RNA editing: a driving force for adaptive evolution?

Can transcriptome size and off-target effects explain the contrasting evolution of mitochondrial vs nuclear RNA editing?

Mitochondrial RNA editing has evolved independently in numerous eukaryotic lineages, where it generally restores conserved sequences and functional reading frames in mRNA transcripts derived from altered or disrupted mitochondrial protein-coding genes. In contrast to this "restorative" RNA editing in mitochondria, most editing of nuclear mRNAs introduces novel sequence variants and diversifies the proteome. This Perspective addresses the hypothesis that these completely opposite effects of mitochondrial vs. nuclear RNA editing arise from the enormous difference in gene number between the respective genomes. Because mitochondria produce a much smaller transcriptome, they likely create less opportunity for off-target editing, which has been supported by recent experimental work expressing mitochondrial RNA editing machinery in foreign contexts. In addition, there is recent evidence that the size and complexity of RNA targets may slow the kinetics and reduce efficiency of on-target RNA editing. These findings suggest that efficient targeting and a low risk of off-target editing have facilitated the repeated emergence of disrupted mitochondrial genes and associated restorative RNA editing systems via (potentially non-adaptive) evolutionary pathways that are not feasible in larger nuclear transcriptomes due to lack of precision.

The Taihangia mitogenome provides new insights into its adaptation and organelle genome evolution in Rosaceae

MAIN CONCLUSION

We present the first Taihangia mitogenome, uncovering frequent rearrangements and significant length variation in Rosaceae, likely driven by hybridization and repeat content, alongside widespread mito-chloroplast phylogenetic conflicts. Taihangia, an ancient and endangered monotypic genus within the subfamily Rosoideae of the family Rosaceae, is endemic to cliffs and serves as an ideal material for studying the adaptations of cliff-dwelling plants and the evolutionary processes of the Rosaceae family. In this study, the mitogenome and plastome of T. rupestris var. ciliata were assembled, with lengths of 265,633 bp and 155,467 bp, both exhibiting typical circular structures. Positive selection was detected in the nad4L and sdh4 genes, likely playing a role in adaptation to harsh environments. Comparative genomic analysis indicated that repetitive sequences are likely the main contributors to genome size variation in Rosaceae and also influence horizontal gene transfer between organelle genomes. In T. rupestris var. ciliata, 20 mitochondrial plastid DNA sequences were identified, including 16 complete plastid genes. Moreover, frequent rearrangements were observed in the non-coding regions of mitogenome within the subfamily Rosoideae, potentially linked to the complex evolutionary history and the presence of repetitive sequences. In contrast, coding regions remained highly conserved (over 83% similarity) to maintain essential mitochondrial functions. Phylogenomic analysis of the two organelle genomes revealed conflicts in the phylogenetic relationships within Rosaceae, potentially due to the inconsistent mutation rates and frequent hybridization events in the evolutionary history of the family. In conclusion, the organelle genome analysis of Taihangia provides crucial genomic resources for understanding the evolution and adaptation of Rosaceae species.

Using Zebrafish Models to Study Epitranscriptomic Regulation of CNS Functions

Epitranscriptomic regulation of cell functions involves multiple post-transcriptional chemical modifications of coding and non-coding RNA that are increasingly recognized in studying human brain disorders. Although rodent models are presently widely used in neuroepitranscriptomic research, the zebrafish (Danio rerio) has emerged as a useful and promising alternative model species. Mounting evidence supports the importance of RNA modifications in zebrafish CNS function, providing additional insights into epitranscriptomic mechanisms underlying a wide range of brain disorders. Here, we discuss recent data on the role of RNA modifications in CNS regulation, with a particular focus on zebrafish models, as well as evaluate current problems, challenges, and future directions of research in this field of molecular neurochemistry.

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.

Narrowing down the candidates of beneficial A-to-I RNA editing by comparing the recoding sites with uneditable counterparts

Adar-mediated adenosine-to-inosine (A-to-I) RNA editing mainly occurs in nucleus and diversifies the transcriptome in a flexible manner. It has been a challenging task to identify beneficial editing sites from the sea of total editing events. The functional Ser>Gly auto-recoding site in insect Adar gene has uneditable Ser codons in ancestral nodes, indicating the selective advantage to having an editable status. Here, we extended this case study to more metazoan species, and also looked for all Drosophila recoding events with potential uneditable synonymous codons. Interestingly, in D. melanogaster, the abundant nonsynonymous editing is enriched in the codons that have uneditable counterparts, but the Adar Ser>Gly case suggests that the editable orthologous codons in other species are not necessarily edited. The use of editable versus ancestral uneditable codon is a smart way to infer the selective advantage of RNA editing, and priority might be given to these editing sites for functional studies due to the feasibility to construct an uneditable allele. Our study proposes an idea to narrow down the candidates of beneficial recoding sites. Meanwhile, we stress that the matched transcriptomes are needed to verify the conservation of editing events during evolution.

The mitochondrial genome of Lavandula angustifolia Mill. (Lamiaceae) sheds light on its genome structure and gene transfer between organelles

BACKGROUND

Lavandula angustifolia holds importance as an aromatic plant with extensive applications spanning the fragrance, perfume, cosmetics, aromatherapy, and spa sectors. Beyond its aesthetic and sensory applications, this plant offers medicinal benefits as a natural herbal remedy and finds use in household cleaning products. While extensive genomic data, inclusive of plastid and nuclear genomes, are available for this species, researchers have yet to characterize its mitochondrial genome. This gap in knowledge hampers deeper understanding of the genome organization and its evolutionary significance.

RESULTS

Through the course of this study, we successfully assembled and annotated the mitochondrial genome of L. angustifolia, marking a first in this domain. This assembled genome encompasses 61 genes, which comprise 34 protein-coding genes, 24 transfer RNA genes, and three ribosomal RNA genes. We identified a chloroplast sequence insertion into the mitogenome, which spans a length of 10,645 bp, accounting for 2.94% of the mitogenome size. Within these inserted sequences, there are seven intact tRNA genes (trnH-GUG, trnW-CCA, trnD-GUC, trnS-GGA, trnN-GUU, trnT-GGU, trnP-UGG) and four complete protein-coding genes (psbA, rps15, petL, petG) of chloroplast derivation. Additional discoveries include 88 microsatellites, 15 tandem repeats, 74 palindromic repeats, and 87 forward long repeats. An RNA editing analysis highlighted an elevated count of editing sites in the cytochrome c oxidase genes, notably ccmB with 34 editing sites, ccmFN with 32, and ccmC with 29. All protein-coding genes showed evidence of cytidine-to-uracil conversion. A phylogenetic analysis, utilizing common protein-coding genes from 23 Lamiales species, yielded a tree with consistent topology, supported by high confidence values.

CONCLUSIONS

Analysis of the current mitogenome resource revealed its typical circular genome structure. Notably, sequences originally from the chloroplast genome were found within the mitogenome, pointing to the occurrence of horizontal gene transfer between organelles. This assembled mitogenome stands as a valuable resource for subsequent studies on mitogenome structures, their evolution, and molecular biology.

Adaptive evolution of A-to-I auto-editing site in Adar of eusocial insects

BACKGROUND

Adenosine-to-inosine (A-to-I) RNA editing is a co-/post-transcriptional modification introducing A-to-G variations in RNAs. There is extensive discussion on whether the flexibility of RNA editing exerts a proteomic diversification role, or it just acts like hardwired mutations to correct the genomic allele. Eusocial insects evolved the ability to generate phenotypically differentiated individuals with the same genome, indicating the involvement of epigenetic/transcriptomic regulation.

