Biotechnologists have discovered some horrifying new consequences of lateral gene transfer: a virus that started making the toxin from black widow spider venom, all by itself. This might be the single combination humanity needed least. No matter what the permutation, viruses plus black widows add up to a whole lot of nope. It’s even less comforting that the virus acquired the ability to make the black widow toxin all on its own.
Thankfully, the virus is a bacteriophage, which is to say it infects bacteria — prokaryotes — and is theoretically restricted to that domain. Specifically, its host of choice is a genus of bacteria collectively called Wolbachia, endemic to “half of all arthropod species, which include insects, spiders and crustaceans.” Wolbachia, though, is an obligate intracellular parasite, so it actually lives inside the cells of the species it infects. It’s frequently found inside the egg cells.
The phage’s name among scientists is “WO,” and it’s found in clumps inside Wolbachia, wherever it lives. As it happens, one of those places is the black widow spider. The researchers, both of whom are named S. Bordenstein (Seth and Sarah), discovered an intact, functional, actively-transcribed region of DNA inside WO that codes for the toxin from black widow spider venom.
What this means is that somehow, WO crossed a profound ecological barrier. While prokaryotes, eukaryotes, and viruses all use DNA, the way they use it is hugely different. Eukaryotes have huge, linear genomes; prokaryotes have shorter, circular DNA; and viruses have specialized machinery to deal with one or the other (or to infect archaea, but they’re outside the scope of this story). Their whole tree of life is set up for different kinds of interactions, specific to each domain.
But somehow, either genes from the spider got into the bacteria and then were taken up by the phage, or genes from the spider were directly siphoned up by the phage. Based on the surrounding genetic detritus, the researchers think it was direct horizontal gene transfer. Either way, this represents the first known viral leap across the barrier between domains of life.
What’s more, WO is tweaking and reconfiguring the stolen genes for its own stealthy purposes. The researchers believe the virus uses them in part to cloak itself in layers of the cell membranes of its hosts — Wolbachia and the spider — so that when it replicates, it isn’t immediately destroyed by host immune defenses. “These sequences are more typical of eukaryotic viruses, not phages,” one of the Bordensteins commented. Which means that the same virus we found that makes black widow venom toxin in bacteria is acting like the kind of virus that infects eukaryotes like fungi, animals, and us.
Normally phages, like WO, carry specialized genes that counter the defenses of the cells they try to infect. But in this case, “the portion of DNA related to the black widow spider toxin gene is intact and widespread in the phage,” said Bordenstein. And it’s a huge gene: more than 14,000 base pairs. Extensive sequencing also revealed “evidence that the phage makes insecticidal toxins,” but we don’t yet know how they figure into the phage’s process of infection and reproduction. The pair did, though, isolate the “basic toolkit” that WO uses to slipstream itself into the Wolbachia genome.
Because Wolbachia infects so many insect species, WO’s toolkit could conceivably be used to turn Wolbachia into a biological control agent, by manipulating the function of the bacteria and thereby manipulating the host species. This would make it a major asset in the endless coevolutionary battle against pests and microbes.
Now, if you’ll excuse me, I will be over here noping my way into next week.