Each week Quanta Magazine explains one of the most important ideas driving modern research. This week, our biology staff writer Yasemin Saplakoglu explores the mounting research on horizontal gene transfer.
| | What We Know About How DNA Jumps Between Species By YASEMIN SAPLAKOGLU | | If you have your father's eyes or your grandmother's freckles, you can thank the genes passed down within your family. But researchers have begun to recognize that at a deeper biological level, another type of genetic inheritance also occurs. Genes can cha-cha-slide between individuals — or even species — through a process known as horizontal gene transfer. It may be far from an everyday occurrence in complex organisms like humans, but on an evolutionary timescale, it may be happening far more often than anyone thought.
Horizontal gene transfers are relatively common in the bacterial world, where they play an important role in evolution and adaptation, and in the spread of antibiotic resistance. In fact, evolutionary biologists struggle to untangle some of the early branches of the tree of life because the high number of horizontal transfers among those ancient single-celled organisms intertwined lineages so thoroughly. Scientists also know very little about how this process might have significantly shaped the genomes of complex organisms like plants and animals.
For many years, scientists who argued that horizontal hops could be happening in multicellular species such as fish were given the side-eye by their peers. Such a migration requires a chain of unlikely events: A gene from one individual must somehow make it into the germline cells that produce the sperm or egg cells of an individual from another species. From there it must get into the nucleus and shimmy into the genome of its new host, who must then produce offspring with those eggs or sperm to pass down that modified genome. Important drivers of this process are often the genetic elements called transposons, or "jumping genes," which can copy and paste themselves to different positions in a genome, or even from one genome to another. Sometimes they seem to do that by catching a ride into a new host's body inside a parasite or virus. It's a journey with a lot of unlikely steps, but molecular biology suggests it happens.
Studies have pinpointed cases of horizontal transfers in a diverse array of animals, including fish, frogs and snakes. Still, it's unclear to what extent complex eukaryotic organisms share genes with other life in this way. The data so far suggests that it's more likely for genes to hop from bacteria into eukaryotes than the other way around: Experiments show that when eukaryotic genes trespass into bacteria, the bacteria often eject them.
Biologists have made many surprising discoveries in recent years about the movement of genes between species.
What's New and Noteworthy
In 2022, researchers reported that a gene called BovB has moved from snakes to frogs independently at least 50 times in various parts of the planet. Oddly, they found that it has happened much more often in Madagascar than elsewhere. It's not clear why. One factor could be the high number of parasites such as leeches that live on the island and move from host to host, carrying acquired sequences of DNA in the blood they've drunk. Evidence of ancient horizontal gene transfers is often scrambled by time, but researchers are now hoping to catch transfers in the act by looking at organisms in hot springs in Yellowstone National Park.
Horizontal gene transfer also seems to have figured in how marine life around the poles evolved — or rather, borrowed — defenses to survive the freezing cold. Researchers found that herrings and smelts, two groups of fish that diverged more than 250 million years ago, use the same gene to make proteins that stop ice crystals from growing in their body. It took decades to convince researchers that the gene must have hopped from the herring to the smelt. How much this kind of horizontal transfer happens among vertebrate cells is unclear, but one study found evidence of at least 975 transfers among 307 vertebrate genomes, mainly in ray-finned fishes.
Gene transfers between species are even relevant to humans, or more specifically to our microbiomes, the sturdy armies of microorganisms that occupy our gut and other parts of the body. A human baby's microbiome first comes from their mother. But surprisingly, those maternal gifts aren't always whole cells. Small snippets of DNA from a mother's bacteria can hop to the baby's bacteria through horizontal gene transfers even months after birth. These genes, which often come from helpful bacterial strains in the mother, could play an important role in the baby's growth and development. Though it's not clear whether the horizontal gene transfers directly benefit the baby by conveying particular functions, they might be indirectly useful by assembling a more capable gut microbiome.
| | | Popular Mechanics described a particularly eerie example of horizontal gene transfer. By stealing genes, parasites known as horsehair worms gained the ability to control the minds of their insect hosts. | | | Trends in Plant Science published a paper that describes how horizontal gene transfer may have shaped the evolution of land plants. | | | Trends in Genetics published an opinion piece by two scientists speculating about how common horizontal gene transfer might be among complex organisms. | | | Neural Academy on YouTube did a deep dive into the topic in a video about the mechanisms of horizontal gene transfer, using fun illustrations to describe the three ways that bacteria can horizontally acquire new genes. | | | | | | |
