National Cancer Institute

Ancient traces of DNA from the family of viruses that cause leukaemia have been discovered in the genomes of bats, filling the “last major gap” in the retrovirus fossil record.

These so-called deltaretroviruses, which include T-lymphotrophic viruses, are said to infect up to 20 million people worldwide and they are to blame for a rare type of Non-Hodgkin Lymphoma called Adult T-Cell Leukaemia/Lymphoma (ATLL). It has long been thought deltaretroviruses have infected humans since prehistoric times but because these viruses have no ‘fossil record’, their deeper origins have remained a mystery, until now.


The research, published in the journal Proceedings of the National Academy of Sciences (PNAS), was conducted by the University of Glasgow and The Czech Academy of Sciences, Prague. A team working under researcher Dr Daniel Elleder identified the remnants of a deltaretrovirus in the genome of ’bent-winged bats’. This sequence was then found in a range of a distantly related Minopterid species, dating back 20 to 45 million years ago.

What are retroviruses? Retroviruses are a family of viruses that can cause cancers and immunodeficiencies in animals. The 'retro' part of their name comes from the fact they are made of RNA, which they can convert into DNA and insert into their host genome - the opposite direction to the normal flow of information in a cell. Such viral sequences, called proviruses, are passed to infected cells as the cells divide meaning proviruses can become inherited. This means they can occasionally be inherited as endogenous retroviruses – retroviruses with an internal origin – forming a virtual genomic fossil record that can be used to look back into their evolutionary history.D eltaretroviruses are the last retroviral genus for which endogenous forms weren't known. The recent research identifies a case of endogenous deltaretrovirus, which entered the genome of long-fingered bat ancestors more than 20 million years ago.

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The bat tissue samples were obtained from museum specimens from the National Museum Prague as parts of the pectoral muscles and from released bats caught during various ecological studies as wing punch biopsies stored in the genetic bank at Charles University, Prague.

The Prague team worked with Dr Robert Gifford from the University of Glasgow Centre for Virus Research to characterise the sequence. This highlighted an unusual – and as yet, unexplained – feature of the virus, which is also found in contemporary deltaretroviruses. The discovery that this characteristic has defined deltaretroviruses for millions of years indicates it is somehow key to their biology, and could help scientists study them in the future.


The retrovirus fossil record contains DNA sequences derived from ancient retroviruses that have been ‘preserved’ in animal genomes. Over recent years, studies of these sequences have revealed the unexpectedly ancient origins of various retrovirus groups, and in doing so, have helped scientists understand the long-term ‘evolutionary arms-race’ between retroviruses and mammals. These latest findings represent the first concrete evidence that the deltaretrovirus group has an ancient origin in mammals.

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“The discovery of this viral sequence fills the last major gap in the fossil record of retroviruses,” said Dr Gifford. “It provides a means of calibrating the timeline of interaction between deltaretroviruses and their hosts.

“Importantly this finding could also be used as a tool for understanding the mechanisms that mammals have evolved specifically to counter the threat from these viruses. Understanding the history of these viruses will help scientists to better understand how they affect people and animals now and in the future.”


The work builds upon, but is separate to, recent research from Oxford University which found that retroviruses - the family of viruses that includes HIV - are almost half a billion years old.

The findings, in the journal Nature Communications, dated the viruses to be several hundred million years older than previously thought, and suggested retroviruses had ancient marine origins, having been with their animal hosts through the transition from sea to land.

The new paper, Discovery of an endogenous Deltaretrovirus, in the genome of long-fingered bats was funded by the Medical Research Council (MRC) and the Czech Ministry of Education, Youth, and Sports under the program.