A Brief History of the Status of Transposable Elements: From Junk DNA to Major Players in Evolution

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L’influence des éléments transposables sur la taille des génomes

La taille des génomes varie fortement entre espèces sans lien direct avec leur complexité. Ce paradoxe, connu sous le nom de « paradoxe de la valeur C », est maintenant expliqué par une abondance différentielle de nombreuses séquences répétées, dont les éléments transposables. Les génomes sont ainsi constitués d’une forte proportion de telles séquences (95 % de l’ADN chez l’Homme dont environ 45 % d’éléments transposables, jusqu’à 99 % de l’ADN chez certaines plantes). Alors que les recherches se sont jusqu’à présent focalisées sur les gènes, ou séquences codantes, qui ne représentent qu'une faible proportion des génomes, notre attention se concentre de plus en plus sur les séquences dites non-codantes. Les éléments transposables, qui sont capables de mouvements dans les génomes, provoquant des mutations, réarrangements chromosomiques, régulations de l’expression des gènes, apparaissent alors comme les éléments majeurs de diversité et d’évolution. Nous présentons une brève revue des connaissances marquantes de ce domaine en pleine expansion

Sequence Divergence Within Transposable Element Families in the Drosophila melanogaster Genome

The availability of the sequenced Drosophila melanogaster genome provides an opportunity to study sequence variation between copies within transposable element families. In this study,we analyzed the 624 copies of 22 transposable element (TE) families (14 LTR retrotransposons, five non-LTR retrotransposons, and three transposons). LTR and non-LTR retrotransposons possessed far fewer divergent elements than the transposons,suggesting that the difference depends on the transposition mechanism. However,there was not a continuous range of divergence of the copies in each class,which were either very similar to the canonical elements,or very divergent from them. This sequence homogeneity among TE family copies matches the theoretical models of the dynamics of these repeated sequences. The sequenced Drosophila genome thus appears to be composed of a mixture of TEs that are still active and of ancient relics that have degenerated and the distribution of which along the chromosomes results from natural selection. This clearly demonstrates that the TEs are highly active within the genome,suggesting that the genetic variability of the Drosophila genome is still being renewed by the action of TEs

The Dynamics of Transposable Elements in Structured Populations

We analyzed the dynamics of transposable elements (TEs) according to Wright's island and continent-island models, assuming that selection tends to counter the deleterious effects of TEs. We showed that migration between host populations has no impact on either the existence or the stability of the TE copy number equilibrium points obtained in the absence of migration. However, if the migration rate is slower than the transposition rate or if selection is weak, then the TE copy numbers in all the populations can be expected to slowly become homogeneous, whereas a heterogeneous TE copy number distribution between populations is maintained if TEs are mobilized in some populations. The mean TE copy number is highly sensitive to the population size, but as a result of migration between populations, it decreases as the sum of the population sizes increases and tends to reach the same value in these populations. We have demonstrated the existence of repulsion between TE insertion sites, which is established by selection and amplified by drift. This repulsion is reduced as much as the migration rate is higher than the recombination rate between the TE insertion sites. Migration and demographic history are therefore strong forces in determining the dynamics of TEs within the genomes and the populations of a species