Horizontal Gene Transfer - Essay Example

?Horizontal Gene Transfer Horizontal Gene Transfer (HGT), also known as Lateral Gene Transfer (LGT), is the process by which genetic material from one organism is passed across to another organism which is not an offspring of that organism. In other words, genetic information is exchanged between distantly-related or even non-related organisms by means other than sexual reproduction. This process is in contrast to the vertical descent of genes where genetic information is passed down to the offspring by conventional heredity mechanisms i.e. sexual and asexual reproduction and only within species or between closely- related individuals. HGT is observed mainly between prokaryotes (single-celled organisms) like bacteria but also can be in Eukaryotes (protists, fungi, plants, animals, humans) to a lesser extent. HGT was first described by Ochiai et al. in 1959, in relation to transfer of antibiotic resistance between different bacterial species. Thereafter many scientists have studied and discussed about this phenomenon and now it is accepted that HGT is not a rare event and what is present in the biological world today is not a result of vertical gene transmission alone but also of HGT. Biological kingdom of earth can be categorized into three domains as bacteria, archea and eukaryotes. During vertical gene transmission, genetic information exchange between members of these domains is restricted to closely-related organisms with homologous DNA sequences. Therefore the ancestry of an organism could be traced back by analyzing their DNA and individuals with similar gene sequences can be grouped together to construct the evolutionary pathway. However, as HGT can transfer genes across wide phylogenetic distances, this picture become obscure. The phylogenetic tree of life (Figure 1) which present the relationship among different biological taxa is thus complicated by HGT as numerous interconnecting branches became obvious due to the presence of homologous DNA sequences in distantly-related organisms (Simonson et al.). Transfer of genes between different biological kingdoms, such as between eucaryotes and bacteria, or between bacteria and insects are some extreme cases of gene exchange between wide phylogenetic distances. An example is the presence of bacterial 'rol' genes from Agrobacterium species in tobacco (Nicotiniana) plants (Intrieri and Buiatti). Figure 1. Phylogenetic tree of life as proposed by Carl Woese In vertical gene transmission, genetic variability of a species mainly arises during meiosis by recombination of genes. According to Mendal’s law of inheritance, alleles of different genes assort independently during gamete formation and thereby produce novel gene combinations which in turn generate variability. Mutations, which are the random changes of gene sequence of a DNA strand, are considered as an error in the vertical gene transmission process. It can happen as a point mutation, chromosomal duplication, breaking and rearrangement of chromosomes and addition or deletion of chromosomes. When such mutant is passed down to the next generation, it is not a mutant anymore, but a variant or a novel type. The variants who survive according to Darwin’s theory of “survival of the fittest”, subsequently pass the novel characters to their offspring by sexual or asexual reproduction. HGT however, is not a mutation and is not involved in gamete formation or sexual reproduction. It moves transposable elements between distantly-related or unrelated genomes and thereby intervenes in the process of evolution by originating different cell types and creating variants in its own way. Two hypotheses are presented here as ‘Continual horizontal transfer hypothesis’ and ‘Early massive horizontal transfer hypothesis.’ (Jain, Rivera and Lake). According to the ‘Continual horizontal transfer hypothesis’, HGT is a continuous process during prokaryotic evolution whereas in the ‘Early massive horizontal transfer hypothesis’, a massive exchange of a few operational genes (genes involved in housekeeping) must have occurred early in prokaryotic evolution before diversification of the modern prokaryotes. As reported by Engels in 1996, (Krishnapillei 224), the P transposon element was present in Drosophila melanogaster for less than 100 years and it has horizontally transferred from another species, D. willistoni indicating that HGT is a continuous process. Natural horizontal gene transfer can happen between different species of bacteria, between bacteria and fungi, between bacteria and plants or bacteria and animals, and is mediated by parasites, epiphytes, virus, mites etc. or by direct cell-to-cell gene transfer. Although the amount of cells thus modified via HGT may be a few in numbers, it can be a significant factor on an evolutionary scale. Genetic engineering activities performed in the laboratories transfer genes horizontally in an artificial manner. Natural ability of the soil bacteria Agrobacterium tumefaciens and A. rhizomatis to transfer their Ti and Ri plasmids respectively into dicot plants is widely used in genetic engineering to produce genetically-modified organisms (GMOs). In prokaryotes, HGT occurs more in smaller operational genes than in informational genes (genes involved in transcription and translation) which are members of large and complex systems. HGT occurs