Transposable elements as drivers of genomic and biological diversity in vertebrates.

Comparative genomics has revealed that major vertebrate lineages contain quantitatively and qualitatively different populations of retrotransposable elements and DNA transposons, with important differences also frequently observed between species of the same lineage. This is essentially due to (i) the differential evolution of ancestral families of transposable elements, with evolutionary scenarios ranging from complete extinction to massive invasion; (ii) the lineage-specific introduction of transposable elements by infection and horizontal transfer, as exemplified by endogenous retroviruses; and (iii) the lineage-specific emergence of new transposable elements, as particularly observed for non-coding retroelements called short interspersed elements (SINEs). During vertebrate evolution, transposable elements have repeatedly contributed regulatory and coding sequences to the host, leading to the emergence of new lineage-specific gene regulations and functions. In all vertebrate lineages, there is evidence of transposable element-mediated genomic rearrangements such as insertions, deletions, inversions and duplications potentially associated with or subsequent to speciation events. Taken together, these observations indicate that transposable elements are major drivers of genomic and biological diversity in vertebrates, with possible important roles in speciation and major evolutionary transitions.