Published online by Cambridge University Press: 06 August 2009
Molecular biologists have long used viruses, plasmids, transposons, and other “vectors” as tools to directly manipulate the genetic makeup of experimental organisms. In nature, these tool vectors originated in species, usually bacteria, as facilitators of horizontal (also known as lateral) gene transfer (HGT). In contrast to vertical inheritance, where the transmission of genetic material occurs vertically from parent to offspring, HGT refers to the horizontal exchange of genes between distantly related strains and species. As described in this volume, there are many examples of HGT between species of bacteria, such as that mediated by plasmids and phages, which bear genes responsible for pathogenicity and antibiotic resistance. HGT is also known to occur in eukaryotes; for example, DNA transposons have been suggested as being horizontally transferred between different species of the fruitfly Drosophila (Bushman, 2002). These are examples of HGT on a relatively recent evolutionary timescale. However, HGT might have had a pivotal evolutionary role in more ancient times. Comparative analyses of molecular data that are exploding from genome sequencing projects indicates that HGT might have been the main driving force behind the evolution of cellular life (Brown, 2003).
The reason for believing the occurrence of ancient HGT is relatively simple. In an evolutionary context, genes are not found where they are expected to be. The most fundamental subdivisions of living organisms are the three urkingdoms or domains of life: the Archea (traditionally called “archaebacteria”), Bacteria (traditionally called “eubacteria”), and Eucarya (interchangeable here and elsewhere with the term “eukaryote”; Woese, Kandler, and Wheelis, 1990).
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