Testing The Retroelement Invasion Hypothesis For The Emergence Of The Ancestoral Eukaryotic Cell

A growing body of research has begun to recognize that retroelements (RTEs) may have played a previously unappreciated role in the evolution of complex biological structures and functions. Even so, many open questions remain regarding the dynamics of their propagation, the evolutionary pressures limiting their activity and expression and how these factors determine their differential abundance among eukaryotes, bacteria and archaea. To address this gap in our understanding, we pair experimental methods such as real-time fluorescence tracking of retroelements with mathematical modeling to quantify RTE expression and activity in different host environments. We have quantified the growth effects of the human RTE LINE-1 and the bacterial group II intron Ll.LtrB in Escherichia coli and planktonic Bacillus subtilis. We found that both RTEs chromosomally integrated and that RTE expression is detrimental to both species. We show theoretically, that our results are sufficient to explain the rarity of RTEs in bacteria. We also observe that the host's ability to repair DNA breaks with bacterial non-homologous end joining (NHEJ) systems increases retrotransposition efficiency. Based on this, we hypothesize that the capacity of the last eukaryotic common ancestor for NHEJ may have enabled the proliferation of RTEs and the evolution of eukaryotes. Following these results, we will quantify the dynamics of our RTE constructs in more complex biological systems.