BACKGROUND
rna editing is a process of post-transcriptional rna modification characterised by the insertion, deletion or modification of nucleotides  <cit> .
one of the most prevalent forms of rna editing is mediated by the adar  class of editing molecules, that work both selectively and non-selectively to deaminate adenosine residues into inosines  in double-stranded rna  substrates  <cit> .
this editing can modify and regulate gene product output, for example via codon modification , and influence splice site and small rna functionality  <cit> .
adars and a-to-i editing have been shown or proposed to play a role in diverse biological processes, the extent of which are not yet fully understood.
perhaps the best-studied role of adars is their involvement in editing neuronal receptor and ion channel components in taxa such as flies, squid and vertebrates  <cit> .
adars have also been implicated in regulatory pathway roles, with suggested functions for a-to-i editing in rnai antagonists  <cit> , in pro- or antiviral mechanisms  <cit> , and in the silencing of transposons and related sequences  <cit> .
gene-level regulation may also occur through editing-induced sequestration of transcripts within organelles  <cit>  or modification of splice sites  <cit> .
the primordial functionalities of the earliest adar systems are currently unknown.
adats  are critical proteins found in all eukaryotes.
adat <dig> is equipped with a single adenosine deaminase  domain, and is responsible for deamination of an adenosine in the trna wobble position into inosine  <cit> ; adat <dig> does not play a role in rna editing.
adars appear to have originated via the incorporation of a double-stranded rna binding  domain-encoding region into the adat <dig> coding sequence  <cit> .
duplication of this ancestral adar gene, and subsequent coding sequence and domain architecture diversification, has led to the generation of the adar family.
adar family members exist in bilaterians and cnidarians  <cit> , and were recently identified in the genome of the ctenophore pleurobrachia bachei  <cit> .
they have not been found in the placozoan trichoplax adhaerens, or in several non-metazoan eukaryotes, including choanoflagellates, fungi and plants, although these surveys have been limited in scope  <cit> .
in this paper, we identify and categorise adar protein family members present in the earliest branching metazoan lineages, including thirteen sponge and ten ctenophore species, and thus conclude that the full, or nearly full, repertoire of adar protein family members existed in the last common ancestor to all contemporary animals.
RESULTS
adars are present in the earliest branching metazoan lineages
we identified adars in a number of key opisthokont and eukaryote taxa for which a draft genome is available.
hmm and blast-based search methods were used to identify ad domain-encoding genes, and domain architecture predictions were employed to narrow this list to likely adar candidates .
adar sequences can be partitioned into three categories based on their overall domain architecture : adad-like ; adar2-like ; and adar1-like  and dsrb domains and one ad domain).
these categories are based on homo sapiens gene names and domain architectures.
the h. sapiens adad sequence, while related to adar <dig> and adar <dig>  is not implicated in rna editing.
adat-like sequences were identified in all species analysed .
we did not find evidence in invertebrates for adar3-like sequences, which possess an adar2-like architecture with an additional arginine-rich r-domain  <cit> .figure  <dig>
reconstruction of adar gene and domain evolution.
the table  lists the number of adar family members identified in each species.
adars are classified based on their domain architecture, as shown by the ‘ball-and-stick’ protein models above each adar name.
the z-dna/rna binding  and double-stranded rna binding  domains of the adar <dig> model are marked with an ‘n’ to indicate that multiple copies of these domains may be present in different species.
the domain architectures of all adar1-like proteins are depicted on the far right.
the adar gene counts were used to reconstruct adat/adar evolution, as mapped to the phylogenetic tree as coloured squares .
searches for adenosine deaminase , dsrb and zb domains were performed to determine the phylogenetic positions of whole-genome domain origin and loss events, regardless of adat/adar complement; these events are also mapped to the tree as coloured shapes.
green boxes separate the tree into the main phylogenetic groupings: bilateria , eumetazoa , metazoa , holozoa  and opisthokonta .
for clarity, we present the sponge and ctenophore lineages on equal footing, and depict all three adars as present in the metazoan stem.