METHODS

We obtained the genomes of 104 Hymenoptera insects and the transcriptomes of representative species. Comparative genomic analysis was performed to parse the evolutionary trajectory of a regulatory Ile > Met auto-recoding site in Adar gene.

RESULTS

At genome level, the pre-editing Ile codon is conserved across a node containing all eusocial hymenopterans. At RNA level, the editing events are confirmed in representative species and shows considerable condition-specificity. Compared to random expectation, the editable Ile codon avoids genomic substitutions to Met or to uneditable Ile codons, but does not avoid mutations to other unrelated amino acids.

CONCLUSIONS

The flexibility of Adar auto-recoding site in Hymenoptera is selectively maintained, supporting the flexible RNA editing hypothesis. We proposed a new angle to view the adaptation of RNA editing, providing another layer to explain the great phenotypical plasticity of eusocial insects.

Chloroplast genomes of Eriobotrya elliptica and an unknown wild loquat "YN-1"

The chloroplast genomes of wild loquat can help to determine their place in the history of evolution. Here, we sequenced and assembled two novel wild loquat's chloroplast genomes, one is Eriobotrya elliptica, and the other is an unidentified wild loquat, which we named "YN-1". Their sizes are 159,471 bp and 159,399 bp, respectively. We also assembled a cultivated loquat named 'JFZ', its chloroplast genome size is 159,156 bp. A comparative study was conducted with six distinct species of loquats, including five wild loquats and one cultivated loquat. The results showed that both E. elliptica and "YN-1" have 127 genes, one gene more than E. fragrans, which is psbK. Regions trnF-GAA-ndhJ, petG-trnP-UGG, and rpl32-trnL-UAG were found to exhibit high variability. It was discovered that there was a positive selection on rpl22 and rps12. RNA editing analysis found several chilling stress-specific RNA editing sites, especially in rpl2 gene. Phylogenetic analysis results showed that "YN-1" is closely related to E. elliptica, E. obovata and E. henryi.

Adenosine-to-Inosine RNA editing in cancer: molecular mechanisms and downstream targets

Adenosine-to-Inosine (A-to-I), one of the most prevalent RNA modifications, has recently garnered significant attention. The A-to-I modification actively contributes to biological and pathological processes by affecting the structure and function of various RNA molecules, including double stranded RNA, transfer RNA, microRNA, and viral RNA. Increasing evidence suggests that A-to-I plays a crucial role in the development of human disease, particularly in cancer, and aberrant A-to-I levels are closely associated with tumorigenesis and progression through regulation of the expression of multiple oncogenes and tumor suppressor genes. Currently, the underlying molecular mechanisms of A-to-I modification in cancer are not comprehensively understood. Here, we review the latest advances regarding the A-to-I editing pathways implicated in cancer, describing their biological functions and their connections to the disease.

The first complete mitochondrial genome of Grossulariaceae: Molecular features, structure recombination, and genetic evolution

BACKGROUND

Mitochondria play crucial roles in the growth, development, and adaptation of plants. Blackcurrant (Ribes nigrum L.) stands out as a significant berry species due to its rich nutritional profile, medicinal properties, and health benefits. Despite its importance, the mitochondrial genome of blackcurrant remains unassembled.

RESULTS

This study presents the first assembly of the mitochondrial genome of R. nigrum in the Grossulariaceae family. The genome spans 450,227 base pairs (bp) and encompasses 39 protein-coding genes (PCGs), 19 transfer RNAs (tRNAs), and three ribosomal RNAs (rRNAs). Protein-coding regions constitute 8.88% of the entire genome. Additionally, we identified 180 simple sequence repeats, 12 tandem repeats, and 432 pairs of dispersed repeats. Notably, the dispersed sequence R1 (cotig3, 1,129 bp) mediated genome recombination, resulting in the formation of two major conformations, namely master and double circles. Furthermore, we identified 731 C-to-U RNA editing sites within the PCGs. Among these, cox1-2, nad1-2, and nad4L-2 were associated with the creation of start codons, whereas atp6-718 and rps10-391 were linked to termination codons. We also detected fourteen plastome fragments within the mitogenome, constituting 1.11% of the total length. Phylogenetic analysis suggests that R. nigrum might have undergone multiple genomic reorganization and/or gene transfer events, resulting in the loss of two PCGs (rps2 and rps11) during its evolutionary history.

CONCLUSIONS

This investigation unveils the molecular characteristics of the R. nigrum mitogenome, shedding light on its evolutionary trajectory and phylogenetic implications. Furthermore, it serves as a valuable reference for evolutionary research and germplasm identification within the genus.

De Novo Hybrid Assembly Unveils Multi-Chromosomal Mitochondrial Genomes in Ludwigia Species, Highlighting Genomic Recombination, Gene Transfer, and RNA Editing Events

Biological invasions have been identified as the fifth cause of biodiversity loss, and their subsequent dispersal represents a major ecological challenge. The aquatic invasive species Ludwigia grandiflora subsp. hexapetala (Lgh) and Ludwigia peploides subsp. montevidensis (Lpm) are largely distributed in aquatic environments in North America and in Europe. However, they also present worrying terrestrial forms that are able to colonize wet meadows. To comprehend the mechanisms of the terrestrial adaptation of Lgh and Lpm, it is necessary to develop their genomic resources, which are currently poorly documented. We performed de novo assembly of the mitogenomes of Lgh and Lpm through hybrid assemblies, combining short reads (SR) and/or long reads (LR) before annotating both mitogenomes. We successfully assembled the mitogenomes of Lgh and Lpm into two circular molecules each, resulting in a combined total length of 711,578 bp and 722,518 bp, respectively. Notably, both the Lgh and Lpm molecules contained plastome-origin sequences, comprising 7.8% of the mitochondrial genome length. Additionally, we identified recombinations that were mediated by large repeats, suggesting the presence of multiple alternative conformations. In conclusion, our study presents the first high-quality mitogenomes of Lpm and Lgh, which are the only ones in the Myrtales order found as two circular molecules.

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.

REDH: A database of RNA editome in hematopoietic differentiation and malignancy

BACKGROUND

The conversion of adenosine (A) to inosine (I) through deamination is the prevailing form of RNA editing, impacting numerous nuclear and cytoplasmic transcripts across various eukaryotic species. Millions of high-confidence RNA editing sites have been identified and integrated into various RNA databases, providing a convenient platform for the rapid identification of key drivers of cancer and potential therapeutic targets. However, the available database for integration of RNA editing in hematopoietic cells and hematopoietic malignancies is still lacking.

METHODS

We downloaded RNA sequencing (RNA-seq) data of 29 leukemia patients and 19 healthy donors from National Center for Biotechnology Information (NCBI) Gene Expression Omnibus (GEO) database, and RNA-seq data of 12 mouse hematopoietic cell populations obtained from our previous research were also used. We performed sequence alignment, identified RNA editing sites, and obtained characteristic editing sites related to normal hematopoietic development and abnormal editing sites associated with hematologic diseases.

