Several studies have shown that genomes contain a mixture of transposable elements, some of which are still active and others ancient relics that have degenerated. This is true for the non-LTR retrotransposon Helena, of which only degenerate sequences have been shown to be present in some species (Drosophila melanogaster), whereas putatively active sequences are present in others (D. simulans). Combining experimental and population analyses with the sequence analysis of the 12 Drosophila genomes, we have investigated the evolution of Helena, and propose a possible scenario for the evolution of this element.
Carareto, C. M. A., Granzotto, A., Lopes, F. R., Lerat, E., & Vieira, C. (2009). The evolutionary dynamics of the Helena retrotransposon revealed by sequenced Drosophila genomes. BMC Evolutionary Biology, 9(1). https://doi.org/10.1186/1471-2148-9-174