Origins and Evolution of Retrotransposons

This chapter summarizes the similarities and differences of retrotransposons, as well as the phylogenetic relationships of the various elements that have been characterized from each of the two classes of retrotransposons. It discusses what can be inferred about the ages and origins of retroelements and their relationship to other cellular components. In the most unusual group of long terminal repeat (LTR), retrotransposons, complete elements have unusual inverted or split terminal repeats and do not encode an integrase. One of the most interesting aspects of the evolution of the LTR retrotransposons is their close relationship to retroviruses, caulimoviruses, and hepadnaviruses. The simplicity of the target-primed reverse transcription (TPRT) mechanism for inserting new copies appears to permit considerable structural flexibility in the evolution of different non-LTR retrotransposons. The authors suggest that it is more likely that the LTR retrotransposons evolved from the same lineages of eubacterial sequences as all other retroelements. They have attempted in this chapter to provide an overview of the diversity and mode of evolution of eukaryotic retrotransposable elements and to trace their origins back to prokaryotic sequences.