RESULTS

We established a new database, "REDH", represents RNA editome in hematopoietic differentiation and malignancy. REDH is a curated database of associations between RNA editome and hematopoiesis. REDH integrates 30,796 editing sites from 12 murine adult hematopoietic cell populations and systematically characterizes more than 400,000 edited events in malignant hematopoietic samples from 48 cohorts (human). Through the Differentiation, Disease, Enrichment, and knowledge modules, each A-to-I editing site is systematically integrated, including its distribution throughout the genome, its clinical information (human sample), and functional editing sites under physiological and pathological conditions. Furthermore, REDH compares the similarities and differences of editing sites between different hematologic malignancies and healthy control.

CONCLUSIONS

REDH is accessible at http://www.redhdatabase.com/ . This user-friendly database would aid in understanding the mechanisms of RNA editing in hematopoietic differentiation and malignancies. It provides a set of data related to the maintenance of hematopoietic homeostasis and identifying potential therapeutic targets in malignancies.

Insights into the multi-chromosomal mitochondrial genome structure of the xero-halophytic plant Haloxylon Ammodendron (C.A.Mey.) Bunge ex Fenzl

BACKGROUND

Haloxylon ammodendron holds significance as an ecological plant, showcasing remarkable adaptability to desert conditions, halophytic environments, and sand fixation. With its potential for carbon sequestration, it emerges as a promising candidate for environmental sustainability. Furthermore, it serves as a valuable C4 plant model, offering insights into the genetic foundations of extreme drought tolerance. Despite the availability of plastid and nuclear genomes, the absence of a mitochondrial genome (mitogenome or mtDNA) hinders a comprehensive understanding of its its mtDNA structure, organization, and phylogenetic implications.

RESULTS

In the present study, the mitochondrial genome of H. ammodendron was assembled and annotated, resulting in a multi-chromosomal configuration with two circular chromosomes. The mtDNA measured 210,149 bp in length and contained 31 protein-coding genes, 18 tRNA and three rRNA. Our analysis identified a total of 66 simple sequence repeats along with 27 tandem repeats, 312 forward repeats, and 303 palindromic repeats were found. Notably, 17 sequence fragments displayed homology between the mtDNA and chloroplast genome (cpDNA), spanning 5233 bp, accounting for 2.49% of the total mitogenome size. Additionally, we predicted 337 RNA editing sites, all of the C-to-U conversion type. Phylogenetic inference confidently placed H. ammodendron in the Amaranthacea family and its close relative, Suaeda glacum.

CONCLUSIONS

H. ammodendron mtDNA showed a multi-chromosomal structure with two fully circularized molecules. This newly characterized mtDNA represents a valuable resource for gaining insights into the basis of mtDNA structure variation within Caryophyllales and the evolution of land plants, contributing to their identification, and classification.

Complete mitochondrial genome of Syzygium samarangense reveals genomic recombination, gene transfer, and RNA editing events

Wax apple (Syzygium samarangense) is a commercial fruit that belongs to one of the most species-rich tree genera in the world. We report here the first complete S. samarangense mitogenome obtained using a hybrid assembly strategy. The mitogenome was a 530,242 bp circular molecule encoding 61 unique genes accounting for 7.99% of the full-length genome. Additionally, 167 simple sequence repeats, 19 tandem repeats, and 529 pairs of interspersed repeats were identified. Long read mapping and Sanger sequencing revealed the involvement of two forward repeats (35,843 bp and 22,925 bp) in mediating recombination. Thirteen homologous fragments in the chloroplast genome were identified, accounting for 1.53% of the mitogenome, and the longest fragment was 2,432 bp. An evolutionary analysis showed that S. samarangense underwent multiple genomic reorganization events and lost at least four protein-coding genes (PCGs) (rps2, rps7, rps11, and rps19). A total of 591 RNA editing sites were predicted in 37 PCGs, of which nad1-2, nad4L-2, and rps10-2 led to the gain of new start codons, while atp6-1156, ccmFC-1315 and rps10-331 created new stop codons. This study reveals the genetic features of the S. samarangense mitogenome and provides a scientific basis for further studies of traits with an epistatic basis and for germplasm identification.

The impacts of thermal heterogeneity across microhabitats on post-settlement selection of intertidal mussels

Rapid genetic selection is critical for allowing natural populations to adapt to different thermal environments such as those that occur across intertidal microhabitats with high degrees of thermal heterogeneity. To address the question of how thermal regimes influence selection and adaptation in the intertidal black mussel Mytilisepta virgata, we continuously recorded environmental temperatures in both tidal pools and emergent rock microhabitats and then assessed genetic differentiation, gene expression patterns, RNA editing level, and cardiac performance. Our results showed that the subpopulations in the tidal pool and on emergent rocks had different genetic structures and exhibited different physiological and molecular responses to high-temperature stress. These results indicate that environmental heterogeneity across microhabitats is important for driving genetic differentiation and shed light on the importance of post-settlement selection for adaptively modifying the genetic composition and thermal responses of these intertidal mussels.

High-level RNA editing diversifies the coleoid cephalopod brain proteome

Coleoid cephalopods (octopus, squid and cuttlefish) have unusually complex nervous systems. The coleoid nervous system is also the only one currently known to recode the majority of expressed proteins through A-to-I RNA editing. The deamination of adenosine by adenosine deaminase acting on RNA (ADAR) enzymes produces inosine, which is interpreted as guanosine during translation. If this occurs in an open reading frame, which is the case for tens of thousands of editing sites in coleoids, it can recode the encoded protein. Here, we describe recent findings aimed at deciphering the mechanisms underlying high-level recoding and its adaptive potential. We describe the complement of ADAR enzymes in cephalopods, including a recently discovered novel domain in sqADAR1. We further summarize current evidence supporting an adaptive role of high-level RNA recoding in coleoids, and review recent studies showing that a large proportion of recoding sites is temperature-sensitive. Despite these new findings, the mechanisms governing the high level of RNA recoding in coleoid cephalopods remain poorly understood. Recent advances using genome editing in squid may provide useful tools to further study A-to-I RNA editing in these animals.

Genome-Wide Analysis on Driver and Passenger RNA Editing Sites Suggests an Underestimation of Adaptive Signals in Insects

Adenosine-to-inosine (A-to-I) RNA editing leads to a similar effect to A-to-G mutations. RNA editing provides a temporo-spatial flexibility for organisms. Nonsynonymous (Nonsyn) RNA editing in insects is over-represented compared with synonymous (Syn) editing, suggesting adaptive signals of positive selection on Nonsyn editing during evolution. We utilized the brain RNA editome of Drosophila melanogaster to systematically study the LD (r2) between editing sites and infer its impact on the adaptive signals of RNA editing. Pairs of editing sites (PESs) were identified from the transcriptome. For CDS PESs of two consecutive editing sites, their occurrence was significantly biased to type-3 PES (Syn-Nonsyn). The haplotype frequency of type-3 PES exhibited a significantly higher abundance of AG than GA, indicating that the rear Nonsyn site is the driver that promotes the editing of the front Syn site (passenger). The exclusion of passenger Syn sites dramatically amplifies the adaptive signal of Nonsyn RNA editing. Our study for the first time quantitatively demonstrates that the linkage between RNA editing events comes from hitchhiking effects and leads to the underestimation of adaptive signals for Nonsyn editing. Our work provides novel insights for studying the evolutionary significance of RNA editing events.

