A virus that infects a bacteria found in spiders and insects shares DNA similarities with a black widow spider’s toxin gene. What are the implications?

If studying viruses that infect bacteria isn’t your lifelong passion, you still might be interested to know that a bacteriophage is harboring DNA that appears to be linked to a spider toxin.

If you are a biologist who studies these types of viruses, this discovery may come as a total surprise.

That’s probably how a pair of researchers at Vanderbilt University felt when they sequenced the genome of phage WO and discovered DNA that resembled the gene for latrotoxin.

This toxin, found in black widow spiders, fatally punches holes in the cells of its victims.

The most surprising part, though, isn’t that the virus had bits of DNA similar to another organism.

Scientists already know that viruses can steal DNA from their hosts.

This is true for both viruses that infect eukaryotes — creatures with a nucleus in their cells — and those that infect bacteria. But viruses usually only have segments of DNA similar to their direct host.

Phage WO is different.

The researchers found that this virus contained DNA that resembles not that of its host — a bacterial parasite called Wolbachia — but DNA from its host’s host. Namely, the black widow spider.

Wolbachia doesn’t just infect black widow spiders. It is also found in the cells of more than 40 percent of all arthropods in the world, including insects, crustaceans, and other spiders.

The discovery puts phage WO in a unique place among viruses.

“To our knowledge, this is the first report of animal-like DNA found in a bacterial virus,” author Sarah Bordenstein, a senior research specialist at Vanderbilt University, wrote in a blog post about the finding.

She and Seth Bordenstein, Ph.D., an associate professor of biological sciences and pathology, microbiology, and immunology at Vanderbilt, published their study Oct. 11 in the journal Nature Communications.

So far, phage WO is unique, but that doesn’t mean scientists won’t eventually find other bacteriophages with animal-like DNA.

“I am optimistic that researchers will find more cases as additional viral genomes are revealed,” said Sarah Bordenstein, “especially now that we are on the lookout, but this may not be the case.”

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It’s tempting to say that the virus “stole” DNA from the black widow spider.

But there are many ways the two organisms could have ended up with similar DNA.

They could have evolved the genes separately, what’s known as convergent evolution. The close link between virus, bacterium, and arthropods, though, points toward some sort of gene transfer.

“Phage WO exists in a unique niche because it infects the cells of highly successful bacteria which infect the cells of insects,” said Sarah Bordenstein. “This provides more opportunities for genetic exposure and transfer.”

So if it was a gene transfer, how did it happen?

Phage WO may have picked up the DNA directly from the black widow spider. Or Wolbachia — or another virus or bacteria — may have picked up the spider’s DNA and passed it onto phage WO.

Or DNA sequences could have flowed the other way.

“Simply stated, we aren’t absolutely confident in the direction of transfer. But wouldn’t either option be interesting?” Sarah Bordenstein told Healthline. “Either the spider passed along a useful chunk of DNA to a bacterial virus, or the phage contributed to the evolution of spider venom.”

On her blog, Sarah Bordenstein writes that the evidence “leans toward spider to virus, possibly via a yet-to-be-discovered intermediary.”

Phage WO’s genome doesn’t include the entire spider toxin gene. But it has DNA that is similar to the protoxin part of the gene — this segment is thought to be involved in the rupture of the spider cells that produce toxin.

The researchers also found other types of animal-like DNA in the phage’s genome. This DNA resembled genes used by animal cells to sense pathogens or evade immune responses.

These pieces of animal-like DNA may help the virus break through both the bacterial and animal cell membranes, or survive in the cellular environment of the animal host.

More research is needed to know what purpose these DNA segments have, if any, for phage WO.

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The researchers also identified how phage WO inserts itself into the genome of its host.

This, wrote Sarah Bordenstein, “offers a potential method of accessing the Wolbachia chromosome in order to unlock its secrets” — aka genetic engineering of the virus.

This could be useful in combating viruses that affect humans.

One of the hosts of Wolbachia is the mosquito that carries viruses like dengue and Zika. Scientists have found that Wolbachia can reduce the ability of mosquitoes to transmit these harmful viruses.

“A major advantage [of this method] is that it is self-sustaining,” Matthew Aliota, Ph.D., an assistant scientist at the University of Wisconsin-Madison, told Healthline. “Wolbachia are maternally inherited, so once you establish Wolbachia in a population you no longer have to continuously release mosquitoes.”

Scientists are using this approach to combat mosquito-borne diseases in Brazil and other countries.

The strain of Wolbachia used by the Eliminate Dengue Program is already effective, which makes genetic engineering less enticing. But that may not always be the case.

“Wolbachia biocontrol of dengue and Zika appears to be extremely promising,” said Aliota. “However, with that said, there is always room for improvement. History has taught us that microbes constantly adapt.”

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