GENETIC VERSITY AS A TRANSPOSABLE ELEMENT:

In 1970’s the emergence of hybrid agencies phenomenon namely Drosophila melangogaster led to various genetics changes, including sterility and increased mutation and recombination rates.

It was some year before these effects were finally shown to be associated with the mobilization of specific TEs P element and I elements (See Picard et al 1978 Kidwell 1979: Rubin et al 1982: Engel 1988). The element P shown in the DNA transposons and I was non-long terminals repeat (LTR) (Finnegan 1992).

LTR retrotransposons, had been dived into various sub-families TYI-Copia-like(Pseudoviridae) TY3-gypsy-like(Metaviride) and Pao-BEL-like depending on their sequence similarity and order of the gene products that they encode.

The terms Dnieper transposons is used for the DNA based TE’s and the term “retrotranposons” either with or without a long terminal repeat at their extremities) for the RNA-based TEs.(Wicker et al 2007 and Kapitovo and Jurka 2008 proposed new TE classification based on sequences similarities and structural relationships to include the new TE classes that have emerged.

The P and I elements which were discovered made us realize that they were able to move within the genome. These P and I elements have gradually invaded in all known population o D. Melanogaster within the span of 50 years Canxolabehere et al 1988)

50 years is a very short period for the evolution but enough for population genetics .The mobilization of the P and I elements as a result of crosses have confirmed the idea that TEs are selfish they produce only detrimental effects on organisms (Doolitte and Sapienza 1980) . This idea showed the significant of evolution of TE’s. Russian scientists referred the first TEs of Drosophila as, mobile dispersed genes”

In the next phase of exploration chromosomal insertions were localized to visualize on the polygene chromosomes of Drosophila, using SIFU hybridization. The Drosophila salivary gland polythene chromosomes showed strong labeling in a specific band in one chromosome but no labeling on its homolog (the two chromosomeshad been separated on the squash as sometime happens). The ’in silu” hybridization technique led to estimate the number of copies of various TEs and detection of the polymorphism of their insertion in different strains and population. Such polymorphism provided diverts evidence of the continuing mobility of these elements.

The parameter of tranposans involved are excision rates, selection against the insertion of the TEs and effective population size- (Charles worth Charlesworth 1983) The population based models were imperfect and once simplified, because TE dynamics of asymptotic values with little understanding to them because of copy number our long stretches of time. But these models were useful for scientists to reanalyze their data, to estimate and reconsider the parameters that they were using in the models and new protocols’.

“TE copies (the ectopic exchange model of Langley et al 1988) or the slightly elrection effects of TE insertions that reduce host fitness with (Charlesworthet all 1977) and humans (Song and Boissinot 2006) in which the reproductive system (Hickely 1982: Dolg in et al 2008) demography(Cock et al 2008) and population size (Lynch and Contery 2003) seem to play major roles.

It was debatable that TEs rose from retroviruses, because Retroviruses and the members of the LTR retro-transpose family had a common ancestors. TEs could not be in par with viruses because, it was easier to recognize and admit the idea of presence of thousands of copies of junk DNA actually inside our human genome. Some infectious retroviruses were discovered in Drosophila (Kim et al 1994, Song et al 1994) such infectious retroviruses was also identified in plants (Wright and Voytas 1998). Because infectious retro viruses can be transmitted between organisms, this greatly broadened the notion of the possible horizontal transfer of TEs.

TE,S ELEMENT AS PLAYERS IN EVOLUTION:

The interest in molecular analyse of TE’s faded between 1900–2000, because they were used as potential genetic tool only. The population approach was not really understood .Scientist were interest in mechanism by which TE activity and copy number are regulated, rather than global process such as those involved at the level of population. The large number of TEs present in these organisms was assumed that TEs were purely selfish and to envisage instead that they may have evolved toward genomic function.

