How do viruses swap genes?

If there’s a disease epidemic in the country that’s literally just a few miles down the road from you, but what you’re worried about is how viruses might exchange genetic information...

...You might just be a scientist.

Yes, everyone’s been worried about the swine flu. But what has been bugging me are comments that this swine flu strain was a mix of human, bird, and pig viruses. I read this in New Scientist:

(F)lu surface proteins come in 16 different families, and viruses interbreed and swap genes.

I knew that this had to be a simplification. And that’s fine, because New Scientist is not a technical journal. But I couldn’t quite wrap my head around the concept of viruses “interbreeding.”

Interbreeding implies two organisms loving each other very much in a very special way, which is to say, sex. In organisms like ourselves, sex at the genetic level requires the fusion of sperm and egg cells. Single celled organisms, like bacteria, don’t have to mess around with all that, but just exchange bits of DNA directly.

Viruses, though, are not cells at all. They have two main bits: genetic material (either DNA or RNA) and some proteins. Viral particles infect living cells, and they take over the machinery of infected cells to make new genetic material and proteins. This cycle is shown below.


I could see how you could get genetic variation in viruses by mutation. In particular, a lot of viruses have very sloppy mechanisms for duplicating their genetic information, and so they throw up a huge variety of variations. This is why it’s been so hard to get a handle on, say the human immunodeficiency virus (HIV): it mutates at very high rates, so it’s almost never the same exact virus.

I could also see how a virus could pick up some DNA from an infected cell by some sort of freak mistake, I suppose.

I went digging through Google Scholar, and fairly quickly confirmed that yes, viruses can intermix their genetic material, but the papers I saw made no mention of how they do this. But then I saw the key word I’d been looking for: “reassortment.” That’s what experts call viruses picking up new genetic information, not “interbreeding.”

Even once I started zooming in, I was surprised by how many papers said that reassortment occurs, but said nothing about how. Finally, I found a couple of articles (one older, one newer) that seemed to indicate what I suspected.

It seems that viruses can pick up new genetic information from each other when two viruses infect the same individual simultaneously. I’m still sketchy on the details, but I think what’s going on is that when two viruses are in the same cell, virus #1 can accidentally pick up genetic material or proteins being generated by virus #2. It’s probably not deliberate, just the nature of random chance at the unpredictable molecular level.

Ironically, literally as I was writing this post, it turns out that the idea that the current swine flu virus has a mix of bird, pig, and human viruses may not be correct. Nevertheless, I learned something very new and interesting in my efforts to sate my curiosity.