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.