Learning from the Codon Table: Convergent Recoding Provides Novel Understanding on the Evolution of A-to-I RNA Editing.

Adenosine-to-inosine (A-to-I) RNA editing recodes the genetic information. Apart from diversifying the proteome, another tempting advantage of RNA recoding is to correct deleterious DNA mutation and restore ancestral allele. Solid evidences for beneficial restorative editing are very rare in animals. By searching for "convergent recoding" under a phylogenetic context, we proposed this term for judging the potential restorative functions of particular editing site. For the well-known mammalian Gln>Arg (Q>R) recoding site, its ancestral state in vertebrate genomes was the pre-editing Gln, and all 470 available mammalian genomes strictly avoid other three equivalent ways to achieve Arg in protein. The absence of convergent recoding from His>Arg, or synonymous mutations on Gln codons, could be attributed to the strong maintenance on editing motif and structure, but the absence of direct A-to-G mutation is extremely unexpected. With similar ideas, we found cases of convergent recoding in Drosophila genus, reducing the possibility of their restorative function. In summary, we defined an interesting scenario of convergent recoding, the occurrence of which could be used as preliminary judgements for whether a recoding site has a sole restorative role. Our work provides novel insights to the natural selection and evolution of RNA editing.

New comparative genomic evidence supporting the proteomic diversification role of A-to-I RNA editing in insects.

Adenosine-to-inosine (A-to-I) RNA editing, resembling A-to-G mutation, confers adaptiveness by increasing proteomic diversity in a temporal-spatial manner. This evolutionary theory named "proteomic diversifying hypothesis" has only partially been tested in very few organisms like Drosophila melanogaster, mainly by observing the positive selection on nonsynonymous editing events. To find additional genome-wide evidences supporting this interesting assumption, we retrieved the genomes of four Drosophila species and collected 20 deep-sequenced transcriptomes of different developmental stages and neuron populations of D. melanogaster. We systematically profiled the RNA editomes in these samples and performed meticulous comparative genomic analyses. Further evidences were found to support the diversifying hypothesis. (1) None of the nonsynonymous editing sites in D. melanogaster had ancestral G-alleles, while the silent editing sites had an unignorable fraction of ancestral G-alleles; (2) Only very few nonsynonymous editing sites in D. melanogaster had corresponding G-alleles derived in the genomes of sibling species, and the fraction of such situation was significantly lower than that of silent editing sites; (3) The few nonsynonymous editing with corresponding G-alleles had significantly more variable editing levels (across samples) than other nonsynonymous editing sites in D. melanogaster. The proteomic diversifying nature of RNA editing in Drosophila excludes the restorative role which favors an ancestral G-allele. The few fixed G-alleles in sibling species might facilitate the adaptation to particular environment and the corresponding nonsynonymous editing in D. melanogaster would introduce stronger advantage of flexible proteomic diversification. With multi-Omics data, our study consolidates the nature of evolutionary significance of A-to-I RNA editing sites in model insects.

Comparative genomic analyses reveal evidence for adaptive A-to-I RNA editing in insect Adar gene.

Although A-to-I RNA editing leads to similar effects to A-to-G DNA mutation, nonsynonymous RNA editing (recoding) is believed to confer its adaptiveness by 'epigenetically' regulating proteomic diversity in a temporospatial manner, avoiding the pleiotropic effect of genomic mutations. Recent discoveries on the evolutionary trajectory of Ser>Gly auto-editing site in insect Adar gene demonstrated a selective advantage to having an editable codon compared to uneditable ones. However, apart from pure observations, quantitative approaches for justifying the adaptiveness of individual RNA editing sites are still lacking. We performed a comparative genomic analysis on 113 Diptera species, focusing on the Adar Ser>Gly auto-recoding site in Drosophila. We only found one species having a derived Gly at the corresponding site, and this occurrence was significantly lower than genome-wide random expectation. This suggests that the Adar Ser>Gly site is unlikely to be genomically replaced with G during evolution, and thus indicating the advantage of editable status over hardwired genomic alleles. Similar trends were observed for the conserved Ile>Met recoding in gene Syt1. In the light of evolution, we established a comparative genomic approach for quantitatively justifying the adaptiveness of individual editing sites. Priority should be given to such adaptive editing sites in future functional studies.

Conserved A-to-I RNA editing with non-conserved recoding expands the candidates of functional editing sites.

Adenosine-to-inosine (A-to-I) RNA editing recodes the genome and confers flexibility for the organisms to adapt to the environment. It is believed that RNA recoding sites are well suited for facilitating adaptive evolution by increasing the proteomic diversity in a temporal-spatial manner. The function and essentiality of a few conserved recoding sites are recognized. However, the experimentally discovered functional sites only make up a small corner of the total sites, and there is still the need to expand the repertoire of such functional sites with bioinformatic approaches. In this study, we define a new category of RNA editing sites termed 'conserved editing with non-conserved recoding' and systematically identify such sites in Drosophila editomes, figuring out their selection pressure and signals of adaptation at inter-species and intra-species levels. Surprisingly, conserved editing sites with non-conserved recoding are not suppressed and are even slightly overrepresented in Drosophila. DNA mutations leading to such cases are also favoured during evolution, suggesting that the function of those recoding events in different species might be diverged, specialized, and maintained. Finally, structural prediction suggests that such recoding in potassium channel Shab might increase ion permeability and compensate the effect of low temperature. In conclusion, conserved editing with non-conserved recoding might be functional as well. Our study provides novel aspects in considering the adaptive evolution of RNA editing sites and meanwhile expands the candidates of functional recoding sites for future validation.