How Virus-like ‘Jumping Genes’ Became Our Partners in Evolution

You might imagine that the DNA in your cells has a simple history. Even though it’s been recombined in every generation through sex and gained the occasional mutation, on the whole the genome has been stable and has been passed down reliably from your ancestors. But that’s not the entire story.

Nearly half of your genome is a wild drama: mobile, repetitive, disruptive, even viral. This half is the result of genetic material that can clip itself out of the DNA sequence, float off, and re-root somewhere else. These sequences can multiply and expand, inflating the genome from within.

They can hop into the middle of another sequence and break it. They can also be fertile soil for new adaptations to grow. These unruly genetic fragments are known as transposable elements, or transposons for short.

Often called jumping genes for their ability to relocate in a genome, they may seem pathological — indeed, many have viral origins — or perhaps little more than junk. But transposons are increasingly understood to be a key feature of many genetic tool kits. Their connections to the evolution of everything from moths to wombs, and even to the fundamental biological processes that turn genes off and on, suggest that the relationship between host genome and transposon is best understood as a deep coevolutionary entanglement.

The first hints of transposons’ existence were uncovered more than 80 years ago by the geneticist Barbara McClintock while she was studying color variation in corn kernels at the Cold Spring Harbor Laboratory in New York. She worked with a corn strain whose kernels were typically solid purple, but some were speckled, with purple pigment spattering a yellow base. She hoped to explain how genes produced this color variation.

McClintock’s explanation would challenge geneticists’ understanding of how the genome works. She discovered genetic elements that could move: They could excise themselves from one location and insert themselves into another on the same chromosome or a different one. Sometimes, these genetic acrobats would jump into the middle of a purple pigment gene and interfere with its function, producing a speckled cell.