the loss and gain of the adar1-like gene is marked with a question mark to illustrate the uncertainty in reconstructing these evolutionary events, which are elaborated upon further in figure  <dig> and additional file  <dig>
we identified novel candidate adar genes in the genomic sequences of representative species of two of the earliest-branching animal lineages – sponges  and ctenophores ; our methodology also isolated the adar candidates recently reported from the ctenophore pleurobrachia bachei  <cit> .
we identified one each of an adar1- and adar2-like gene in a. queenslandica, a single adar1-like gene in o. carmela, and two adad- and three adar2-like m. leidyi genes .
of the previously identified p. bachei adar candidates  <cit> , we categorised two sequences as adad-like and one as adar2-like, based on our domain architecture criteria .
analysis of the sycon ciliatum unpublished genome reveals that this calcarean sponge possesses adad-, adar2- and adar1-like genes .
the presence of multiple adar types in sponges, ctenophores and other invertebrates is consistent with the idea that the metazoan last common ancestor was already equipped with a suite of adars comparable to the repertoire that exists in humans and other modern bilaterians, and that adar gene and domain loss occurred independently in multiple metazoan lineages .
adars in the metazoan last common ancestor
sponges and ctenophores are of significant evolutionary interest because they are considered the two earliest-branching metazoan lineages.
however, questions remain as to whether sponges or ctenophores are the sister group to the rest of the metazoa  <cit> .
although both taxa have multiple adar family members, all four examined species, a. queenslandica, o. carmela, m. leidyi and p. bachei, differ in their complement of adar genes.
to facilitate a reconstruction of the evolution of the adar family, we searched for candidate adar sequences within the transcriptomes of an additional eleven sponge and eight ctenophore species .
across the analysed sponge species, we identified candidate transcripts belonging to all three adar categories, adad-, adar2- and adar1-like.
in no instance did a single species possess transcripts belonging to all three adar types ; adad-, adar2- and adar1-like genes are however present in the s. ciliatum genome .
in ctenophores, no adar1-like transcripts were identified in any species; only adad- and adar2-like transcripts were identified, either together or separately.
it should be noted, as these searches were performed on transcriptome data, that the failure to identify adar family members in particular species is not necessarily indicative that these sequences are absent from the genome; the overall lineage-specific trends do however allow insight into the taxonomic distribution of this protein family.figure  <dig>
adar family member distribution in sponges and ctenophores.
as in figure  <dig>  the number of candidate adar family members identified in each sponge and ctenophore genome  or transcriptome is shown.
the domain architectures of adar1-like sequences are given on the far right.
the phylogenetic relationships within the ctenophore  and sponge  lineages are depicted to the left.
adar <dig> sequences indicated by a ^ are predicted to encode three dsrb domains.
amphimedon queenslandica and pseudospongosorites suberitoides are abbreviated to conserve space.
until the relative phyletic positions of sponges and ctenophores are fully resolved, multiple reconstructions of adar evolution are obtained depending if sponges or ctenophores are the earlier-branching phylum.
adad-, adar2- and adar1-like proteins are all present in the sponge lineage, but adar1-like proteins, and indeed zb domains entirely , are absent in ctenophores.
from this we conclude that adat-, adad- and adar2-like sequences were all present in the metazoan ancestor.
adar1-like proteins were either present and subsequently lost in the ctenophore lineage, or gained later.
if ctenophores branch first, the adar1-like gene was either lost in this taxon, along with the zb domain  or gained in the sponge + eumetazoan clade after diverging from ctenophores .
alternatively, if sponges are the most basal metazoans, the adar1-like gene was either lost in ctenophores  or gained independently in both the sponge and eumetazoan groups .
scenario iv appears to be less likely, as it would require adar1-like genes to evolve twice.
a phylogenetic analysis of the adar family-associated ad domains from all analysed non-bilaterian genomes provided poor resolution regarding the evolutionary relationships between adad-, adar <dig> and adar1-like sequences .
however, as in earlier phylogenetic analyses of eumetazoan ad domains  <cit> , the ad domains from non-bilaterian adar1-like sequences were found to form a cluster with reasonable bootstrap support, suggesting that the adar1-like gene has undergone little diversification across evolutionary history.
interestingly, the ad domain of an m. leidyi adad-like gene is also present in this adar1-like ad domain cluster .
