Madagascar is home to a unique biodiversity with a large number of endemic species, among these many species of lemurs, including mouse lemurs. This diversity is also found in their retroviruses, a team led by scientists from the Leibniz Institute of Zoo and Wildlife Research (Leibniz-IZW) and the University of Stirling reports in the journal “Virus Evolution”. They analyzed the mouse lemur genome and identified viruses from two classes that represent ancient germline infections of the mouse lemur. The viruses now behave similarly to lemur genes and are therefore called endogenous retroviruses (ERVs). It was surprising that some of the retroviruses identified were closely related to viruses found in other very different mammals such as polar bears or domestic sheep. This suggests an intriguing and complex pattern of retrovirus host switching, much more complex than previously thought.
For their analysis, the team took blood samples from four species of Malagasy mouse lemurs and screened them using high-throughput sequencing. Scientists have identified two gamma and three beta retrovirus sequences in lemur genomes, representing ancient germline infections of mouse lemurs. Since then, virus DNA has been incorporated into host genomes and viruses are no longer active or infectious. “We were surprised to find that one of the two identified gamma retroviruses was linked to an ERV described in polar bears,” says Dr. Sharon Kessler, German Academic Exchange Service (DAAD)-supported scientist and assistant professor at the University of Stirling. The polar bear virus is evolutionarily young while the lemur virus is old. “How these related viruses infect such geographically separated species is unclear,” Kessler said.
There were other surprises among beta retroviruses. A virulent retrovirus that infects domestic sheep called Jaagsiekte sheep retrovirus (JSRV), which also forms VREs in domestic sheep, is thought to be a virus confined to domestic sheep, goats and their relatives — the first cloned sheep “Dolly” had to be euthanized after JSRV infection and subsequent illness. Mouse lemurs have a closely related JSRV-like virus in their genome. “This suggests that JSRV-like viruses have been more widespread in mammals and are considerably older than previously thought. – IZW Department of Wildlife Diseases, where the testing of the samples was carried out. Similarly, the team also identified a virus in mouse lemurs related to retroviruses found in squirrel monkeys, vampire bats and marsupials. “This group of viruses becomes more interesting over time as more examples of similar viruses are found in many places, including very young ones that may still have exogenous counterparts currently infectious in nature. ”, says Greenwood.
Much of the mouse lemur retroviral diversity observed is associated with non-primate viruses, suggesting a complex pattern of viral host switching around the time lemur ancestors colonized Madagascar. Further studies of viral diversity will help clarify the complex history of retroviral transmission in mammals.
Dr. Kessler and Prof. Greenwood collaborated with Prof. Solofonirina Rasoloharijaona from the University of Mahajanga (Madagascar), Prof. Ute Radespiel from the Hannover University of Veterinary Medicine, Foundation (Germany) and Dr. Kyriakos Tsangaras from the University of Nicosia (Cyprus) in this scientific investigation which was also made possible by funding from the American Association of Physical Anthropologists.
Retroviruses are viruses that replicate by incorporating their genetic material into the genome of a host cell. If the infected cell is a germ cell, the retrovirus can then be transmitted as an “endogenous” retrovirus and spread through a population as part of the host genome. Repeated infections have resulted in the ubiquity of endogenous retroviruses in mammalian genomes, sometimes constituting significant parts of the host genome. However, most retrovirus integrations are very old and already degraded and therefore inactive – their initial impact on host health reduced by millions of years of evolution.
