Leptothrix
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LATERAL GENE TRANSFER Hans-Joachim Fritz University of Göttingen, Göttingen, Germany
Synonyms Horizontal gene transfer Definition The use of the term in the literature is not entirely unambiguous. For the purpose of this entry, lateral gene transfer (LGT) is defined as the stable transfer of genetic material (DNA, possibly RNA) from one organism to another where the two organisms belong to lineages which are genetically too distant to allow a mutual act of homologous recombination. Introduction Vegetative proliferation is characterized by a vertical line of genetic transfer from one cell to two daughter cells. Upon zygote formation, in contrast, sexual reproduction mixes the genetic outfit of two parent organisms. Fertile offspring, however, is only produced if the two parents are genetically close enough to allow for gamete formation (i.e., meiosis with mandatory homologous recombination) in the offspring. This limit defines a breeding pool which, in turn, can serve as a useful approach to the term “species.” In the bacterial and the archaeal domains of life, the species concept is much more contentious – for reasons partly illustrated by this article. Nevertheless, the homologous recombination criterion can be upheld at least in cases like Escherichia coli, in which genetic traits are transferred by conjugation between two pre-existing cells. Conjugation proceeds by transient cell fusion between an Hfrþ and an Hfr cell, followed by physical transfer of
DNA from the former to the latter, followed by homologous recombination. Natural transformation in Streptococcus pneumoniae provides is a similar example. Against this backdrop, LGT can be viewed as any transfer of genetic material between two organisms that are separated by the homologous recombination barrier. This criterion separates, for example, E. coli from the closely related Salmonella typhimurium; in cases with no known system of homologous recombination, the criterion has to be replaced by corresponding and somewhat arbitrary measures such as sequence distance. There are two hallmarks of LGT which may be detected simultaneously or alternatively in any particular single case. 1. Presence of genes in a genome that are not present in genomes of otherwise closely related organisms. 2. Topologically different branching patterns obtained in attempts to delineate phylogenetic relationships between a host of lineages, based on sequence comparisons within more than one set of (seemingly) orthologous genes. Other, less clear-cut indicators of LGT include G/C content and codon usage that deviate from those of the bulk genome and also physical gene linkages that differ from those observed with close relatives. These statements together make it immediately understandable why the intensity of the debate of LGT developed in parallel with the unfolding of the genome analysis era that followed the publication of the Haemophilus influenzae sequence in 1995 (Fleischmann et al., 1995). To date, several hundred genomic sequences have been determined – mostly of bacteria an
