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R E V I E W S
Horizontal gene transfer: building the web of life
Shannon M.Soucy1, Jinling Huang2 and Johann Peter Gogarten1,3
Abstract | Horizontal gene transfer (HGT) is the sharing of genetic material between organisms that are not in a parentoffspring relationship. HGT is a widely recognized mechanism for adaptation in bacteria and archaea. Microbial antibiotic resistance and pathogenicity are often associated with HGT, but the scope of HGT extends far beyond disease-causing organisms. In this Review, we describe how HGT has shaped the web of life using examples of HGT among prokaryotes, between prokaryotes and eukaryotes, and even between multicellular eukaryotes. We discuss replacement and additive HGT, the proposed mechanisms of HGT, selective forces that influence HGT, and the evolutionary impact of HGT on ancestral populations and existing populations such as the human microbiome.
Horizontal gene transfer (HGT) was first described in microorganisms in the late 1940s1, and around 20years later it was speculated to have a role in the adaptation of multicellular eukaryotes specifically plants2. Since then, methods to detect HGT have improved, and these have revealed the surprising extent and relevance of HGT to the variation of viral, prokaryotic and eukaryotic gene content. Many apparent gene duplications, for example, are now known to be the result of HGT, not autochthonous gene duplication, resulting in a web of life rather than in a steadily bifurcating tree3,4.
For a transferred gene to survive in the recipient lineage for long periods of time, the gene usually needs to provide a selective advantage either to itself (in the case of a selfish genetic element) or to the recipient, and research on HGT initially focused on such genes. However, it is now known that many of the genes that have been identified as transferred through comparative genomics between close relatives have neutral or nearly neutral effects in the recipient in both prokaryotic and eukaryotic organisms5. One rule for transferred genes seems to be first do no harm genes that are successfully integrated into a recipient are often expressed at low levels and encode functions at the periphery of metabolism6. These neutral acquisitions, however, can later provide novel combinations of genetic material for selection to act on in some cases, the transferred material becomes domesticated over...