TEs are considered to be integrated component of genomes, which have played a major role in evolution. The genome should be viewed as an ecological eco system in which the TE families and sub familiesn (Brookfield 2005: Mauricio 2005: Rouzic et all 2007: Venna et al 2009). As it is considered as “Controlling Elements” proposed by B.MC, Clintock. TE’s could control some genes appeared more reasonable when some insertions such as LTR possess as promoters were found in permanent positions with respect to specific genes. However this does not eliminate the possibility that some TE insertions sites may also be involved in gene network regulation and in species adaptation (Gonzalez et al 2010) The existence of TEs accounts for most of the early reports of “spontaneous” mutations observed in some natural populations or laboratory line in which” mutator genes or mutator systems” has been suggested (Demerec 1927, Duseeva 1948: Tinyakov 1939: Green 1973)

The mutational capacity of the TEs their power to regulate genetic systems and their sensitivity to environmental stress that has been shown to mobilization them. TEs could not only generate genetic polymorphism favoring population adaptation but also could promote speciation (Flavell 1982: Georgie et al 1983: MC Donald 1983: Syvanen 1984)

Do the genomes really need TE? Will be answered only if a comparative analyses of genomes of various organism and various populations of the same species are available. The growing body of genomic sequence data will provide precise information about the exact localization and precise DNA sequences of the TEs will be studied.

While analyzing the DNA sequencing in large scale it is easy to identify the new TE families which helps to explain why only some TE families have invaded certain cytosines, histone modifications and RNA interference all interdependent mechanisms associated with chromatin conformation can switch TEs on and off. These process not only explains the defending the genome against invasion by TEs and retroviruses but also in the complex interactions involved in gene radiation throughout development (Hudn et al 2010) is reported for neuronal development.

The TE has a great role in the spread of antibiotic resistance genes in bacterial population which has a great impact o human health and their role through their epigeneric activation- inactivation in the effects of the early environs (i.e nutrition, ultra violet light, temperature) to adult hood (Bulletin and Baccarelli 2010) making them significant players in the interaction between genotype and phenotype.

New modles of TE copy number dynamics have to be developed on the basis of the new epigenetic regulations that involve RNA interference (LU and Clark 2010). These modles will have their utility at the level of population which structuring populations may have a greater impact on TE dynamics then preciously imagined (Deceliere et al 2005). The number of TE sequences to be analyzed will be larger than the SILU hybridization and Southren blots (Strobel et all 1979: Charlesworth et al 1992: Beaumont et al 1994) TE family is defined s a “ Set of phylogenetically close copies that share >80% sequence identity” (Wicker et al 2008)

Due to the sequencing of the genomes of a wide variety of organisms, huge amounts of data would be analysed and will enter a cycle of complexity in which the gnome will be considered in its entirely. TEs and all other repeated sequences of the genome play a vital role in deciphering this complexity (Shapiro 2010). Finally when we tend to concentrate on the human genome and on the genome of organisms of industrial or argronoms interest. The historical model organism like Drosoprila yeast, nematodes, Arabidopsics should not be ignores, because the knowledge gained by such study and researches constitutes a wide knowledge gained by such study and researches’ constitutes a wide knowledge and information that will help us to pursue further in studying our own genome.

CONCLUSION:

TEs which were initially considered just a junk DNA slowly gained its importance when its influence on recombination rates and chromosomal rearrangements, as mutators and gene regulators their ability to be domesticated by the genome which transforms them into a new genes

It was found that TEs and epigenetics and epigenomics play a major role than one would have anticipated few years ago.

REFRENCES:

· Ananiev, E. V., and Y. V. Ilyin, 1981 A comparative study of the location of mobile dispersed genes in salivary gland and midgut polytene chromosomes of Drosophila melanogaster. Chromosoma 82: 429–435.

· Anxolabe´he`re, D., M. G. Kidwell and G. Periquet,1988 Molecular characteristics of diverse populations are consistent with the hypothesis of a recent invasion of Drosophila melanogaster by mobile P elements. Mol. Biol. Evol. 5: 252–269.

