Koala Secrets
Geneticists have figured out koalas' love for poisonous eucalyptus leaves
Ekaterina Rusakova, N+1
Scientists have sequenced the complete koala genome for the first time and analyzed more than 26 thousand active genes, according to Nature Genetics (Johnson et al., Adaptation and conservation insights from the koala genome). This allowed scientists to understand why marsupial bears can eat poisonous eucalyptus leaves without harm to themselves, how they chose a suitable diet for themselves and how they learned to protect cubs and adults from infections.
Koala (Phascolarctos cinereus) is currently the only representative of the koala family that lives in Australia. Koalas are the closest relatives of wombats, their common ancestor lived about 30-40 million years ago. In ancient times, there were 15-20 species of these animals on the continent, and the current species appeared about 350 thousand years ago. To date, three subspecies are known. One of them lives in Queensland, in the northeast of the continent, the other two - in the south and southeast of the country. Previous studies (though conducted more than 20 years ago) have shown that two of the three subspecies have low genetic diversity and a high percentage of closely related crossbreeding.
A female koala gives birth to an underdeveloped cub after a 35-day pregnancy, and the baby spends the next six months in the mother's brood bag.
Koala with a cub. Author: Brian Dell, Wikipedia
Koalas are very selective in food: they feed almost exclusively on eucalyptus leaves, and out of 600 species of eucalyptus prefer about 30. Animals get almost all their water from leaves, so they include in the diet those types of eucalyptus whose leaves contain at least 55 percent water. Since the leaves are low in calories, animals need to eat up to 400 grams of leaves per day and save energy. They sleep about 20 hours a day, and eat most of the remaining four hours. Eucalyptus leaves are not too attractive food, not only because of their low calorie content. They contain compounds that are extremely toxic to most other animals. Koalas have adapted to them, and thus almost avoided food competition. However, how they adapted to the poisonous food, and how they distinguish suitable trees among the huge variety of eucalyptus species, was still unclear.
To answer these and other questions (for example, how koalas protect themselves from infections and how the species can be preserved in the future), scientists from the Koala Genome consortium sequenced the complete genome of the marsupial bear. The team of researchers, currently consisting of 54 scientists from seven countries under the leadership of Dr. Rebecca Johnson, began work in 2013 and has already published some of the results.
In the new work, scientists present directly the results of genome sequencing and analysis of 26,558 active genes, of which it consists. The koala genome turned out to be larger than a human (3.42 versus 3.2 billion pairs of nucleotides), but consists of fewer chromosomes (16 versus 23 pairs).
Scientists have discovered how marsupial bears have adapted to their poisonous diet. They turned out to have much more genes encoding proteins from the cytochrome P450 family than other animals. These enzymes oxidize various substances, turning them into water-soluble metabolites that are rapidly excreted in the urine. It turned out that in koalas cytochromes are produced in many tissues, including the liver. However, the protection turned out to have a downside – cytochromes quickly break down antibiotics that are given to sick koalas.
Also, genes allowed us to explain the ability of animals to recognize the right types of eucalyptus. Koalas turned out to have 24 genes responsible for recognizing bitter taste – the largest number among Australian marsupials. In addition, they turned out to have six genes encoding vomeronasal receptors capable of detecting the smell of not too volatile substances. For comparison, the marsupial devil and the gray short-tailed possum each have one such gene, and the platypus and wallaby do not have them at all. Koalas are able to feel and "taste water" – to recognize the water content in eucalyptus leaves. They learned this by increasing the number of genes of the protein aquaporin 5, which forms pores in the cell membrane through which water penetrates into cells.
Researchers have found that koalas protect their cubs from infections while they are sitting in a bag, with the help of breast milk. It contains koala-specific enzymes with antimicrobial action. They protect young animals from a number of bacterial and fungal infections, including Chlamydia pecorum, which cause diseases of the eyes and genitourinary system. Adult koalas are saved from infections with the help of numerous proteins of the immune system – immunoglobulins, proteins of the main histocompatibility complex, T-lymphocytes.
In addition, scientists have found new genetic markers and with their help made sure that subspecies, which according to old studies, had low genetic diversity and a large percentage of closely related crossing due to the isolation of populations, actually mix with each other and their genetic diversity is quite high.
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