horizontal gene transfer
/ hor-ih-ZON-tul JEEN TRANS-fer /
We usually think of genes as flowing straight down a family tree: parents pass them to children, who pass them to grandchildren. That top-to-bottom flow is called vertical inheritance. But microbes have another, sideways route. A bacterium can pick up genes from a neighbor that is not its parent or its offspring — sometimes from a completely different species. That sideways jump is horizontal (or lateral) gene transfer.
Horizontal gene transfer happens by three main routes. In conjugation, a donor cell passes DNA (often a plasmid) directly through a bridge to a recipient. In transformation, a cell simply scoops up loose DNA fragments floating in its surroundings, often released by dead cells. In transduction, a virus that infects bacteria accidentally carries a piece of one cell's DNA into the next cell it infects. By any of these routes, a useful gene can spread through a population far faster than waiting for it to be inherited generation by generation.
Horizontal gene transfer is one of the biggest forces shaping the microbial world. It is the main reason antibiotic resistance spreads so quickly — a resistance gene can hop between unrelated bacteria in days. It also blurs the neat 'tree of life,' since organisms can acquire genes from distant relatives, making microbial evolution look more like a tangled web than a clean tree. The important takeaway: for bacteria and archaea, evolution is not only about who your ancestors were, but also about what genes your neighbors are willing to share.
The gene that lets some gut bacteria digest a kind of seaweed sugar appears to have jumped, by horizontal transfer, from ocean microbes into the gut bacteria of people in Japan who historically ate a lot of nori. Their microbes literally borrowed a tool from the sea.
A seaweed-digesting gene that jumped sideways from ocean microbes into human gut bacteria.
Horizontal transfer moves genes sideways between coexisting organisms; it is not the same as ordinary inheritance from parent to offspring. It is common in microbes but rare in animals and plants.