Koalas and the Toxic Eucalyptus Leaves
Australia
The modern koala (Phascolarctos cinereus) is a well-known arboreal Australian marsupial and the only extant species of the family Phascolarctos. It is classified as ‘Vulnerable’ by the IUCN Red List of Threatened Species due to habitat loss, urbanisation, climate change, and disease. The quick decline in the koala population over recent decades, throughout most areas of Australia, is a result of Eucalyptus forest clearing for development and the resulting spread of diseases, like Chlamydia.
Koalas are specialist folivores consuming nearly exclusively Eucalyptus spp. This diet is fatal or toxic to the majority of other mammals due to the toxic molecules it contains. Therefore, there is a lack of competition over food resources. Despite their assessment of ideal leaves, Eucalyptus is still very low in caloric content, so koalas lack energy and thus can sleep and rest up to 22 hours a day. The understanding behind how koalas detoxify Eucalyptus and protect their young has been vague up until 2018 when their genome was sequenced. Genomes are important for understanding and tracking genetic diversity in a species.
KOALA YOUNG
As a marsupial, koala offspring are born undeveloped after 35 days of gestation, weighing less than 0.5g, about the size of a kidney bean. It then further develops in their mothers’ pouch for 6-7 months, attached to a teat. After it has left the pouch, it will continue to suck until it is roughly a year old. Koalas give birth to offspring lacking immune organs or tissues, therefore, while young are in the pouch, they depend on their mother’s milk to provide immunological protection whilst their immune system develops.
"Koalas ability to detoxify and get rid of toxic Eucalyptus molecules is very advantageous"
CHOOSING THE LEAST TOXIC LEAVES
As selective feeders, koalas choose which leaves to consume by smelling them before tasting, targeting nutrients, and avoiding highly toxic leaves which contain high concentrations of plant secondary metabolites. The role of smell as well as taste aids in specialist detection of leaves, which helps alongside chemical defences, like detoxification. Genome sequencing identified multiple gene expansions that would aid in its ability to make food choices. One of which is an extra copy of the gene that enhances its ability to ‘taste water’, helping to assess a leaf’s moisture and nutrient content. Another is a gene expansion associated with the ability to detect non-volatile odorants, discriminating plant secondary metabolites and, therefore, how toxic a leaf is, optimising ingestion of less toxic leaves. There are six of these genes in Koalas, compared to one in the Tasmanian devil and zero in humans. Past gene duplications have led to the koala possessing more ‘bitter’ taste genes than any other marsupial and the most out of all mammals, including humans.
TOLERATING TOXIC DIET
This also explains its ability to recognise toxins found in Eucalyptus. These adaptations are important given the koalas’ narrow diet, making them very vulnerable to climate change, enabling koalas to select suitable trees to forage and survive.
Once Eucalyptus foliage has been consumed, koalas can detoxify it as they have high enzyme levels for detoxification due to an expansion in a specific gene family, with high expressions found in the liver consistent with a detoxification role. This results in koalas flushing out toxins from their system rather rapidly, enabling them to continue eating without falling ill.
DISEASE SUSCEPTIBILITY
Koalas’ ability to detoxify and get rid of the toxic Eucalyptus molecules from their body is very advantageous, apart from when they need medicine as these get flushed out as quickly as the toxins. This process is much faster in koalas than in humans. An issue arises when koalas need to be treated by vets for diseases, like Chlamydia, and cannot be put on the same medicines as humans, or other marsupials, for the same condition as it will not work. Also, some diseases require antibiotics that alter the koala’s gut microbiome, affecting their ability to break down Eucalyptus leaves, leading to starvation. For many years, a vaccine to prevent Chlamydia in koalas has been researched, but now with a sequenced genome, it provides an insight into koalas’ genetic susceptibility to Chlamydia, presenting the genomic foundation to create a vaccine. It also helps with conservation management solutions, involving their population and genetic structure, like aiding gene flow through connecting habitats.