| Horizontal Gene Transfer |
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"Increasingly, studies of genes and genomes are indicating that considerable horizontal transfer has occurred between Prokaryote s." Horizontal gene transfer is called by some "A New Paradigm for Biology " [http://www.esalenctr.org/display/confpage.cfm?confid=10&pageid=105&pgtype=1 and emphasised by others as an important factor in "The Hidden Hazards of Genetic Engineering". "While horizontal gene transfer is well-known among bacteria, it is only within the past 10 years that its occurrence has become recognized among higher plants and animals. The scope for horizontal gene transfer is essentially the entire biosphere, with bacteria and viruses serving both as intermediaries for gene trafficking and as reservoirs for gene multiplication and recombination (the process of making new combinations of genetic material)." [http://online.sfsu.edu/~rone/GEessays/horizgenetransfer.html]. PROKARYOTES Horizontal gene transfer is common among Bacteria , even very distantly-related ones. This process is thought to be a significant cause of increased Drug Resistance ; when one bacterial cell acquires resistance, it can quickly transfer the resistance genes to many species. Enteric bacteria appear to exchange genetic material with each other within the Gut in which they live. There are three common mechanisms for horizontal gene transfer:
EUKARYOTES Analysis of DNA Sequence s suggests that horizontal gene transfer has also occurred within Eukaryote s, from their chloroplast and mitochondrial genome to their nuclear genome. As stated in the Endosymbiotic Theory , chloroplasts and mitochondria probably originated as bacterial Endosymbiont s of a progenitor to the eukaryotic cell. Horizontal transfer of genes from bacteria to some Fungi , especially the yeast '' Saccharomyces Cerevisiae '' has been well documented. There is also recent evidence that the Adzuki Bean Beetle has somehow acquired genetic material from its (non-beneficial) endosymbiont '' Wolbachia ''. "Sequence comparisons suggest recent horizontal transfer of many Gene s among diverse Species including across the boundaries of Phylogenetic "domains". Thus determining the phylogenetic history of a species can not be done conclusively by determining evolutionary trees for single genes." {Link without Title} Another exemple is the hemoglobin family: Fabaceae are the only plants to produce a oxygen-fixer hemoprotein closely resembling Hemoglobin , called Leghemoglobin . It is thought to result from an HGT, possibly through bacteria. EVOLUTIONARY THEORY Horizontal gene transfer is a potential Confounding Factor in inferring Phylogenetic Tree s based on the Sequence of one Gene . For example, given two distantly related bacteria that have exchanged a gene, a Phylogenetic Tree including those species will show them to be closely related because that gene is the same, even though most other genes have substantially diverged. For this reason, it is often ideal to use other information to infer robust phylogenies, such as the presence or absence of genes, or, more commonly, to include as wide a range of genes for phylogenetic analysis as possible. For example, the most common gene to be used for constructing phylogenetic relationships in Prokaryote s is the 16s RRNA gene, since its sequences tend to be conserved among members with close phylogenetic distances, but variable enough that differences can be measured. However, in recent years it has also been argued that 16s rRNA genes can also be horizontally transferred. Although this may be infrequent, validity of 16s rRNA-constructed phylogenetic trees must be reevaluated. Biologist Gogarten suggests "the original metaphor of a tree no longer fits the data from recent genome research" therefore "biologists use the metaphor of a mosaic to describe the different histories combined in individual genomes and use [the metaphor of a net to visualize the rich exchange and cooperative effects of HGT among microbes." [http://www.esalenctr.org/display/confpage.cfm?confid=10&pageid=105&pgtype=1] "Using single Gene s as Phylogenetic Marker s, it is difficult to trace organismal Phylogeny in the presence of HGT gene transfer . Combining the simple Coalescence model of Cladogenesis with rare HGT gene transfer events suggest there was no single Last Common Ancestor that contained all of the genes ancestral to those shared among the three domains of Life . Each contemporary Molecule has its own history and traces back to an individual molecule Cenancestor . However, these molecular ancestors were likely to be present in different organisms at different times." [http://web.uconn.edu/gogarten/articles/TIG2004_cladogenesis_paper.pdf] ''Uprooting the Tree of Life'' by W. Ford Doolitte ('' tree based upon the encoding for the Enzyme HMGCoA Reductase - the organism in question is a definite Archaean, with all the cell lipids and transcription machinery that are expected of an Archaean, but whose HMGCoA genes are actually of bacterial origin. Again on p.76, the article continues with: : "The weight of evidence still supports the likelihood that Mitochondria in Eukaryote s derived from alpha-proteobacterial cells and that Chloroplast s came from ingested Cyanobacteria , but it is no longer safe to assume that those were the only lateral gene transfers that occurred after the first eukaryotes arose. Only in later, multicellular eukaryotes do we know of definite restrictions on horizontal gene exchange, such as the advent of separated (and protected) Germ Cell s." The article continues with: :"If there had never been any lateral gene transfer, all these individual gene trees would have the same topology (the same branching order), and the ancestral genes at the root of each tree would have all been present in the last universal common ancestor, a single ancient cell. But extensive transfer means that neither is the case: gene trees will differ (although many will have regions of similar topology) ''and'' there would never have been a single cell that could be called the last universal common ancestor. :"As Woese has written, 'the ancestor cannot have been a particular organism, a single organismal lineage. It was communal, a loosely knit, diverse conglomeration of primitive cells that evolved as a unit, and it eventually developed to a stage where it broke into several distinct communities, which in their turn became the three primary lines of descent ( Bacteria , Archaea and Eukaryote s)' In other words, early cells, each having relatively few genes, differed in many ways. By swapping Gene s freely, they shared various of their talents with their contemporaries. Eventually this collection of eclectic and changeable cells coalesced into the three basic domains known today. These domains become recognisable because much (though by no means all) of the gene transfer that occurs these days goes on within domains." SEE ALSO REFERENCES
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