The exaptation of nitrate/carbon stress-induced smRNAs and their targets from transposable elements in the unicellular green alga Chlamydomonas reinhardtii

Transposable elements (TEs) are acknowledged sources of genetic change within organisms. The effects of transposition can range from the disruption or creation of a single gene to large-scale genome rearrangements. Transposition events can result in beneficial mutations which allow an organism to adapt to a new environment. In the last three years, several studies have reported that some miRNAs, small RNAs involved in post-transcriptional gene regulation, have evolved from TEs. miRNAs play an important role in the stress responses of many organisms. Interestingly, TEs are derepressed under the same stress conditions that miRNAs are known to ameliorate. The observation that miRNAs are known to evolve from TEs and that TEs are derepressed under stress conditions lead me to question whether TEs play a role in environmental adaptation through the creation of small RNA networks. To test this idea, Chlamydomonas reinhardtii cultures were grown under low carbon, nitrate enriched conditions and the small RNA pool was analyzed. I found that these conditions do stimulate the expression of novel small RNAs and that some of these RNAs and their targets are derived from transposition events.