Koala genome reveals bitter-sweet secrets

The recent announcement that the koala genome has been sequenced has sent a ripple of excitement throughout the scientific community. The koala genome is the first of any marsupial to be sequenced to this extent, as extensively sequenced as the human genome.1

Led by an Australian research team from the Australian Museum and the University of Sydney, sequencing the koala genome was a mammoth effort involving a team of 54 international researchers from 29 institutions across seven countries.2

Koalas are plagued by several complex factors that threaten their existence. While populations in the northern regions of Australia are vulnerable to habitat destruction, South Australian populations are paradoxically threatened by overpopulation and lack of genetic diversity. Koalas are also afflicted by two specific diseases: koala retrovirus (KoRV) and Chlamydia.

Already, researchers have used the genome to establish how koalas are able to survive exclusively on eucalyptus leaves, a diet that would kill most other species. They scoured the genome for genes with a detoxifying function and identified a family of genes that were expressed in a wide range of koala tissues, and
in particular in the liver. What’s more, variations of the genes within this family were identified as being involved in meloxicam metabolism, explaining why this non-steroidal anti-inflammatory drug (NSAID) is so rapidly metabolised in koalas compared with humans and other mammals.

Koalas are notoriously fussy eaters, sniffing leaves before tasting them, suggesting an olfactory function, in addition to taste, in selecting appropriate leaves. Using this hypothesis, researchers searched the genome for genes associated with taste and smell.

Lo and behold, they found a gene family associated with the detection of non-volatile odorants. Interestingly, where other marsupials, such as the Tasmanian devil and gray short-tailed opossum, only carry one of these genes (humans and other mammals such as mouse, dog, platypus carry none), koalas have six.

The researchers also found an expanded repertoire of genes associated with taste receptors, and in particular those associated with bitter tastes, enabling koalas to identify leaves with higher moisture and nutrient content and lower concentrations of toxic metabolites. Taken together, it appears that koalas can smell and taste the most nutritious, least toxic leaves and detoxify them, enabling to survive in their niche environment.

This is all amazing stuff and gives us an understanding of how koalas have adapted to take advantage of a niche food source with almost no competition. But we’re not done yet; the koala genome offers even more insights. Using the sequenced genome, the study authors found that koala mothers can adjust milk protein composition during lactation in accordance with the life stage of their pouch young.

Like other marsupials such as the platypus and Tasmanian devils, koala milk has antimicrobial properties to help protect pouch young. The gene encoding these antimicrobials is located next to the marsupial milk gene, suggesting that these genes are expressed together, so that when the koala lactates, she also secretes the antimicrobial. Significantly, these antimicrobials could offer novel therapies for antimicrobial-resistant infections in humans.

Given their vulnerability to habitat destruction, genetic variation and diseases that threaten various populations, the koala genome will be an extremely valuable tool. We have already gained unprecedented insights into how these unique marsupials have adapted to such a niche environment. Access to the full genome will also provide researchers with potential cure or vaccine targets for the diseases that afflict koalas by looking at their immune response pathways. It will also aid in conservation efforts by allowing researchers to better assess genetic diversity.

Nidhi Sodhi
Science Writer

1. Johnson RN, O’Meally D, Chen Z et al. Adaptation and conservation insights from the koala genome. Nat Genet 2018. DOI: https://doi.org/10.1038/s41588-018-0153-5.
2. University of Sydney. Cracking the genetic code of koalas. ScienceDaily
2018. www.sciencedaily.com/releases/2018/07/180702111201.htm.

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