Autorecoding A-to-I RNA editing sites in the Adar gene underwent compensatory gains and losses in major insect clades

As one of the most prevalent RNA modifications in animals, adenosine-to-inosine (A-to-I) RNA editing facilitates the environmental adaptation of organisms by diversifying the proteome in a temporal-spatial manner. In flies and bees, the editing enzyme Adar has independently gained two different autorecoding sites that form an autofeedback loop, stabilizing the overall editing efficiency. This ensures cellular homeostasis by keeping the normal function of target genes. However, in a broader range of insects, the evolutionary dynamics and significance of this Adar autoregulatory mechanism are unclear. We retrieved the genomes of 377 arthropod species covering the five major insect orders (Hemiptera, Hymenoptera, Coleoptera, Diptera, and Lepidoptera) and aligned the Adar autorecoding sites across all genomes. We found that the two autorecoding sites underwent compensatory gains and losses during the evolution of two orders with the most sequenced species (Diptera and Hymenoptera), and that the two editing sites were mutually exclusive among them: One editable site is significantly linked to another uneditable site. This autorecoding mechanism of Adar could flexibly diversify the proteome and stabilize global editing activity. Many insects independently selected different autorecoding sites to achieve a feedback loop and regulate the global RNA editome, revealing an interesting phenomenon during evolution. Our study reveals the evolutionary force acting on accurate regulation of RNA editing activity in insects and thus deepens our understanding of the functional importance of RNA editing in environmental adaptation and evolution.

Differential adaptive RNA editing signals between insects and plants revealed by a new measurement termed haplotype diversity

BACKGROUND

C-to-U RNA editing in plants is believed to confer its evolutionary adaptiveness by reversing unfavorable DNA mutations. This "restorative hypothesis" has not yet been tested genome-wide. In contrast, A-to-I RNA editing in insects like Drosophila and honeybee is already known to benefit the host by increasing proteomic diversity in a spatial-temporal manner (namely "diversifying hypothesis").

METHODS

We profiled the RNA editomes of multiple tissues of Arabidopsis thaliana, Drosophila melanogaster, and Apis melifera. We unprecedentedly defined the haplotype diversity (HD) of RNA molecules based on nonsynonymous editing events (recoding sites).

RESULTS

Signals of adaptation is confirmed in Arabidopsis by observing higher frequencies and levels at nonsynonymous editing sites over synonymous sites. Compared to A-to-I recoding sites in Drosophila, the C-to-U recoding sites in Arabidopsis show significantly lower HD, presumably due to the stronger linkage between C-to-U events.

CONCLUSIONS

C-to-U RNA editing in Arabidopsis is adaptive but it is not designed for diversifying the proteome like A-to-I editing in Drosophila. Instead, C-to-U recoding sites resemble DNA mutations. Our observation supports the restorative hypothesis of plant C-to-U editing which claims that editing is used for fixing unfavorable genomic sequences.

Evolutionary honing in and mutational replacement: how long-term directed mutational responses to specific environmental pressures are possible

Recent results have shown that the human malaria-resistant hemoglobin S mutation originates de novo more frequently in the gene and in the population where it is of adaptive significance, namely, in the hemoglobin subunit beta gene compared to the nonresistant but otherwise identical 20A[Formula: see text]T mutation in the hemoglobin subunit delta gene, and in sub-Saharan Africans, who have been subject to intense malarial pressure for many generations, compared to northern Europeans, who have not. This finding raises a fundamental challenge to the traditional notion of accidental mutation. Here, we address this finding with the replacement hypothesis, according to which preexisting genetic interactions can lead directly and mechanistically to mutations that simplify and replace them. Thus, an evolutionary process under selection can gradually hone in on interactions of importance for the currently evolving adaptations, from which large-effect mutations follow that are relevant to these adaptations. We exemplify this hypothesis using multiple types of mutation, including gene fusion mutations, gene duplication mutations, A[Formula: see text]G mutations in RNA-edited sites and transcription-associated mutations, and place it in the broader context of a system-level view of mutation origination called interaction-based evolution. Potential consequences include that similarity of mutation pressures may contribute to parallel evolution in genetically related species, that the evolution of genome organization may be driven by mutational mechanisms, that transposable element movements may also be explained by replacement, and that long-term directed mutational responses to specific environmental pressures are possible. Such mutational phenomena need to be further tested by future studies in natural and artificial settings.

Experimental evidence for the functional importance and adaptive advantage of A-to-I RNA editing in fungi

Adenosine-to-inosine (A-to-I) editing is the most prevalent type of RNA editing in animals, and it occurs in fungi specifically during sexual reproduction. However, it is debatable whether A-to-I RNA editing is adaptive. Deciphering the functional importance of individual editing sites is essential for the mechanistic understanding of the adaptive advantages of RNA editing. Here, by performing gene deletion for 17 genes with conserved missense editing (CME) sites and engineering underedited (ue) and overedited (oe) mutants for 10 CME sites using site-specific mutagenesis at the native locus in Fusarium graminearum, we demonstrated that two CME sites in CME5 and CME11 genes are functionally important for sexual reproduction. Although the overedited mutant was normal in sexual reproduction, the underedited mutant of CME5 had severe defects in ascus and ascospore formation like the deletion mutant, suggesting that the CME site of CME5 is co-opted for sexual development. The preediting residue of Cme5 is evolutionarily conserved across diverse classes of Ascomycota, while the postediting one is rarely hardwired into the genome, implying that editing at this site leads to higher fitness than a genomic A-to-G mutation. More importantly, mutants expressing only the underedited or the overedited allele of CME11 are defective in ascosporogenesis, while those expressing both alleles displayed normal phenotypes, indicating that concurrently expressing edited and unedited versions of Cme11 is more advantageous than either. Our study provides convincing experimental evidence for the long-suspected adaptive advantages of RNA editing in fungi and likely in animals.

The complete chloroplast genome of Hibiscus syriacus using long-read sequencing: Comparative analysis to examine the evolution of the tribe Hibisceae

Hibiscus syriacus, a member of the tribe Hibisceae, is considered an important ornamental and medicinal plant in east Asian countries. Here, we sequenced and assembled the complete chloroplast genome of H. syriacus var. Baekdansim using the PacBio long-read sequencing platform. A quadripartite structure with 161,026 base pairs was obtained, consisting of a pair of inverted repeats (IRA and IRB) with 25,745 base pairs, separated by a large single-copy region of 89,705 base pairs and a short single-copy region of 19,831 base pairs. This chloroplast genome had 79 protein-coding genes, 30 transfer RNA genes, 4 ribosomal RNA genes, and 109 simple sequence repeat regions. Among them, ndhD and rpoC1, containing traces of RNA-editing events associated with adaptive evolution, were identified by analysis of putative RNA-editing sites. Codon usage analysis revealed a preference for A/U-terminated codons. Furthermore, the codon usage pattern had a clustering tendency similar to that of the phylogenetic analysis of the tribe Hibisceae. This study provides clues for understanding the relationships and refining the taxonomy of the tribe Hibisceae.

