Where do you get your genes?
The obvious answer is from your parents, whose egg and sperm fused to create the unique combination of genetic material that makes you, you. But surprising new research throws a wrench into that well-known story: It turns out that large quantities of genetic material found in humans actually jumped from other species sometime in the past, and this process may be a major driver of evolution in animals from platypuses to humans.
According to the researchers, the idea that a significant amount of DNA transfers horizontally, rather than vertically, could change our understanding of how humans and other animals came to be. “It shows that this foreign DNA that could have come from anywhere could somehow end up in us and start changing things,” says Atma Ivancevic, a post-doctoral researcher in bioinformatics at the University of Adelaide in Australia and the lead author of a study recently published in Genome Biology.
Let's start at the beginning. First off, jumping genes aren’t really genes. They’re transposable gene elements, the non-coding genetic material that sits between genes. Humans are packed with the stuff—more than half of our genome is made up of transposable elements—but a lot of what it actually does is still a mystery. “Its one role seems to be to replicate itself as much as it can,” Ivancevic says.
David Adelson, Ivancevic’s supervisor at the University of Adelaide and a coauthor on the paper, had previously published research finding that transposable elements called Bovine-B (BovB) were jumping around among animals as diverse as rhinos, lizards and platypuses. To see what was gonig on, the team looked for BovBs and another transposable element called L1 in the genomes of 759 species of animals, plants and fungi whose fully mapped genomes were already available online.
“We wanted to shed some more light and see if we could understand why they were moving around in the genome and how far they could spread,” Ivancevic says. “We tried to look for similar matches of elements between very distant species.”
Since they knew that BovB elements could transfer between species, they tracked that type of genetic material first. They discovered some strange bedfellows: some BovBs had transferred at least twice between frogs and bats, and Ivancevic says BovBs that originated in snakes made up at least 25 percent of the genome of cows and sheep.
They also tracked L1 elements, which make up about 17 percent of the human genome and are probably much older than BovB elements, according to Ivancevic. They found for the first time that L1s, too, could be horizontally transferred: they were present in many animal and plant species, and all mammals they examined other than platypus and echidna (the only two egg-laying mammals, or monotremes, alive on the planet).
This led the team to conclude that the transposable elements were likely never present in monotremes—instead, they must jumped into a common ancestor of the rest of mammals between 160 and 191 million years ago.
Ivancevic even has a mechanism in mind. Critically, BovBs were also found in pests like bed bugs and leeches while L1s were found in aquatic parasites like sea worms and oysters. This led Ivancevic and her colleagues to believe that transposable elements may enter the DNA of diverse creatures by using these parasites, or other blood-sucking creatures like ticks or mosquitoes, as their vehicles.
Bats, too, could play a role. Transposable elements are inactive in many fruit bat species, which may be due to the fact that their insect diet made them particularly susceptible to horizontal genetic transfer. In other words, bats seem to have developed an increased ability to suppress these kinds of elements inside their own bodies—while at the same time acting as hosts capable of transferring them to other species.
Not that all these transposable elements are inherently bad. Ivancevic notes that while L1s may be related to cancer or neurological disorders like schizophrenia, other transposable elements may also be involved in placenta formation or helping the immune system. “We have evidence that they are doing good and bad things, almost accidentally,” she says, adding that many of L1s in humans are also inactive. “It’s almost like the genome tries to make use of them, or silence them to its own effect.”
Chiara Boschetti, a lecturer in biological sciences at the University of Plymouth in the U.K. who studies horizontal gene transfer, says that this kind of study shows that what scientists used to consider "junk" elements could actually play important roles in the function or regulation of genes. In some cases, it could even influence how the DNA is divided or replicated, and how chromosomes work.
“I think it does have the potential somehow of changing the recipient genome,” says Boschetti, who was not involved in Ivancevic’s work. “It’s very likely that there are effects.” She adds that the new research opens up new questions, such as how quickly these transposable elements transfer, and how active they are in genomes.
Scientists have long known that genetic material can be passed between bacteria horizontally; this is how they develop antibiotic resistance so quickly. But the discovery that more complex organisms also do this is becoming more important, and prompting more research into the concept of genetic inheritance, she says. “It’s kind of cool in a way," she says. "It adds a random dynamic element to everything."