World's largest koala family tree genome database to protect the population of the endangered species

A major challenge for these animals, for example, is the koala retrovirus (KoRV), which increases susceptibility to bacterial infections, leukaemia and other types of cancer. All koalas in zoological gardens in North America and Europe, as well as almost all wild koalas in Australia, carry this virus.

How to help koalas survive: The San Diego Zoo Wildlife Alliance, the Californian research company Illumina, the University of Sydney, the Australian Museum Research Institute, the University of Nottingham, the Leibniz-IZW and the ZooParc de Beauval in France want to understand the health consequences of koala viruses and thus ensure the long-term survival of the species. Almost all koalas - both in their natural habitats in Australia and in zoological gardens - are naturally infected with one or more retroviruses, most notably the koala retrovirus (KoRV). In some koalas, these viral infections cause serious diseases such as cancer, which jeopardise the population. Scientists from several biological and veterinary disciplines are working together worldwide to improve the understanding of KoRV. To understand how retroviruses are transmitted and change from generation to generation, a research team led by the San Diego Zoo Wildlife Alliance in collaboration with the Leibniz-IZW studied five generations of koalas at the San Diego Zoo and sequenced the genomes of all individuals.

At the Leibniz-IZW, Prof Alex D. Greenwood, head of the Department of Wildlife Diseases, has been studying the retrovirus KoRV and its effects on the health of koalas for many years. While we have learnt a lot about KoRV integrations into the koala somatic cell genome and their effects on the health of wild koalas, we still don't really know how KoRV is transmitted from one generation to the next;We still don't really know how KoRV is transmitted from one generation to the next, how new integrations of KoRV arise in the koala genome and which integrations are most important for koala health," says Greenwood. Whole genomes of wild koalas have already been sequenced, but information about their ancestry and health status was lacking. The San Diego Zoo Wildlife Alliance's Koala Genome Project aims to provide this new information.

The koala population living in human care in North America was established between 1976 and 1981 with 14 koalas from Australia. More than 40 years later, the San Diego Zoo Wildlife Alliance cares for 30 koalas - the largest colony outside of Australia. „The koalas in this study are part of one of the most comprehensive phylogenies in the world, with familial relationships that have been reliably documented over decades of careful care and maintenance, including with the help of biobanks“ says David Alquezar, Ph.D., Manager of the Australian Centre for Wildlife Genomics, Australian Museum Research Institute. „The study of CoRV integrations in this 'closed system' of related individuals provides a unique opportunity to explore CoRV mobility and illuminate the impact of these integrations on the disease history of individual animals. This is particularly important for koala populations in human care, where ideally a reservoir of robust and healthy individuals can be bred for use in future reintroductions.

„For animal health and welfare, a key question is whether we can reduce the impact of CoRV-induced diseases on koala populations through testing and breeding programmes,

said Dr Rachael Tarlinton from the University of Nottingham's Faculty of Veterinary Medicine and Science. Most viruses are transmitted by infection „horizontally“ (between different individuals within a generation) „and are therefore exogenous. They inject their genetic code into the somatic cells of a host organism, causing them to produce copies of the virus, which can then infect other host organisms. However, some viruses inject their genetic code directly into the germline cells of a host organism, i.e. into sperm or egg cells. When this happens, the viral genetic material can become a permanent part of the host's genome: an endogenous virus that is not transmitted horizontally within a generation, but vertically from one (host) generation to the next.

The vertical transmission of retroviruses is a relatively common process, according to the researchers. All living organisms, including humans, have incorporated retroviral DNA into their own genome. The remains of endogenous retroviruses make up eight per cent of the human genome. In the vast majority of species, however, these viral integrations occurred millions of years ago and the DNA is now so degraded that it can no longer infect other hosts or cause health problems. The koala retrovirus is a special case. It has only started endogenising in koalas in the last 50,000 years. On an evolutionary timescale, this is practically yesterday, according to the researchers. Some of the KoRVs are still exogenous subtypes and therefore far from harmless. These subtypes regularly cause serious health problems in koalas living in the wild and in human care.

„We don't know what happens to the host when a retrovirus is exogenous and it initially becomes a relatively harmless fellow traveller,

says Alex Greenwood. „We also don't know how the virus changes during this time to better adapt to the host, if it ever does.“ Rachael Tarlinton adds: „In some cases, these viruses are also beneficial to their hosts by altering gene expression and introducing new genetic diversity. We just don't know how this process works and how quickly the changes in host and virus level off.

All koalas carry a high risk of cancer. Cora Singleton, chief veterinarian at the San Diego Zoo Wildlife Alliance, explains: "Koalas are very good at hiding early stages of some cancers. They can appear healthy one day and then suddenly show a subtle change in behaviour the next. We then examine their blood and bone marrow and often find that they have late-stage cancer.

At the moment, there is no adequate treatment for the affected koalas.

Press release of the "idw - Informationsdienst Wissenschaft" from 18 January 2024