Chloroplast C-to-U RNA editing in vascular plants is adaptive due to its restorative effect: testing the restorative hypothesis

The adaptiveness of nonsynonymous RNA editing (recoding) could be conferred by the flexibility of the temporal-spatially controllable proteomic diversity, or by its restorative effect which fixes unfavorable genomic mutations at the RNA level. These two complementary hypotheses, namely, the diversifying hypothesis and the restorative hypothesis, have distinct predictions on the landscape of RNA editing sites. We collected the chloroplast C-to-U RNA editomes of 21 vascular plants (11 angiosperms, four gymnosperms, and six ferns) from a previous study, aiming to testify whether the plant editomes typically conform to the restorative hypothesis. All predictions made by the restorative hypothesis are verified: (i) nonsynonymous editing sites are more frequent and have higher editing levels than synonymous sites; (ii) nonsynonymous editing levels are extremely high and show weak tissue-specificity in plants; (iii) on the inferred genomic sites with recent T-to-C mutations, nonsynonymous sites but not synonymous sites are compensated by C-to-U RNA editing. In conclusion, nonsynonymous C-to-U RNA editing in plants is adaptive due to its restorative effects. The recoding levels are high and are constantly required across the whole plant so that the recoding events could perfectly mimic DNA mutations. The evolutionary significance of plant RNA editing is systematically demonstrated at the genome-wide level.

RNA epitranscriptomics dysregulation: A major determinant for significantly increased risk of ASD pathogenesis

Autism spectrum disorders (ASDs) are perhaps the most severe, intractable and challenging child psychiatric disorders. They are complex, pervasive and highly heterogeneous and depend on multifactorial neurodevelopmental conditions. Although the pathogenesis of autism remains unclear, it revolves around altered neurodevelopmental patterns and their implications for brain function, although these cannot be specifically linked to symptoms. While these affect neuronal migration and connectivity, little is known about the processes that lead to the disruption of specific laminar excitatory and inhibitory cortical circuits, a key feature of ASD. It is evident that ASD has multiple underlying causes and this multigenic condition has been considered to also dependent on epigenetic effects, although the exact nature of the factors that could be involved remains unclear. However, besides the possibility for differential epigenetic markings directly affecting the relative expression levels of individual genes or groups of genes, there are at least three mRNA epitranscriptomic mechanisms, which function cooperatively and could, in association with both genotypes and environmental conditions, alter spatiotemporal proteins expression patterns during brain development, at both quantitative and qualitative levels, in a tissue-specific, and context-dependent manner. As we have already postulated, sudden changes in environmental conditions, such as those conferred by maternal inflammation/immune activation, influence RNA epitranscriptomic mechanisms, with the combination of these processes altering fetal brain development. Herein, we explore the postulate whereby, in ASD pathogenesis, RNA epitranscriptomics might take precedence over epigenetic modifications. RNA epitranscriptomics affects real-time differential expression of receptor and channel proteins isoforms, playing a prominent role in central nervous system (CNS) development and functions, but also RNAi which, in turn, impact the spatiotemporal expression of receptors, channels and regulatory proteins irrespective of isoforms. Slight dysregulations in few early components of brain development, could, depending upon their extent, snowball into a huge variety of pathological cerebral alterations a few years after birth. This may very well explain the enormous genetic, neuropathological and symptomatic heterogeneities that are systematically associated with ASD and psychiatric disorders at large.

A full repertoire of Hemiptera genomes reveals a multi-step evolutionary trajectory of auto-RNA editing site in insect Adar gene

Adenosine-to-inosine (A-to-I) RNA editing, mediated by metazoan ADAR enzymes, is a prevalent post-transcriptional modification that diversifies the proteome and promotes adaptive evolution of organisms. The Drosophila Adar gene has an auto-recoding site (termed S>G site) that forms a negative-feedback loop and stabilizes the global editing activity. However, the evolutionary trajectory of Adar S>G site in many other insects remains largely unknown, preventing us from a deeper understanding on the significance of this auto-editing mechanism. In this study, we retrieved the well-annotated genomes of 375 arthropod species including the five major insect orders (Lepidoptera, Diptera, Coleoptera, Hymenoptera and Hemiptera) and several outgroup species. We performed comparative genomic analysis on the Adar auto-recoding S>G site. We found that the ancestral state of insect S>G site was an uneditable serine codon (unSer) and that this state was largely maintained in Hymenoptera. The editable serine codon (edSer) appeared in the common ancestor of Lepidoptera, Diptera and Coleoptera and was almost fixed in the three orders. Interestingly, Hemiptera species possessed comparable numbers of unSer and edSer codons, and a few 'intermediate codons', demonstrating a multi-step evolutionary trace from unSer-to-edSer with non-synchronized mutations at three codon positions. We argue that the evolution of Adar S>G site is the best genomic evidence supporting the 'proteomic diversifying hypothesis' of RNA editing. Our work deepens our understanding on the evolutionary significance of Adar auto-recoding site which stabilizes the global editing activity and controls transcriptomic diversity.

Mosaic translation hypothesis: chimeric polypeptides produced via multiple ribosomal frameshifting as a basis for adaptability

How many different proteins can be produced from a single spliced transcript? Genome annotation projects overlook the coding potential of reading frames other than that of the reference open reading frames (refORFs). Recently, alternative open reading frames (altORFs) and their translational products, alternative proteins, have been shown to carry out important functions in various organisms. AltORFs overlapping refORFs or other altORFs in a different reading frame may be involved in one fundamental mechanism so far overlooked. A few years ago, it was proposed that altORFs may act as building blocks for chimeric (mosaic) polypeptides, which are produced via multiple ribosomal frameshifting events from a single mature transcript. We adopt terminology from that earlier discussion and call this mechanism mosaic translation. This way of extracting and combining genetic information may significantly increase proteome diversity. Thus, we hypothesize that this mechanism may have contributed to the flexibility and adaptability of organisms to a variety of environmental conditions. Specialized ribosomes acting as sensors probably played a central role in this process. Importantly, mosaic translation may be the main source of protein diversity in genomes that lack alternative splicing. The idea of mosaic translation is a testable hypothesis, although its direct demonstration is challenging. Should mosaic translation occur, we would currently highly underestimate the complexity of translation mechanisms and thus the proteome.

The occurrence, characteristics, and adaptation of A-to-I RNA editing in bacteria: A review

A-to-I RNA editing is a very important post-transcriptional modification or co-transcriptional modification that creates isoforms and increases the diversity of proteins. In this process, adenosine (A) in RNA molecules is hydrolyzed and deaminated into inosine (I). It is well known that ADAR (adenosine deaminase acting on RNA)-dependent A-to-I mRNA editing is widespread in animals. Next, the discovery of A-to-I mRNA editing was mediated by TadA (tRNA-specific adenosine deaminase) in Escherichia coli which is ADAR-independent event. Previously, the editing event S128P on the flagellar structural protein FliC enhanced the bacterial tolerance to oxidative stress in Xoc. In addition, the editing events T408A on the enterobactin iron receptor protein XfeA act as switches by controlling the uptake of Fe³⁺ in response to the concentration of iron in the environment. Even though bacteria have fewer editing events, the great majority of those that are currently preserved have adaptive benefits. Interestingly, it was found that a TadA-independent A-to-I RNA editing event T408A occurred on xfeA, indicating that there may be other new enzymes that perform a function like TadA. Here, we review recent advances in the characteristics, functions, and adaptations of editing in bacteria.