· Baldari, C. T., and F. Amaldi, 1976 DNA reassociation kinetics in relation to genome size in four amphibian species. Chromosoma 59: 13–22.

· Bie´mont, C., 2009 Are transposable elements only silenced or are they under house arrest? Trends Genet. 25: 333–334.

· Bie´mont, C., 2010 From genotype to phenotype. What do epigenomics and epigenetics tell us? Heredity 105: 1–3.

· Bie´mont, C., and C. Vieira, 2006 Junk DNA as an evolutionary force. Nature 443: 521–524.

· Bie´mont, C., A. Aouar and C. Arnault, 1987 Genome reshuffling of the copia element in a Drosophila melanogaster inbred line. Nature 329: 742–744.

· Bie´mont, C., C. Arnault, A. Heizmann and S. Ronsseray,1990 Massive changes in genomic locations of P elements in an inbred line of Drosophila melanogaster. Naturwissenschaften 77: 485–488.

· Bie´mont, C., F. Lemeunier, M. Garcia Guerreiro, J. F. Brookfield, C. Gautier et al., 1994 Population dynamics of the copia, mdg1, mdg3, gypsy and P transposable elements in a natural population of Drosophila melanogaster. Genet. Res. 63: 197–212.

· Bollati, V., and A. Baccarelli, 2010 Environmental epigenetics. Heredity 105: 105–112.

· Brookfield, J. F. Y., 2005 The ecology of the genome: mobile DNA elements and their hosts. Nat. Rev. Genet. 6: 128–136.

· Charlesworth, B., and D. Charlesworth, 1983 The population dynamics of transposable elements. Genet. Res. 42: 1–27.

· Charlesworth, B., A. Lapid and D. Canada, 1992 The distribution of transposable elements within and between chromosomes in a population of Drosophila melanogaster. I. Element frequencies and distribution. Genet. Res. 60: 103–114.

· Charlesworth, B., C. H. Langley and P. Sniegowski,1997 Transposable element distribution in Drosophila. Genetics 147: 1993–1995.

· Crain, W. R., E. H. Davidson and R. J. Britten, 1976 Contrasting patterns of DNA sequence arrangement in Apis mellifera (honeybee)and Musca domestica (housefly). Chromosoma 59: 1– 12.

· Daniels, S. B., K. R. Peterson, L. D. Strausbaugh, M. G. Kidwell and A. Chovnick, 1990 Evidence for horizontal transmission of the P transposable element between Drosophila species. Genetics

· 124: 339–355.

· Deceliere, G., S. Charles, C. Vieira and C. Bie´mont, 2005 The dynamics of transposable elements in structured populations Genetics 169: 467–474.

· Demerec, M., 1927 The behavior of mutable gene. Proceedings of the Fifth International Congress ofGenetics Inheritance, Berlin, pp. 183–193.

· Dolgin, E. S., B. Charlesworth and A.D. Cutter, 2008 Population frequencies of transposable elements in selfing and out crossing Caenorhabditis nematodes. Genet. Res. 90: 317–329.

· Doolittle, W. F., and C. Sapienza, 1980 Selfish genes, the phenotype paradigm and genome evolution. Nature 284: 601–603.

· Duseeva, N. D., 1948 on the distribution of high mutability in populations of Drosophila melanogaster. Dokl. Akad. Nauk SSSR 59:151–159.

· Engel, W. R., 1988 P elements in Drosophila, pp. 437–484 in Mobile DNA, edited by D. E. Berg and M. Howe. ASM publications, Washington, DC.

· Fedoroff, N. V., 1994 Barbara Mc Clintock ( June 16, 1902–September 2, 1992). Genetics 136: 1–10.

· Feschotte, C., 2008 Transposable elements and the evolution of regulatory networks. Nat. Rev. Genet. 9: 397–405.

