IN CONTEXT
Biology
1928 Frederick Griffith shows that one strain of bacteria can be transformed into another, by the transfer of what is later found to be DNA.
1946 Joshua Lederberg and Edward Tatum discover the natural exchange of genetic material in bacteria.
1959 Tomoichiro Akiba and Kunitaro Ochia report that antibiotic-resistant plasmids (rings of DNA) can move between bacteria.
1993 American geneticist Margaret Kidwell identifies instances where genes have crossed species boundaries in complex organisms.
2008 American biologist John K Pace and others present evidence of horizontal gene transfer in vertebrates.
The continuity of life – the growth, reproduction, and evolution of organisms – is widely seen as a vertical process, driven by genes passed down from parents to offspring. But in 1985, American microbiologist Michael Syvanen proposed that, rather than being simply passed down, genes could also be passed horizontally between species, independently of reproduction, and that horizontal gene transfer (HGT) plays a key role in evolution.
Back in 1928, British physician Frederick Griffith was studying the bacteria implicated in pneumonia. He found that a harmless strain could be made dangerous simply by mixing its living cells with the dead remnants of a heat-killed virulent one. He attributed his results to a transforming “chemical principle” that had leaked from the dead cells into the living ones. A quarter of a century before DNA’s structure was unlocked by James Watson and Francis Crick, Griffith had found the first evidence that DNA could pass horizontally between cells of the same generation, as well as vertically between generations.
In 1946, American biologists Joshua Lederberg and Edward Tatum demonstrated that bacteria exchange genetic material as part of their natural behaviour. In 1959, a team of Japanese microbiologists led by Tomoichiro Akiba and Kunitaro Ochia showed that this kind of DNA transfer explains how resistance to antibiotics can spread through bacteria so quickly.
Transforming microbes
Bacteria have small, mobile rings of DNA called plasmids that pass from cell to cell when they come into direct contact – taking their genes with them. Some bacteria contain genes that make them resist the action of certain types of antibiotics. The genes are copied whenever the DNA replicates, and can spread through a population of bacteria as the DNA is transferred.
This sort of horizontal gene transfer can also happen via viruses, as Lederberg’s student Norton Zinder discovered. Viruses are even smaller than bacteria and can invade living cells – including bacteria. They may interfere with the host genes, and when they move from host to host, they may take host genes with them.
"The flow of genes between different species represents a form of genetic variation whose implications have not been fully appreciated."
Michael Syvanen
Genes for development
From the mid-1980s, Syvanen set HGT in a wider context. He noted similarities in how the development of embryos is genetically controlled at a cellular level – even between distantly related species – and attributed this to genes moving between different organisms in evolutionary history. He argued that the genetic control of animal development had evolved to be similar in different groups because this maximized the chances that gene-swapping would work.
As genome sequences are completed for more species, and as the fossil record is re-examined, evidence suggests that HGT may occur in not only microbes but also more complex organisms, in both plants and animals. Darwin’s tree of life may look more like a net, with multiple ancestors rather than a last universal common ancestor. With potential implications for taxonomy, disease and pest control, and genetic engineering, HGT’s full significance is still unfolding.
DNA plasmids, coloured blue on this micrograph, are independent of a cell’s chromosomes, yet they can replicate genes and be used to insert new genes into organisms.
MICHAEL SYVANEN
Michael Syvanen trained in chemistry and biochemistry at the universities of Washington and Berkeley, California, before going on to specialize in the field of microbiology. He was appointed professor of microbiology and molecular genetics at Harvard Medical School in 1975, where he undertook research in the development of antibiotic resistance in bacteria, and insecticide resistance in flies. His findings led him to publish his theory of horizontal gene transfer (HGT) and its role in adaptation and evolution.
Since 1987, Syvanen has been professor of medical microbiology and immunology at the School of Medicine in the University of California at Davis.
Key works
1985 Cross-species Gene Transfer: Implications for a New Theory of Evolution
1994 Horizontal Gene Transfer: Evidence and Possible Consequences