Adaptive evolution at mRNA editing sites in soft-bodied cephalopods

Abstract

The bulk of variability in mRNA sequence arises due to mutation - change in DNA sequence which is heritable if it occurs in the germline. However, variation in mRNA can also be achieved by post-translational modification including mRNA editing, changes in mRNA nucleotide sequence that mimic the effect of mutations. Such modifications are not inherited directly; however, as the processes affecting them are encoded in the genome, they have a heritable component, and therefore can be shaped by selection. In soft-bodied cephalopods, adenine-to-inosine RNA editing is very frequent, and much of it occurs at nonsynonymous sites, affecting the sequence of the encoded protein. Here, we show that mRNA editing of individual nonsynonymous sites in cephalopods originates in evolution through substitutions at regions adjacent to these sites. As such substitutions mimic the effect of the substitution at the edited site itself, we hypothesize that they are favored by selection if the inosine is selectively advantageous to adenine at the edited position. Consistent with this hypothesis and with observations on Drosophila and human editing sites, we show that edited adenines are more frequently substituted with guanine, an informational analogue of inosine, in the course of evolution than their unedited counterparts, and for heavily edited adenines, these transitions are favored by positive selection. Thus, our study shows that coleoid editing sites may enhance adaptation, which, together with distinct evolutionary features of Drosophila and human editing sites, points at a general role of RNA editing in the molecular evolution of metazoans.