in prokaryotes through ‘bacterial transformation’ (direct uptake of extracellular DNA), ‘transduction’ (transfer of bacterial DNA from one bacterium to another by a phage virus) and ‘bacterial conjugation’ (cell-to-cell contacts) (Krishnapillai 226). Short DNA fragments are transferred during transformation, whereas larger DNA fragments are transferred between more distantly-related species of bacteria or between bacteria and eukaryotic cells during conjugation. Transduction is limited to gene transfer between close relations as they need to have common cell surface receptors. In higher plants, HGT happens through bacterial infections (as in Agrobacterium-mediated transformation) or by ‘jumping genes’ (mobile DNA) as observed by Diao Xianmin, Micheal Freeling and Damon Lisch in rice and millet (“Jumping genes cross Plant Species Boundaries.”). Mitochondria in cells play a significant role in horizontal gene transfer than chloroplast and nuclear genomes. However, transfer of rubisco genes (rbcL, rbcS) from bacteria to red algae and transfer of a gene for enzyme phosphoglucose isomrase from Poa genus to sheep’s fescue Festuca ovina are the results of chloroplast and nuclear genomes involvements in HGT respectively (“Horizontal gene transfer”). Generating new types of plants, animals and useful pathogens are some of the beneficial effects of HGT. Whilst vertical descent of genes is responsible for passing genetic information down the generations (parents to the offspring), it is the HGT which exchange various information across far distances of the phylogeny and modify the genome. Therefore HGT can be considered as a mechanism which accelerate novel genetic combinations and thereby increase the genetic diversity of an organism. Conversely, HGT can initiate some deleterious effects on the environment as well as to the life on earth. Developing antibiotic - resistant bacterial pathogens make antibiotic treatment of diseases ineffective. By cross-pollination with transgenic plants, by cross-breeding with transgenic animals and through bees carrying pollen of transgenic plants, there is always the danger of spreading transgenic DNA to wider distances and affects a greater proportion of living organisms. As HGT is designed to cross all genetic barriers and travel widely across genomes, it may develop new types of viruses and bacteria that cause infectious untreatable diseases. Therefore it is essential to have a control over HGT. Breaking down or inactivating foreign genetic material by chemical modification and enzymatic digestion of the foreign DNA material is a natural mechanism that prevent deleterious DNA fragments entering into cell genomes (Krishnapillei 227). Host specificity of the protein coat of the attacking virus creates a species barrier which limits the number of hosts that a virus could attack and cell wall in plant cells efficiently prevent entering of foreign genetic material into the plant cells. However, as genetic engineering protocols in laboratories can open up plant cell walls and insert DNA efficiently, it is important to control the genetic modification activities of humans with firm rules and regulations as it can lead to a disastrous end. Increasing evidences imply that horizontal gene transfer is not a new activity, nor an error or a mutant, but a definite biological process that has being active simultaneously with vertical gene transmission, perhaps from the beginning of life. Even though the precise role of HGT is still uncertain, the objective might be to increase genetic diversity, to increase the chance of survival of an organism by providing some ‘extra’ genes or, most importantly, since horizontal gene transfer is mainly active in prokaryotes who are the early form of living organisms, HGT might be the section of the evolutionary process dealing with creation of exclusively novel gene combinations to ensure future life on earth. Works Cited “Horizontal gene transfer.” (15 Feb 2010). Citizendium. [Accessed 08 April 2011] Intrieri M C and M Buiatti. “The horizontal transfer of Agrobacterium rhizogenes genes and the evolution of the genus Nicotiana.” Mol. Phylogen. Evol. 20.1(2001):100-10. PubMed.gov. [Accessed 10April 2011] Jain, Ravi, Maria C. Rivera and James A. Lake. “Horizontal gene transfer among genomes: the complexicity hypothesis.” Proc Natl Acad Sci USA. 96.7(1999):3801 – 06. PubMed.gov. [Accessed 10 April 2011] “Jumping genes cross Plant Species Boundaries.” PLoS Biology 4.1(2006): e35.doi:10.1371/journal.pbio.0040035. PLos Biology.org. [Accessed 06 April 2011] Krishnapillai, Viji. “Horizontal Gene Transfer: Review Article.” J. Genet. 75.2 (1996):219 – 32 Ochiai K, T. Yamanaka, K. Kimura and O. Sawada. “Inheritance of drug resistance (and its transfer) between Shigells strains and between Shigella and E. coli strains (in Japanese).” Hihon Iji Shimpor 1861(1959):34. Wikipedia. 04 April 2011. [Accessed 05 April 2011] Simenson Anne B., J. A. Servin, R.G. Skophammer, C.W. Herbold, M.C. Rivera and J. A. Lake. “Decoding the genomic tree of life.” Proc. Natl. Sci. Acad. USA. 102 Suppl. 1(2005):6608 – 13. PubMed.gov [Accessed 11 April 2011] Woese, Carl, O. Kandler and M Wheels. “Towards a natural system of organisms: proposal for the domains Archaea, Bacteria and Eucarya.”Proc Natl Acad Sci USA. 87.12 (1990):4576 – 9. Carl Woes:wikis [Accessed 07 April 2011] Read More