How does the koala genome explain their specialised, ‘toxic’ diet?
What does the koala genome tell us about their interesting taste for low calorie, high toxin plants?
Wild koala eating while climbing a tree in French Island, Victoria
Photographer: Greta Frankham © Australian Museum
Koalas eat a diet largely of eucalyptus leaves (20 of the >600 species), which have a high level of toxins. The consortium found that koalas have two large expansions in a gene family known to be integral to detoxification, the Cytochrome P450 gene family of metabolic enzymes, which oxidise molecules that can then be excreted as water soluble metabolites. These detox genes are expressed in many koala tissues, especially the liver. The evolution of these extra detox genes is undoubtedly what allowed koalas to become dietary specialists, and thus evade competition from other species which could not detoxify as effectively.
There must have been strong selection for koalas to be able to metabolise this food source. The Consortium found that koala detox genes have experienced diversifying selection (creating more detox genes via expansion through tandem duplicates) and purifying selection (which keeps the genes functioning, presumably because they play a very important role).
Knowledge of the koalas’ detox genes is important for veterinary care of koalas because this gene family is known to metabolise non-steroidal anti-inflammatory drugs (typically used in pain relief) and possibly antibiotics used to treat koala diseases like chlamydia.
In addition to this, Eucalyptus leaves are very low in calories, so koalas typically need to eat 600-800g per day. They are often touted as the most expensive animal to feed in captivity outside of Australia. The koala genome offers us clues to the basis of koala ‘choosiness’.
The consortium found that koalas have an expanded ‘bitter taste’ repertoire of 24 genes, the most of any Australian marsupial. Bitter taste receptors recognise structural toxins (such as terpenes, phenols and glycosides which are found in high levels in Eucalyptus), so this would allow them to optimise their leaf choice by targeting nutrients and minimising toxins.
Koalas derive most of their water from their diet, eating only leaves with more than 55% water content. The genome points to the koala’s ability to ‘water taste’ as it has a duplication in the aquaporin 5 gene thought to be a central component to sense water concentration.
Dr Rebecca Johnson , Director, Australian Museum Research Institute