An experimental test of the emergence of Cellular Complexity by Constructive Neutral Evolution
Sam Elley, Alicia Sook-Wei Lai, Nicole E. Wheeler, Anthony M. Poole
Biomolecular Interaction Centre, School of Biological Sciences, University of Canterbury, Christchurch, New Zealand
Small, asexual populations are known to accumulate sublethal mutations over successive generations through a process called Muller’s Ratchet. Under such conditions, sublethal mutations can become fixed, which can lead to fitness decline. We subjected lines of E. coli to single-cell bottlenecks and found that RNA slippage-type ‘editing’ can evolve under conditions favouring genetic drift. This observation fits the model of Covello and Gray, which proposes that the emergence of RNA editing is a product of constructive neutral evolution (Covello & Gray, 1993). This model proposes that complex cellular systems can originate by non-adaptive evolution.
We are now screening for the emergence of a broader range of cellular systems by constructive neutral evolution. Our lab group has evolved a lineage of E.coli that has been subjected to multiple population bottlenecks (which favours genetic drift) and has subsequently accumulated numerous sub-lethal mutations. We have subjected this line to further evolution under bottleneck relief (i.e. where natural selection can operate efficiently). We have analysed genomic data and screened for the emergence of altered substrate utilisation capacities using phenotype arrays, comparing data collected from the original ancestral strains, both pre-bottleneck and following the bottleneck, as well as data following bottleneck relief. In addition, we used a newly-described method (delta-bitscore) to assess the functional severity of mutations and mapped these onto cellular pathways (Wheeler, Barquist, Ashari Ghomi, Kingsley, & Gardner, 2015). We present the results of these analyses, and report on the impact of neutral evolutionary processes in shaping biomolecular systems.