December 2016

This fall researchers announced a surprising discovery: a virus carrying genes for a toxin from black widow spider venom. While that conjures up frightening thoughts (a flu that causes a breakout of festering welts!), rest assured that this virus cannot infect humans. In fact, it infects only a particular type of bacterium that itself only infect insects, spiders, and their close relatives (i.e., arthropods). How did this creepy mash-up occur in the first place? A black widow virus might sound like the product of genetic engineering — like strawberries bearing fish genes or goats that make spider-silk proteins – but it is actually the result of evolution…

Where's the evolution?

Researchers think it most likely that the virus (which infects the bacterium Wolbachia) got the genes from spiders through a process called horizontal transfer and subsequent natural selection, favoring virus particles that could use the toxin to break through animal cell walls. This discovery highlights the different ways that new genetic variation can arise in a population.

We usually think of genetic variation (new genes or different versions of old genes) in a population arising through either de novo mutations popping up at random in the genome or via migration and breeding — where an individual or group of individuals carrying different gene versions mates with individuals in the population, and the next generation carries some of those gene versions as well as the versions that previously existed in the population. In these ways, gene versions are passed “vertically” from parent to offspring.

However, gene versions can also be passed horizontally — that is, DNA can be passed directly from one organism to another organism that already has its own DNA and may not even be closely related to the DNA donor. This is what seems to have occurred with the black widow and the virus. Viruses undergo horizontal transfer readily. However, normally they only pick up genetic material from the host cells that they infect. In this case, it seems that the virus has picked up DNA from the host of its host.

Increasingly, research has uncovered the role that horizontal transfer plays in generating genetic variation within lineages. Genetic variation obtained through horizontal transfer appears to have been particularly important in the evolution of viruses, bacteria, and many other single-celled organisms. It’s also likely that horizontal transfer was common during the early evolution of life on Earth. However, its role in the evolution larger, multicellular organisms is still unclear — though exciting to consider! After all, if a virus deploys a spider toxin for its personal use, what molecular tools might have been co-opted by our ancestors?

In any case, it’s important to note that horizontal transfer is not a new mechanism of evolution allowing organisms to pick and choose genes from their environment. Organisms “pick up” genes and gene versions at random depending on what’s in the environment what genes are carried by other organisms they interact with, not based upon what would be useful. This is just another way that lineages get genetic variation, and this variation then serves as the raw material upon which undirected evolutionary processes like natural selection act. Natural selection shapes which genetic variants are retained by the lineage (because they benefit survival and reproduction) and which are lost (because they do not).

Some media reports on the black widow virus have described chunks of arachnid DNA being “stolen” by the virus to help it punch through animal cells. However, this makes the process sound intentional, when it was not. In fact, over the course of the virus’s evolutionary history, it’s likely that a wide variety of spider genes were incorporated into its genome but most either did not boost survival or did harm and so were not preserved in the viral lineage. Horizontal transfer is an exciting and interesting means through which a lineage acquires new genetic variation, but it doesn’t change the basic rules of evolution.