Last updated April 17, 2018 at 11:08 am
Viral universe expands. Viruses are everywhere.
Zika virus in blood with red blood cells, a type of RNA virus.
Exploring new avenues in research can sometimes be a long, unrewarding experience. But sometimes it can reveal the rich, ancient history of RNA viruses.
A new discovery that opens up our knowledge of RNA viruses was made by an international team of researchers from the University of Sydney, the China Center for Disease Control and Prevention, and the Shanghai Public Health Clinical Centre.
Greater diversity in RNA
A lot of research on the diversity and evolution of vertebrate RNA viruses (where the genomic material is RNA rather than DNA) has been centred around mammalian and avian species. However, if one really wants to understand the true diversity, one needs to look broader at undersampled species.
Researchers have discovered 214 vertebrate-associated viruses in reptiles, amphibians, lungfish, ray-finned fish, cartilaginous fish and jawless fish. 196 of these are vertebrate-specific.
In total, they looked at more than 186 host species, sequencing their RNA and screening for RNA viruses. To narrow their analysis, they focused on vertebrate-associating viruses and vector-borne viruses which can infect both vertebrate and invertebrate hosts.
“This study reveals some groups of virus have been in existence for the entire evolutionary history of the vertebrates – it transforms our understanding of virus evolution,” said Professor Eddie Holmes, from the University of Sydney.
“For the first time we can definitely show that RNA viruses are many millions of years old, and have been in existence since the first vertebrates existed.”
Incredibly every RNA viral family or genus already known to infect mammals and birds were also detected in amphibians, reptiles or fish. This highlights the evolutionary history of RNA viruses.
“Fish, in particular, carry an amazing diversity of viruses, and virtually every type of virus family detected in mammals is now found in fish. We even found relatives of both Ebola and influenza viruses in fish,” points out Prof Holmes.
The evolutionary relationship between virus and host became very evident through co-divergence. Family trees patterns of both host and virus mirrored each other. Vertebrates hosts which diverged as certain points was mirrored by vertebrate viruses which also diverged as similar points. This also revealed certain examples of host switching, previously unknown.
Tracking the evolution of RNA viruses. Shi et al. sequenced RNA viruses present in various classes of vertebrate, and constructed trees of virus evolution. Over a period of 525 million years, vertebrates branched off into several classes. The beginning of each coloured blocked arrow indicates the divergence between a vertebrate group and that below it in the figure; the beginning of the darker shading indicates the time that the most recent common ancestor of currently extant members of a class arose. The authors found that RNA viruses co-diverged with their vertebrate hosts (black lines indicate virus evolution). Each vertebrate class is dominated by its own set of RNA viruses; however, occasional cross-species transmissions occur (dashed arrows), introducing new viruses into a particular class. This phylogenetic tree is a simplified schematic to exemplify RNA-virus evolution as a whole, and does not reflect precise dates or cross-species transmission events found by the authors. Credit: Nature
This study reveals that RNA viruses in vertebrates are present in greater numbers and diversity than previously thought.
“This study emphasises just how big the universe of viruses – the virosphere – really is. Viruses are everywhere,” said Prof Holmes.
“It is clear that there are still many millions more viruses still to be discovered.”
This research was published in Nature. A supplementary editorial piece was published in Nature News & Views.
