Talk about "the stranger within"-- scientists have discovered what appears to be the entire genetic code of one species residing inside the DNA of another. The finding might have serious repercussions for DNA sequencing projects, as this ScienCentral News video reports.
Foreign Genes
In a discovery reminiscent of the movie, "The Fly," scientists have discovered that there can be "fusion at genetic-molecular level" between two completely different organisms, and that it's actually common. Fortunately, this doesn't appear to be possible in humans.
Jack Werren and Michael Clark of the University of Rochester were studying fruit flies, which are commonly infected with a bacteria that infect about 70 percent of all insects worldwide. It’s called Wolbachia. In one type of fruit fly, the bacteria live inside the sperm and egg cells, allowing it to pass to the next generation of fruit flies.
So, planning to sequence the genome of the fly, they first fed the flies an antibiotic to eliminate the bacteria. But when their study collaborators at the J. Craig Venter Institute analyzed the flies’ DNA, they also found bacterial DNA.
Clark recalls, “So, I assumed that I failed to cure them—maybe I didn’t use enough antibiotic, maybe they might be resistant, or something else went wrong. So I repeated that again and tested for the same bacterial genes.”
A: bacteria in fruit fly tissue
B: absence of bacteria
C: bacterial DNA inserted into fly DNA image courtesy: Dr. Jack Werren
But they got the same results. To confirm that the bacteria themselves were eliminated,the scientists made the bacteria glow green inside the tissue of the flies and compared “before” and “after” antibiotic treatment photos.But as the image above shows, the bacterial DNA had inserted itself into the DNA of the fly.
“The big news is that we’ve shown that DNA from bacteria routinely gets into the genome of an animal," says Werren. Prior to this study, the scientific community did not believe this was a widespread occurrence.
“I think it was pretty surprising, because scientists have known for a while that bacteria transfer DNA back and forth between each other. As a matter of fact, this is one reason why antibiotic resistance can spread so rapidly among bacteria-- is because they’re very promiscuous in exchanging their DNA," he says.
But, looking amused, Werren goes on to explain why it shouldn’t be that surprising.
“In another sense, it’s logical, because bacteria live within the germ line, the germ line in animals; and therefore the chance for exchanges are really high.”
As they wrote in the journal Science, it shows a whole new way that insects may be acquiring genes.
“It’s probably almost inevitable that some of these transfers are gong to lead to new functions for animals that pick up this DNA," says Werren.
As Werren and Clark work to find out what those functions might be, they explain that scientists who study genomes will need to be aware that some of the DNA they find in an animal may not just be contamination, but a new part of its genetic code. Werren says that this discovery could one day help in pest control as well as pharmacology.
He adds that while this type of gene transfer is common from bacteria to insects, it is highly unlikely that this could happen in humans. This is because in humans, bacteria do not infect the sperm and eggs.
“Although this is probably not going on in people today, in our very distant ancestors it definitely happened," he says, "because we have a very special structure that lives within our cells called the mitochondria. It’s the powerhouse. It’s really vitally important for our cells. And the cells of all plants and animals have these mitochondria, and it’s now overwhelmingly evident that those mitochondria came from a bacteria in the very distant past, and then there were a lot of transfers from that bacteria into our genomes. So, now today, we do have genes that were bacterial in origin a long, long time ago… And the same is true for chloroplasts, which are what make plants green. Chloroplasts have their own DNA and they came from a bacteria.”
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