Background: Non-LTR retrotransposons often exhibit base composition that is markedly different from the nucleotide content of their host's gene. For instance, the mammalian L1 element is AT-rich with a strong A bias on the positive strand, which results in a reduced transcription. It is plausible that the A-richness of mammalian L1 is a self-regulatory mechanism reflecting a trade-off between transposition efficiency and the deleterious effect of L1 on its host. We examined if the A-richness of L1 is a general feature of non-LTR retrotransposons or if different clades of elements have evolved different nucleotide content. We also investigated if elements belonging to the same clade evolved towards different base composition in different genomes or if elements from different clades evolved towards similar base composition in the same genome.
Results: We found that non-LTR retrotransposons differ in base composition among clades within the same host but also that elements belonging to the same clade differ in base composition among hosts. We showed that nucleotide content remains constant within the same host over extended period of evolutionary time, despite mutational patterns that should drive nucleotide content away from the observed base composition.
Conclusions: Our results suggest that base composition is evolving under selection and may be reflective of the long-term co-evolution between non-LTR retrotransposons and their host. Finally, the coexistence of elements with drastically different base composition suggests that these elements may be using different strategies to persist and multiply in the genome of their host.