this raises the possibility of a fifth evolutionary scenario of adar evolution  where the metazoan ancestor encoded all three adar family members, and that domain loss events converted a ctenophore adar1-like protein into a protein with adad-like architecture leaving ctenophores with two genes classifiable as adad-like.
however, due to the poor bootstrap support for this tree overall, and as no p. bachei domain sequences are present in this cluster , it is currently unclear whether this result is evolutionarily significant.figure  <dig>
possible scenarios for adar evolution in the metazoan ancestor.
five different scenarios of gene gain and loss events could explain the adar family distribution observed in sponges, ctenophores and eumetazoans, depending on whether sponges or ctenophores are the earliest-branching metazoan lineage.
filled and blank shapes represent gene  or zb domain  gain and loss events, respectively.
in panel v, the arrow represents the possible conversion of an adar1-like sequence to an adad-like architecture via domain loss.
domain architecture of the adar1-like genes
adar1-like genes were identified in a diverse set of metazoans, and are present in a variety of domain conformations .
human and other vertebrate adar <dig> genes encode two zb, three dsrb, and one ad domain, while the sea urchin strongylocentrotus purpuratus genome encodes a protein equipped with three zb, one dsrb and one ad domain.
the nematostella vectensis adar <dig> protein possesses two zb , one dsrb and one ad domain.
all adar1-like proteins identified in the other studied non-deuterostome eumetazoan taxa encode one copy each of the zb, dsrb and ad domains.
interestingly, a diversity of domain architectures are encoded amongst the adar1-like genes and transcripts of sponges.
in a. queenslandica, the adar1-like gene encodes three zb, one dsrb and one ad domain, identical to the architecture of the s. purpuratus adar <dig>  while the o. carmela gene encodes the vertebrate-like domain complement of two zb, three dsrb and one ad domain ; the unpublished s. ciliatum genome encodes an adar1-like protein with two zb, one dsrb and one ad domain .
we also identified adar1-like transcripts from ephydatia muelleri and corticium candelabrum.
these sequences both possess one dsrb and one ad domain, and the e. muelleri sequence contains one zb domain while the c. candelabrum sequence has two .
the diversity of adar1-like architectures present in modern sponges complicates the resolution of the ancestral adar1-like form.
however, a combination of one zb, one dsrb and one ad domain remains the most parsimonious ancestral conformation; this form is seen within the sponge lineage  and in all analysed non-deuterostome eumetazoan species except n. vectensis.
adar1-like domain diversification has occurred in the sponge lineage, perhaps indicative of molecular tinkering allowing the testing and retaining in various species of different adar1-like domain architecture combinations.
it is currently unknown whether similar levels of interspecies diversity exist in other phyla or classes.
origin of the metazoan adar protein family
adat genes are present throughout eukaryotes and are responsible for the deamination of adenosine into inosine for trna functionality  <cit> .
although ad and dsrb domains evolved prior to eukaryotic cladogenesis , the first evidence of these domains coming together to form an ancestral adar exists in the lineage leading to the crown metazoa.
this is likely to have occurred when a duplicated adat gene was coupled to a gene or part of a gene encoding one – or possibly more – dsrb domains, via domain shuffling.
it appears most plausible that the first adar had one copy each of a dsrb and ad domain and thus was adad-like.
this new gene then duplicated and incorporated a second dsrb domain, forming an adar2-like gene.
the formation of the adar1-like gene involved the incorporation of one or more zb domains into either an adad- or adar2-like gene.
it is not clear which of these two family members was the original acceptor for the zb domain, however, the combination of a single zb and dsrb domain together in a number of species  suggests the former is more likely.
the adar suite was thus in place early in metazoan history.
minor alterations, namely gene loss and duplication events, have occurred in some animal lineages , but dramatic expansion and diversification events do not characterise the evolutionary history of the adar family.
CONCLUSIONS
the ancestral role of the adars is currently unknown.
indeed, the biochemical functionality of basal metazoan adar protein family members in a-to-i editing remains to be tested experimentally.
the existence of a diversified gene family in the earliest branching lineages of animals, but not in their close unicellular holozoan and fungal relatives, is consistent with this gene family being an animal-specific innovation.
the evolution of metazoan multicellularity and complexity was accompanied by a wide range of genomic innovations  <cit> .
the origin and expansion of the adar gene family prior to the diversification of crown metazoans is similar to other regulatory gene families, including micrornas and piwirnas, and many transcription factor and signalling pathway families  <cit> .
the maintenance of the adar gene family in most modern phyla suggests that rna editing was and remains an essential part of the metazoan regulatory toolkit.

