The koala genome has been sequenced, making the cuddly creatures only the fourth marsupial to get this treatment. The work could assist in saving the threatened species from two of its greatest threats, and has provided an abundance of surprising results biologists will explore for decades. However, geneticists had to overcome an unexpected obstacle when they discovered the DNA in koala blood is unusually hard to sequence.
Not long ago revealing the entire genome of an animal was such an enormous enterprise there was a major debate as to whether to make the opossum or the Tamar wallaby the first marsupial to have the honor. The preponderance of research facilities in North America overcame the wallaby's status as a more representative species, but both are now done.
Sequencing has become easier and more accurate since, but Dr Rebecca Johnson of the Australian Museum told IFLScience it was still a challenge to collect the resources needed to add koalas to the list. The paper in Nature Genetics announcing the near-complete sequence has 54 authors.
The koala’s status as both iconic and threatened got it prioritized, but Johnson and her co-authors hit a snag when they tried to use DNA from koalas’ blood, as usually occurs. The low-quality jumble produced was inadequate for the third generation sequencing the team hoped to apply, forcing them to wait until sick koalas were euthanized so they could extract DNA from their organs. Johnson told IFLScience the reasons normal techniques don't work are unclear, but may relate to their blood's high lipid concentration.
Eventually, three unfortunate beasts were found that were to be put down, in two cases because they were so ravaged with chlamydia. Out of this woe, Johnson and co-authors made something positive. Nevertheless, the difficulties contributed to the sequenced animals' limited diversity.
The chlamydia threat is one reason koalas were prioritized for sequencing, as an understanding of the genes for their immune systems should assist vaccine design. The project could be even more useful against a less well-understood threat, the koala retrovirus. The virus inserts itself into the koala genome, sometimes in more than a hundred places, and the sequencing could help us distinguish between dangerous and less harmful versions.
Unfortunately, there is no way sequencing can address the single biggest threat koalas face, destruction and fragmentation of their habitat, particularly in northern parts of their range.
The project revealed the koala has around 26,000 genes, slightly more than humans, and not surprisingly those aspects dedicated to detoxifying eucalyptus oils are prominent, with heavy expression in the liver.
Like other marsupials, koalas are born exceptionally underdeveloped and vulnerable, lacking a functioning immune system to protect them. "We identified genes that allow the koala to finetune milk protein composition across the stages of lactation, to meet the changing needs of their young," author Professor Kathy Belov of the University of Sydney said in a statement.
Some of the proteins found in the milk have never been seen before in other mammals and may protect the joey against bacterial and fungal diseases. Replicating these could improve the survival prospects of orphaned joeys, but may also provide a starting point for those looking for new classes of antibiotics.
The study also revealed koalas went through a sharp population decline 30-40,000 years ago, around the time much of Australia's megafauna died out, although Johnson told IFLScience the causes are unclear.
The use of third generation sequencing techniques means the program is estimated at 95.1 percent complete, higher than any other marsupial, and almost matching the human genome, despite the vast work we have put into studying ourselves.
1