Unveiling the functional and evolutionary landscape of RNA editing in chicken using genomics and transcriptomics

The evolutionary and functional features of RNA editing are well studied in mammals, cephalopods, and insects, but not in birds. Here, we integrated transcriptomic and whole-genomic analyses to exhaustively characterize the expansive repertoire of adenosine-to-inosine (A-to-I) RNA editing sites (RESs) in the chicken. In addition, we investigated the evolutionary status of the chicken editome as a potential mechanism of domestication. We detected the lowest editing level in the liver of chickens, compared to muscles in humans, and found higher editing activity and specificity in the brain than in non-neural tissues, consistent with the brain's functional complexity. To a certain extent, specific editing activity may account for the specific functions of tissues. Our results also revealed that sequences critical to RES secondary structures remained conserved within avian evolution. Furthermore, the RNA editome was shaped by purifying selection during chicken domestication and most RESs may have served as a selection pool for a few functional RESs involved in chicken domestication, including evolution of nervous and immune systems. Regulation of RNA editing in chickens by adenosine deaminase acting on RNA (ADAR) enzymes may be affected by non-ADAR factors whose expression levels changed widely after ADAR knockdown. Collectively, we provide comprehensive lists of candidate RESs and non-ADAR-editing regulators in the chicken, thus contributing to our current understanding of the functions and evolution of RNA editing in animals.

Gene product diversity: adaptive or not?

One gene does not equal one RNA or protein. The genomic revolution has revealed numerous different RNA and protein molecules that can be produced from one gene, such as circular RNAs generated by back-splicing, proteins with residues mismatching the genomic encoding because of RNA editing, and proteins extended in the C terminus via stop codon readthrough in translation. Are these diverse products results of exquisite gene regulations or imprecise biological processes? While there are cases where the gene product diversity appears beneficial, genome-scale patterns suggest that much of this diversity arises from nonadaptive, molecular errors. This finding has important implications for studying the functions of diverse gene products and for understanding the fundamental properties and evolution of cellular life.

A transcriptomic atlas underlying developmental plasticity of seasonal forms of Bicyclus anynana butterflies

Organisms residing in regions with alternating seasons often develop different phenotypes, or forms, in each season. These forms are often adaptations to each season and result from an altered developmental response to specific environmental cues such as temperature. While multiple studies have examined form-specific gene expression profiles in a diversity of species, little is known about how environments and developmental transitions, cued by hormone pulses, alter post-transcriptional patterns. In this study, we examine how gene expression, alternative splicing, and miRNA-mediated gene silencing in Bicyclus anynana butterfly hindwing tissue, varies across two rearing temperatures at four developmental timepoints. These timepoints flank two temperature-sensitive periods that coincide with two pulses of the insect hormone 20E. Our results suggest that developmental transitions, coincident with 20E pulses, elicit a greater impact on all these transcriptomic patterns than rearing temperatures per se. More similar transcriptomic patterns are observed pre-20E pulses than those observed post-20E pulses. We also found functionally distinct sets of differentially expressed and differentially spliced genes in the seasonal forms. Furthermore, around 10% of differentially expressed genes are predicted to be direct targets of, and regulated by, differentially expressed miRNAs between the seasonal forms. Many differentially expressed genes, miRNAs, or differentially spliced genes potentially regulate eyespot size plasticity, and we validated the differential splicing pattern of one such gene, daughterless. We present a comprehensive and interactive transcriptomic atlas of the hindwing tissue of both seasonal forms of B. anynana throughout development, a model organism of seasonal plasticity.

Extent and complexity of RNA processing in honey bee queen and worker caste development

The distinct honeybee (Apis mellifera) worker and queen castes have become a model for the study of genomic mechanisms of phenotypic plasticity. Here we performed a nanopore-based direct RNA sequencing with exceptionally long reads to compare the mRNA transcripts between queen and workers at three points during their larval development. We found thousands of significantly differentially expressed transcript isoforms (DEIs) between queen and worker larvae. These DEIs were formatted by a flexible splicing system. We showed that poly(A) tails participated in this caste differentiation by negatively regulating the expression of DEIs. Hundreds of isoforms uniquely expressed in either queens or workers during their larval development, and isoforms were expressed at different points in queen and worker larval development demonstrating a dynamic relationship between isoform expression and developmental mechanisms. These findings show the full complexity of RNA processing and transcript expression in honey bee phenotypic plasticity.

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Honeybee caste differentiation has a complexity of RNA processing

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Isoforms differentially express between queens and workers during larval development

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Isoforms are formatted by a flexible alternative splicing system

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Poly(A) tails are negatively correlated with isoform expression

mRNA editing of kainate receptor subunits: what do we know so far?

Kainate receptors (KARs) are considered one of the key modulators of synaptic activity in the mammalian central nervous system. These receptors were discovered more than 30 years ago, but their role in brain functioning remains unclear due to some peculiarities. One such feature of these receptors is the editing of pre-mRNAs encoding GluK1 and GluK2 subunits. Despite the long history of studying this phenomenon, numerous questions remain unanswered. This review summarizes the current data about the mechanism and role of pre-mRNA editing of KAR subunits in the mammalian brain and proposes a perspective of future investigations.

A hierarchy in clusters of cephalopod mRNA editing sites

RNA editing in the form of substituting adenine with inosine (A-to-I editing) is the most frequent type of RNA editing in many metazoan species. In most species, A-to-I editing sites tend to form clusters and editing at clustered sites depends on editing of the adjacent sites. Although functionally important in some specific cases, A-to-I editing usually is rare. The exception occurs in soft-bodied coleoid cephalopods, where tens of thousands of potentially important A-to-I editing sites have been identified, making coleoids an ideal model for studying of properties and evolution of A-to-I editing sites. Here, we apply several diverse techniques to demonstrate a strong tendency of coleoid RNA editing sites to cluster along the transcript. We show that clustering of editing sites and correlated editing substantially contribute to the transcriptome diversity that arises due to extensive RNA editing. Moreover, we identify three distinct types of editing site clusters, varying in size, and describe RNA structural features and mechanisms likely underlying formation of these clusters. In particular, these observations may explain sequence conservation at large distances around editing sites and the observed dependency of editing on mutations in the vicinity of editing sites.

The edited UPF1 is correlated with elevated asparagine synthetase in pancreatic ductal adenocarcinomas

BACKGROUND

Pancreatic ductal adenocarcinomas (PDACs) is a malignant disorder and is the most common pancreatic cancer type. The malignant cells depend on the uptake of asparagine (Asn) for growth. The synthesis of Asn occurs through the enzyme asparagine synthetase (ASNS). Interestingly, ASNS is known as is direct target of nonsense-mediated RNA decay (NMD). We have previously reported that NMD major factor UPF1 mutations in the pancreatic tumors. However, the relationship between NMD and the level of ASNS is unknown.

METHOD

We constructed point mutations by site-specific mutagenesis. To evaluate NMD magnitude, we assessed the expression ratio of an exogenously expressed wild-type and mutated β-globin mRNA with N39 allele, and five known NMD targets. Then, reverse transcription-polymerase chain reaction (RT-PCR), RT-qPCR and western bolt to determine RNA or protein levels, after knockdown of endogenous UPF1 by small RNA interference in the cells.

RESULTS

An RNA editing event (c.3101 A > G) at UPF1 transcripts resulting in an Asparagine (p.1034) changed to a Serine is found in one primary PDAC patient. The edited UPF1 increases the ability of degrading of NMD provoking transcripts, such as β-globin mRNA with N39 allele and 5 out of 5 known endogenous NMD substrate mRNAs, including ASNS. In addition, ASNS mRNA is subjected to NMD degradation by virtue of its possessing uORFs at the 5'UTR. A reduction of endogenous ASNS RNA and the increased protein expression level is found either in the PDAC patient or in the cells with edited UPF1 at c.3101 A > G relative to the controls.

