What transposable elements tell us about genome organization and evolution: the case of Drosophila.

Transposable elements (TEs) have been identified in every organism in which they have been looked for. The sequencing of large genomes, such as the human genome and those of Drosophila, Arabidopsis, Caenorhabditis, has also shown that they are a major constituent of these genomes, accounting for 15% of the genome of Drosophila, 45% of the human genome, and more than 70% in some plants and amphibians. Compared with the 1% of genomic DNA dedicated to protein-coding sequences in the human genome, this has prompted various researchers to suggest that the TEs and the other repetitive sequences that constitute the so-called "noncoding DNA", are where the most stimulating discoveries will be made in the future (Bromham, 2002). We are therefore getting further and further from the original idea that this DNA was simply "junk DNA", that owed its presence in the genome entirely to its capacity for selfish transposition. Our understanding of the structures of TEs, their distribution along the genomes, their sequence and insertion polymorphisms within genomes, and within and between populations and species, their impact on genes and on the regulatory mechanisms of genetic expression, their effects on exon shuffling and other phenomena that reshape the genome, and their impact on genome size has increased dramatically in recent years. This leads to a more general picture of the impact of TEs on genomes, though many copies are still mainly selfish or junk DNA. In this review we focus mainly on discoveries made in Drosophila, but we also use information about other genomes when this helps to elucidate the general processes involved in the organization, plasticity, and evolution of genomes.

Biémont C ，Vieira C 《-》

Amplification of the 1731 LTR retrotransposon in Drosophila melanogaster cultured cells: origin of neocopies and impact on the genome.

Transposable elements (TEs), represent a large fraction of the eukaryotic genome. In Drosophila melanogaster, about 20% of the genome corresponds to such middle repetitive DNA dispersed sequences. A fraction of TEs is composed of elements showing a retrovirus-like structure, the LTR-retrotransposons, the first TEs to be described in the Drosophila genome. Interestingly, in D. melanogaster embryonic immortal cell culture genomes the copy number of these LTR-retrotransposons was revealed to be higher than the copy number in the Drosophila genome, presumably as the result of transposition of some copies to new genomic locations [Potter, S.S., Brorein Jr., W.J., Dunsmuir, P., Rubin, G.M., 1979. Transposition of elements of the 412, copia and 297 dispersed repeated gene families in Drosophila. Cell 17, 415-427; Junakovic, N., Di Franco, C., Best-Belpomme, M., Echalier, G., 1988. On the transposition of copia-like nomadic elements in cultured Drosophila cells. Chromosoma 97, 212-218]. This suggests that so many transpositions modified the genome organisation and consequently the expression of targeted genes. To understand what has directed the transposition of TEs in Drosophila cell culture genomes, a search to identify the newly transposed copies was undertaken using 1731, a LTR-retrotransposon. A comparison between 1731 full-length elements found in the fly sequenced genome (y(1); cn(1)bw(1), sp(1) stock) and 1731 full-length elements amplified by PCR in the two cell line was done. The resulting data provide evidence that all 1731 neocopies were derived from a single copy slightly active in the Drosophila genome and subsequently strongly activated in cultured cells; and that this active copy is related to a newly evolved genomic variant (Kalmykova, A.I., et al., 2004. Selective expansion of the newly evolved genomic variants of retrotransposon 1731 in the Drosophila genomes. Mol. Biol. Evol. 21, 2281-2289). Moreover, neocopies are shown to be inserted in different sets of genes in the two cell lines suggesting they might be involved in the biological and physiological differences observed between Kc and S2 cell lines.

Maisonhaute C ，Ogereau D ，Hua-Van A ，Capy P ... -  《GENE》

Transposable elements in gene regulation and in the evolution of vertebrate genomes.

Repetitive DNA and in particular transposable elements have been intimately linked to eukaryotic genomes for millions of years. Once overlooked for being only a collection of selfish debris and a nuisance for sequence assembly, genomic repeats are now being recognized as a key driving force in genome evolution. Indeed, by changing the DNA landscape of genomes, transposable elements have been a rich source of innovation in genes, regulatory elements and genome structures. In this review, I will focus on recent advances that demonstrate that genomic repeats have had a global impact on vertebrate gene regulatory networks. I will also summarize results that show how transposable elements have been a major catalyst of structural rearrangements throughout evolution.

Bourque G 《-》

The Trickster in the genome: contribution and control of transposable elements.

In mythology, the Trickster is an archetype who typically behaves selfishly and delights in playing tricks and breaking ordinary rules. In many myths and folktales, however, the Trickster also brings new knowledge and, ultimately, has positive effects on the community. Transposable elements (TEs) might have played such a role in the story of genome evolution. TEs can cause nonroutine genetic events like insertional mutations and ectopic recombination that provide a fundamental source of genetic variation, but they can also be a potential threat to genome integrity. Thus, the activity of TEs is usually controlled by an array of sophisticated mechanisms for genome defense. Recent findings indicate that TEs are important components of eukaryotic genomes, often to a much larger extent than ever anticipated. In this review, I focus on the contributions of TEs to various aspects of genome evolution. In addition, why TEs are specific targets for the genome defense mechanisms is discussed.

Nakayashiki H 《-》

Transposable elements and genome evolution: the case of Drosophila simulans.

Biémont C ，Vieira C ，Borie N ，Lepetit D ... -  《GENETICA》