· Feschotte, C., and E. J. Pritham, 2007 DNA transposons and the evolution of eukaryotic genomes. Annu. Rev. Genet. 41: 331–368.

· Finnegan, D. J., 1992 Transposable elements. Curr. Opin. Genet. Dev. 2: 861–867.

· Flavell, R., 1982 Sequence amplification, deletion and rearrangement :major sources of variation during species divergence, pp.301–323 in Genome Evolution, edited by G. Dover and R. Flavell. Academic Press, London.

· Flavell, R. B., M. D. Bennett, J. B. Smith and D. B. Smith,1974 Genome size and the proportion of repeated nucleotide sequence DNA in plants. Biochem. Genet. 12: 257–269.

· Fontdevila, A., 2005 Hybrid genome evolution by transposition. Cytogenet. Genome Res. 110: 49–55.

· Georgiev, G. P., D. A. Kramerov, A. P. Ryskov, K. G. Skryabin and E. M. Lukanidin, 1983 Dispersed repetitive sequences in eukaryotic genomes and their possible biological significance. Cold Spring Harb. Symp. Quant. Biol. 47: 1109–1121.

· Gerasimova, T. G., L. A. Obolenkova, S. L. Kiselev and N. A. Tchurikov, 1990 Participation of new mobile elements in transposition bursts in Drosophila melanogaster. Dokl. Akad. Nauk SSSR 311: 474–476.

· Gonza´lez, J., T. L. Karasov, P. W. Messer and D. A. Petrov, 2010 Genome-wide patterns of adaptation to temperate environments associated with transposable elements in Drosophila. PLoS Genet. 6: e1000905.

· Grabundzija, I., M. Irgang, L. Ma´te´s, E. Belay, J. Matrai et al.,2010 Comparative analysis of transposable element vector systems in human cells. Mol. Ther. 10: 1200–1209.

· Green, M. M., 1973 Some observations and comments on mutable and mutator genes in Drosophila. Genetics 73: 187–194.

· Gvozdev, V. A., E. S. Belyaeva, Y. V. Ilyin, I. S. Amosova and L. Z.Kaidanov, 1981 Selection and transposition of mobile dispersed genes in Drosophila melanogaster. Cold Spring Harb. Symp. Quant. Biol. 45(Pt. 2): 673–685.

· Herpin, A., I. Braasch, M. Kraeussling, C. Schmidt, E. C. Thoma et al., 2010 Transcriptional rewiring of the sex determining dmrt1 gene duplicate by transposable elements. PLoS Genet.6(2): e1000844.

· Hickey, D. A., 1982 Selfish DNA: a sexually-transmitted nuclear parasite. Genetics 101: 519–531.

· Honeybee Genome Sequencing Consortium, 2006 Insights into social insects from the genome of the honeybee Apis mellifera. Nature 443: 931–949.

· Huda, A., L. Marin˜o-Ramı´rez and I. K. Jordan, 2010 Epigenetic histone modifications of human transposable elements: genome defense versus exaptation. Mob. DNA 1: 2.International Silkworm Genome Consortium, 2008 The genome of a lepidopteran model insect, the silkworm Bombyx mori. Insect Biochem. Mol. Biol. 38: 1036–1045.

· Johnson, L. J., and P. J. Tricker, 2010 Epigenomic plasticity within populations: its evolutionary significance and potential. Heredity 105: 113–121.

· Jurka, J., and A. J. Gentles, 2006 Origin and diversification of minisatellites derived from human Alu sequences. Gene 365: 21–26.

· Kapitonov, V. V., and J. Jurka, 2005 RAG1 core and V (D)J recombination signal sequences were derived from Transib transposons. PLoS Biol. 3(6): e181.

Received on 01.08.2013 Modified on 05.09.2013

Research J. Pharm. and Tech. 6(11): November 2013; Page 1271-1273