennan. “If the Wollondilly koalas breed with those elsewhere in Sydney they could increase genetic diversity. But they may bring chlamydia with them. If the latter happens, individual koalas are unlikely to have enough genetic variation to adapt to the threat.”
“Instead of some individuals being able to naturally clear chlamydia without it progressing to blindness and infertility, it is possible all individuals will contract the infection whereby it progresses to the later stages of the disease.”
In a further twist, illustrating just how complex koala biology can be, the researchers point out it’s also plausible that these SW Sydney koalas have had previous exposure to chlamydia but carry a recently discovered genetic variant — part of the major histocompatibility complex (MHC) — that allows them to clear chlamydiosis infections faster.
So, chlamydia may not be in the population because those koalas deal with the disease more readily, says McLennan, which means these sites could be crucial for understanding how to combat the infection. Confirmation or otherwise of this possibility awaits whole genome resequencing.
At this stage, there is no ready solution to addressing the hazards to Koalas, says McLennan, including anthropogenic threats from climate change and ongoing urbanisation in SW Sydney. Simply improving habitat connectivity to increase genetic diversity may promote chlamydia spread, she adds.
“Beyond SW Sydney, the results show the importance of managing koala populations and their surrounding landscapes. We need to ensure ongoing connectivity between all koala populations to maintain their health and resilience to threats.”
The research highlights an issue faced by conservationists worldwide too. “Without diversity, endangered species risk succumbing to disease outbreaks and environmental threats.”
The findings have been published in Conservation Genetics.
