Animal viruses with separately packaged RNA segments

dose-response-plaque-assayThere are many examples of viruses with segmented genomes – like influenza viruses – but these genomes segments are packaged in  one virus particle. Sometimes the genome segments are separately packaged in virus particles. Such multicomponent viruses are commonly found to infect plants and fungi, but only recently have examples of such viruses that infect animals been discovered (paper link).
Sequence analysis of viruses isolated from Culex mosquitoes in Central and South American Countries revealed six new viruses with segmented RNA genomes, which was confirmed by gel electrophoresis of RNA extracted from virus particles.
Some of the virus isolates appear to lack the fifth RNA segment, and the results of RNA transfection experiments indicated that this RNA is not needed for viral infectivity.
RNA viruses with segmented genomes are common, but in this case, the surprise came when it was found that the dose-response curve of infection for these viruses was not linear. In other words, one virus particle was not sufficient to infect a cell (illustrated). In this case, between 3 and 4 particles were needed to establish an infection. These findings indicate that the viral RNA segments are separately packaged, and must enter a cell together to initiation infection.
These novel viruses, called Guaico Culex virus (GCXV) are distantly related to flaviviruses, a family of non-segmented, + strand RNA viruses. They are part of a clade of RNA viruses with segmented genomes called the Jingmenvirus, which includes a novel tick-borne virus isolated in China (previously discussed on this blog), and a variant isolated from a red colobus monkey in Uganda. These viruses are also likely to have genomes that are separately packaged.
An interesting question is to identify the selection that lead to the emergence of  multicomponent viruses that require multiple particles to initiate an infection. Perhaps transmission of these types of viruses by insect vectors facilitates the introduction of multiple virus particles into a cell. How such viral genomes emerged and persisted remains a mystery that might be solved by the analysis of other viruses with similar genome architectures.

Virology question of the week: why a segmented viral genome?

influenza-reassortmentThis week’s virology question comes from Eric, who writes:

I’m working on an MPH and in one of my classes we are currently studying the influenza virus. I’d forgotten that the genome is in 8 separate parts. Curious, I’ve been searching but can’t find any information as to why that is?

What evolutionary advantage is conferred by having a segmented genome?

Terrific question! Here is my reply:

It’s always hard to have answers to ‘why’ questions such as yours. We answer these questions from a human-centric view of what viruses ‘need’. We might not be right. But I’d guess there are at least two important advantages of having a segmented RNA genome.

Mutation is an important source of RNA virus diversity that is made possible by the error-prone nature of RNA synthesis. Viruses with segmented genome have another mechanism for generating diversity: reassortment (illustrated).

An example of the evolutionary importance of reassortment is the exchange of RNA segments between mammalian and avian influenza viruses that give rise to pandemic influenza. The 2009 H1N1 pandemic strain is a reassortant of avian, human, and swine influenza viruses.

Having a segmented genome is another way to get around the limitation that eukaryotic mRNAs can only encode one protein. Viruses with segmented RNA genomes can produce at least one protein per segment, sometimes more. There are other ways to overcome this limitation – for example by encoding a polyprotein (picornaviruses), or producing subgenomic RNAs (paramyxoviruses).

Other segmented viral genomes include those of reoviruses, arenaviruses, and bunyaviruses.

There are various ways to achieve genetic variation and gene expression, and viruses explore all aspects of this space.