 rna editing is a process of post-transcriptional rna modification characterised by the insertion, deletion or modification of nucleotides. one of the most prevalent forms of rna editing is mediated by the adar class of editing molecules, that work both selectively and non-selectively to deaminate adenosine residues into inosines in double-stranded rna substrates. this editing can modify and regulate gene product output, for example via codon modification, and influence splice site and small rna functionality. adars and a-to-i editing have been shown or proposed to play a role in diverse biological processes, the extent of which are not yet fully understood. perhaps the best-studied role of adars is their involvement in editing neuronal receptor and ion channel components in taxa such as flies, squid and vertebrates. adars have also been implicated in regulatory pathway roles, with suggested functions for a-to-i editing in rnai antagonists, in pro- or antiviral mechanisms, and in the silencing of transposons and related sequences. gene-level regulation may also occur through editing-induced sequestration of transcripts within organelles or modification of splice sites. the primordial functionalities of the earliest adar systems are currently unknown. adats are critical proteins found in all eukaryotes. adat is equipped with a single adenosine deaminase domain, and is responsible for deamination of an adenosine in the trna wobble position into inosine; adat does not play a role in rna editing. adars appear to have originated via the incorporation of a double-stranded rna binding domain-encoding region into the adat coding sequence. duplication of this ancestral adar gene, and subsequent coding sequence and domain architecture diversification, has led to the generation of the adar family. adar family members exist in bilaterians and cnidarians, and were recently identified in the genome of the ctenophore pleurobrachia bachei. they have not been found in the placozoan trichoplax adhaerens, or in several non-metazoan eukaryotes, including choanoflagellates, fungi and plants, although these surveys have been limited in scope. in this paper, we identify and categorise adar protein family members present in the earliest branching metazoan lineages, including thirteen sponge and ten ctenophore species, and thus conclude that the full, or nearly full, repertoire of adar protein family members existed in the last common ancestor to all contemporary animals.  adars are present in the earliest branching metazoan lineages we identified adars in a number of key opisthokont and eukaryote taxa for which a draft genome is available. hmm and blast-based search methods were used to identify ad domain-encoding genes, and domain architecture predictions were employed to narrow this list to likely adar candidates. adar sequences can be partitioned into three categories based on their overall domain architecture: adad-like; adar2-like; and adar1-like and dsrb domains and one ad domain). these categories are based on homo sapiens gene names and domain architectures. the h. sapiens adad sequence, while related to adar and adar is not implicated in rna editing. adat-like sequences were identified in all species analysed. we did not find evidence in invertebrates for adar3-like sequences, which possess an adar2-like architecture with an additional arginine-rich r-domain.figure reconstruction of adar gene and domain evolution. the table lists the number of adar family members identified in each species. adars are classified based on their domain architecture, as shown by the ‘ball-and-stick’ protein models above each adar name. the z-dna/rna binding and double-stranded rna binding domains of the adar model are marked with an ‘n’ to indicate that multiple copies of these domains may be present in different species. the domain architectures of all adar1-like proteins are depicted on the far right. the adar gene counts were used to reconstruct adat/adar evolution, as mapped to the phylogenetic tree as coloured squares. searches for adenosine deaminase, dsrb and zb domains were performed to determine the phylogenetic positions of whole-genome domain origin and loss events, regardless of adat/adar complement; these events are also mapped to the tree as coloured shapes. green boxes separate the tree into the main phylogenetic groupings: bilateria, eumetazoa, metazoa, holozoa and opisthokonta. for clarity, we present the sponge and ctenophore lineages on equal footing, and depict all three adars as present in the metazoan stem. the loss and gain of the adar1-like gene is marked with a question mark to illustrate the uncertainty in reconstructing these evolutionary events, which are elaborated upon further in figure  and additional file we identified novel candidate adar genes in the genomic sequences of representative species of two of the earliest-branching animal lineages – sponges and ctenophores; our methodology also isolated the adar candidates recently reported from the ctenophore pleurobrachia bachei. we identified one each of an adar1- and