CONCLUSIONS

This edited UPF1 found in the PDAC results in hyperactivated NMD, which is tightly correlation to elevated expression level of ASNS. The targeting of knockdown of ASNS may improve the antitumor potency in PDACs.

Sperm-borne sncRNAs: potential biomarkers for semen fertility?

Semen infertility or sub-fertility, whether in humans or livestock species, remains a major concern for clinicians and technicians involved in reproduction. Indeed, they can cause tragedies in human relationships or have a dramatic overall negative impact on the sustainability of livestock breeding. Understanding and predicting semen fertility issues is therefore crucial and quality control procedures as well as biomarkers have been proposed to ensure sperm fertility. However, their predictive values appeared to be too limited and additional relevant biomarkers are still required to diagnose sub-fertility efficiently. During the last decade, the study of molecular mechanisms involved in spermatogenesis and sperm maturation highlighted the regulatory role of a variety of small non-coding RNAs (sncRNAs) and led to the discovery that sperm sncRNAs comprise both remnants from spermatogenesis and post-testicular sncRNAs acquired through interactions with extracellular vesicles along epididymis. This has led to the hypothesis that sncRNAs may be a source of relevant biomarkers, associated either with sperm functionality or embryo development. This review aims at providing a synthetic overview of the current state of knowledge regarding implication of sncRNA in spermatogenesis defects and their putative roles in sperm maturation and embryo development, as well as exploring their use as fertility biomarkers.

Regulatory roles of RNA modifications in breast cancer

Collectively referred to as the epitranscriptome, RNA modifications play important roles in gene expression control regulating relevant cellular processes. In the last few decades, growing numbers of RNA modifications have been identified not only in abundant ribosomal (rRNA) and transfer RNA (tRNA) but also in messenger RNA (mRNA). In addition, many writers, erasers and readers that dynamically regulate the chemical marks have also been characterized. Correct deposition of RNA modifications is prerequisite for cellular homeostasis, and its alteration results in aberrant transcriptional programs that dictate human disease, including breast cancer, the most frequent female malignancy, and the leading cause of cancer-related death in women. In this review, we emphasize the major RNA modifications that are present in tRNA, rRNA and mRNA. We have categorized breast cancer-associated chemical marks and summarize their contribution to breast tumorigenesis. In addition, we describe less abundant tRNA modifications with related pathways implicated in breast cancer. Finally, we discuss current limitations and perspectives on epitranscriptomics for use in therapeutic strategies against breast and other cancers.

Adaptive Proteome Diversification by Nonsynonymous A-to-I RNA Editing in Coleoid Cephalopods

RNA editing by the ADAR enzymes converts selected adenosines into inosines, biological mimics for guanosines. By doing so, it alters protein-coding sequences, resulting in novel protein products that diversify the proteome beyond its genomic blueprint. Recoding is exceptionally abundant in the neural tissues of coleoid cephalopods (octopuses, squids, and cuttlefishes), with an over-representation of nonsynonymous edits suggesting positive selection. However, the extent to which proteome diversification by recoding provides an adaptive advantage is not known. It was recently suggested that the role of evolutionarily conserved edits is to compensate for harmful genomic substitutions, and that there is no added value in having an editable codon as compared with a restoration of the preferred genomic allele. Here, we show that this hypothesis fails to explain the evolutionary dynamics of recoding sites in coleoids. Instead, our results indicate that a large fraction of the shared, strongly recoded, sites in coleoids have been selected for proteome diversification, meaning that the fitness of an editable A is higher than an uneditable A or a genomically encoded G.

Evolutionary driving forces of A-to-I editing in metazoans

Adenosine-to-inosine (A-to-I) RNA editing is an evolutionarily conserved mechanism that converts adenosines to inosines in metazoans' transcriptomes. However, the landscapes of editomes have considerably changed during evolution. Here, we review some of our current knowledge of A-to-I editing in the metazoan transcriptomes, focusing on the possible evolutionary driving forces underlying the editing events. First, we review the evolution of ADAR gene family in animals. Then, we summarize the recent advances in characterizing the editomes of various metazoan species. Next, we highlight several factors contributing to the interspecies differences in editomes, including variations in copy number and expression patterns of ADAR genes, the differences in genomic architectures and contents, and the differences in the efficacy of natural selection. After that, we review the possible diversifying and restorative effects of the editing (recoding) events that change the protein sequences. Finally, we discuss the possible convergent evolution of RNA editing in distantly related clades. This article is categorized under: RNA Evolution and Genomics > RNA and Ribonucleoprotein Evolution RNA Processing > RNA Editing and Modification.

Proteome Diversification by RNA Editing

RNA editing is an RNA modification that alters the RNA sequence relative to its genomic blueprint. The most common type of RNA editing is A-to-I editing by double-stranded RNA-specific adenosine deaminase (ADAR) enzymes. Editing of a protein-coding region within the RNA molecule may result in non-synonymous substitutions, leading to a modified protein product. These editing sites, also known as "recoding" sites, contribute to the complexity and diversification of the proteome. Recent computational transcriptomic studies have identified thousands of recoding sites in multiple species, many of which are conserved within (but not usually across) lineages and have functional and evolutionary importance. In this chapter we describe the recoding phenomenon across species, consider its potential utility for diversity and adaptation, and discuss its evolution.

Genomic Variation, Evolvability, and the Paradox of Mental Illness

Twentieth-century genetics was hard put to explain the irregular behavior of neuropsychiatric disorders. Autism and schizophrenia defy a principle of natural selection; they are highly heritable but associated with low reproductive success. Nevertheless, they persist. The genetic origins of such conditions are confounded by the problem of variable expression, that is, when a given genetic aberration can lead to any one of several distinct disorders. Also, autism and schizophrenia occur on a spectrum of severity, from mild and subclinical cases to the overt and disabling. Such irregularities reflect the problem of missing heritability; although hundreds of genes may be associated with autism or schizophrenia, together they account for only a small proportion of cases. Techniques for higher resolution, genomewide analysis have begun to illuminate the irregular and unpredictable behavior of the human genome. Thus, the origins of neuropsychiatric disorders in particular and complex disease in general have been illuminated. The human genome is characterized by a high degree of structural and behavioral variability: DNA content variation, epistasis, stochasticity in gene expression, and epigenetic changes. These elements have grown more complex as evolution scaled the phylogenetic tree. They are especially pertinent to brain development and function. Genomic variability is a window on the origins of complex disease, neuropsychiatric disorders, and neurodevelopmental disorders in particular. Genomic variability, as it happens, is also the fuel of evolvability. The genomic events that presided over the evolution of the primate and hominid lineages are over-represented in patients with autism and schizophrenia, as well as intellectual disability and epilepsy. That the special qualities of the human genome that drove evolution might, in some way, contribute to neuropsychiatric disorders is a matter of no little interest.

A-to-I RNA editing in honeybees shows signals of adaptation and convergent evolution

Social insects exhibit extensive phenotypic diversities among the genetically similar individuals, suggesting a role for the epigenetic regulations beyond the genome level. The ADAR-mediated adenosine-to-inosine (A-to-I) RNA editing, an evolutionarily conserved mechanism, facilitates adaptive evolution by expanding proteomic diversities. Here, we characterize the A-to-I RNA editome of honeybees (Apis mellifera), identifying 407 high-confidence A-to-I editing sites. Editing is most abundant in the heads and shows signatures for positive selection. Editing behavior differs between foragers and nurses, suggesting a role for editing in caste differentiation. Although only five sites are conserved between bees and flies, an unexpectedly large number of genes exhibit editing in both species, albeit at different locations, including the nonsynonymous auto-editing of Adar. This convergent evolution, where the same target genes independently acquire recoding events in distant diverged clades, together with the signals of adaptation observed in honeybees alone, further supports the notion of recoding being adaptive.

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Nonsynonymous editing sites in honeybees were under positive selection

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Differential editing may contribute to the phenotypic diversity between sub-castes

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Target genes acquire editing in different clades, suggesting convergent evolution

Adaptive evolution at mRNA editing sites in soft-bodied cephalopods

BACKGROUND

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-transcriptional 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.

METHODS

We study selection regimes at coleoid A-to-I editing sites, estimate the prevalence of positive selection, and analyze interdependencies between the editing level and contextual characteristics of editing site.

RESULTS

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, we show that edited adenines are more frequently substituted with guanine, an informational analog of inosine, in the course of evolution than their unedited counterparts, and for heavily edited adenines, these transitions are favored by positive selection. Our study shows that coleoid editing sites may enhance adaptation, which, together with recent observations on Drosophila and human editing sites, points at a general role of RNA editing in the molecular evolution of metazoans.

Comparative analysis of chloroplast genomes in Vasconcellea pubescens A.DC. and Carica papaya L

The chloroplast genome is an integral part of plant genomes in a species along with nuclear and mitochondrial genomes, contributing to adaptation, diversification, and evolution of plant lineages. In the family Caricaceae, only the Carica papaya chloroplast genome and its nuclear and mitochondrial genomes were sequenced, and no chloroplast genome-wide comparison across genera was conducted. Here, we sequenced and assembled the chloroplast genome of Vasconcellea pubescens A.DC. using Oxford Nanopore Technology. The size of the genome is 158,712 bp, smaller than 160,100 bp of the C. papaya chloroplast genome. And two structural haplotypes, LSC_IRa_SSCrc_IRb and LSC_IRa_SSC_IRb, were identified in both V. pubescens and C. papaya chloroplast genomes. The insertion-deletion mutations may play an important role in Ycf1 gene evolution in family Caricaceae. Ycf2 is the only one gene positively selected in the V. pubescens chloroplast genome. In the C. papaya chloroplast genome, there are 46 RNA editing loci with an average RNA editing efficiency of 63%. These findings will improve our understanding of the genomes of these two crops in the family Caricaceae and will contribute to crop improvement.

RNA editing: an overlooked source of fine-scale adaptation in insect vectors?

RNA editing is a source of molecular diversity that regulates the functional repertoire of animal transcriptomes. Multiple studies in Drosophila have revealed that conserved editing events can be a source of evolutionary adaptations, and there is a solid body of evidence linking editing and the fine-tuning of neural genes, which are often targeted by insecticides used in vector control. Yet, despite these suggestive connections, genome-wide analyses of editing in insect vectors are conspicuously lacking. Future advances will require complementing the growing wealth of vector genomes with targeted transcriptome analyses. Here, we review recent investigations of the genetic footprints of adaptive RNA editing in insects and provide an overview of new methodologies applicable to studies of RNA editing in insect vectors.

Editome Disease Knowledgebase (EDK): a curated knowledgebase of editome-disease associations in human

RNA editing, as an essential co-/post-transcriptional RNA modification type, plays critical roles in many biological processes and involves with a variety of human diseases. Although several databases have been developed to collect RNA editing data in both model and non-model animals, there still lacks a resource integrating associations between editome and human disease. In this study, we present Editome-Disease Knowledgebase (EDK; http://bigd.big.ac.cn/edk), an integrated knowledgebase of RNA editome-disease associations manually curated from published literatures. In the current version, EDK incorporates 61 diseases associated with 248 experimentally validated abnormal editing events located in 32 mRNAs, 16 miRNAs, 1 lncRNA and 11 viruses, and 44 aberrant activities involved with 6 editing enzymes, which together are curated from more than 200 publications. In addition, to facilitate standardization of editome-disease knowledge integration, we propose a data curation model in EDK, factoring an abundance of relevant information to fully capture the context of editome-disease associations. Taken together, EDK is a comprehensive collection of editome-disease associations and bears the great utility in aid of better understanding the RNA editing machinery and complex molecular mechanisms associated with human diseases.

A-to-I RNA Editing Uncovers Hidden Signals of Adaptive Genome Evolution in Animals

In animals, the most common type of RNA editing is the deamination of adenosines (A) into inosines (I). Because inosines basepair with cytosines (C), they are interpreted as guanosines (G) by the cellular machinery and genomically encoded G alleles at edited sites mimic the function of edited RNAs. The contribution of this hardwiring effect on genome evolution remains obscure. We looked for population genomics signatures of adaptive evolution associated with A-to-I RNA edited sites in humans and Drosophila melanogaster. We found that single nucleotide polymorphisms at edited sites occur 3 (humans) to 15 times (Drosophila) more often than at unedited sites, the nucleotide G is virtually the unique alternative allele at edited sites and G alleles segregate at higher frequency at edited sites than at unedited sites. Our study reveals that a significant fraction of coding synonymous and nonsynonymous as well as silent and intergenic A-to-I RNA editing sites are likely adaptive in the distantly related human and Drosophila lineages.

The preponderance of nonsynonymous A-to-I RNA editing in coleoids is nonadaptive

A-to-I editing enzymatically converts the base adenosine (A) in RNA molecules to inosine (I), which is recognized as guanine (G) in translation. Exceptionally abundant A-to-I editing was recently discovered in the neural tissues of coleoids (octopuses, squids, and cuttlefishes), with a greater fraction of nonsynonymous sites than synonymous sites subject to high levels of editing. Although this phenomenon is thought to indicate widespread adaptive editing, its potential advantage is unknown. Here we propose an alternative, nonadaptive explanation. Specifically, increasing the cellular editing activity permits some otherwise harmful G-to-A nonsynonymous substitutions, because the As are edited to Is at sufficiently high levels. These high editing levels are constrained upon substitutions, resulting in the predominance of nonsynonymous editing at highly edited sites. Our evidence for this explanation suggests that the prevalent nonsynonymous editing in coleoids is generally nonadaptive, as in species with much lower editing activities.

The neural tissues of coleoids have a greater fraction of nonsynonymous sites than synonymous sites subject to high levels of A-to-I RNA editing, a pattern thought to indicate widespread adaptive editing. Here the authors propose and provide evidence for an alternative, nonadaptive explanation.

Cell lineage inference from SNP and scRNA-Seq data

Several recent studies focus on the inference of developmental and response trajectories from single cell RNA-Seq (scRNA-Seq) data. A number of computational methods, often referred to as pseudo-time ordering, have been developed for this task. Recently, CRISPR has also been used to reconstruct lineage trees by inserting random mutations. However, both approaches suffer from drawbacks that limit their use. Here, we develop a method to detect significant, cell type specific, sequence mutations from scRNA-Seq data. We show that only a few mutations are enough for reconstructing good branching models. Integrating these mutations with expression data further improves the accuracy of the reconstructed models. As we show, the majority of mutations we identify are likely RNA editing events indicating that such information can be used to distinguish cell types.