adar2-like gene in a. queenslandica, a single adar1-like gene in o. carmela, and two adad- and three adar2-like m. leidyi genes. of the previously identified p. bachei adar candidates, we categorised two sequences as adad-like and one as adar2-like, based on our domain architecture criteria. analysis of the sycon ciliatum unpublished genome reveals that this calcarean sponge possesses adad-, adar2- and adar1-like genes. the presence of multiple adar types in sponges, ctenophores and other invertebrates is consistent with the idea that the metazoan last common ancestor was already equipped with a suite of adars comparable to the repertoire that exists in humans and other modern bilaterians, and that adar gene and domain loss occurred independently in multiple metazoan lineages. adars in the metazoan last common ancestor sponges and ctenophores are of significant evolutionary interest because they are considered the two earliest-branching metazoan lineages. however, questions remain as to whether sponges or ctenophores are the sister group to the rest of the metazoa. although both taxa have multiple adar family members, all four examined species, a. queenslandica, o. carmela, m. leidyi and p. bachei, differ in their complement of adar genes. to facilitate a reconstruction of the evolution of the adar family, we searched for candidate adar sequences within the transcriptomes of an additional eleven sponge and eight ctenophore species. across the analysed sponge species, we identified candidate transcripts belonging to all three adar categories, adad-, adar2- and adar1-like. in no instance did a single species possess transcripts belonging to all three adar types; adad-, adar2- and adar1-like genes are however present in the s. ciliatum genome. in ctenophores, no adar1-like transcripts were identified in any species; only adad- and adar2-like transcripts were identified, either together or separately. it should be noted, as these searches were performed on transcriptome data, that the failure to identify adar family members in particular species is not necessarily indicative that these sequences are absent from the genome; the overall lineage-specific trends do however allow insight into the taxonomic distribution of this protein family.figure adar family member distribution in sponges and ctenophores. as in figure  the number of candidate adar family members identified in each sponge and ctenophore genome or transcriptome is shown. the domain architectures of adar1-like sequences are given on the far right. the phylogenetic relationships within the ctenophore and sponge lineages are depicted to the left. adar sequences indicated by a ^ are predicted to encode three dsrb domains. amphimedon queenslandica and pseudospongosorites suberitoides are abbreviated to conserve space. until the relative phyletic positions of sponges and ctenophores are fully resolved, multiple reconstructions of adar evolution are obtained depending if sponges or ctenophores are the earlier-branching phylum. adad-, adar2- and adar1-like proteins are all present in the sponge lineage, but adar1-like proteins, and indeed zb domains entirely, are absent in ctenophores. from this we conclude that adat-, adad- and adar2-like sequences were all present in the metazoan ancestor. adar1-like proteins were either present and subsequently lost in the ctenophore lineage, or gained later. if ctenophores branch first, the adar1-like gene was either lost in this taxon, along with the zb domain or gained in the sponge + eumetazoan clade after diverging from ctenophores. alternatively, if sponges are the most basal metazoans, the adar1-like gene was either lost in ctenophores or gained independently in both the sponge and eumetazoan groups. scenario iv appears to be less likely, as it would require adar1-like genes to evolve twice. a phylogenetic analysis of the adar family-associated ad domains from all analysed non-bilaterian genomes provided poor resolution regarding the evolutionary relationships between adad-, adar and adar1-like sequences. however, as in earlier phylogenetic analyses of eumetazoan ad domains, the ad domains from non-bilaterian adar1-like sequences were found to form a cluster with reasonable bootstrap support, suggesting that the adar1-like gene has undergone little diversification across evolutionary history. interestingly, the ad domain of an m. leidyi adad-like gene is also present in this adar1-like ad domain cluster. this raises the possibility of a fifth evolutionary scenario of adar evolution where the metazoan ancestor encoded all three adar family members, and that domain loss events converted a ctenophore adar1-like protein into a protein with adad-like architecture leaving ctenophores with two genes classifiable as adad-like. however, due to the poor bootstrap support for this tree overall, and as no p. bachei domain sequences are present in this cluster, it is currently unclear whether this result is evolutionarily significant.figure possible scenarios for adar evolution in the metazoan ancestor. five different scenarios of gene gain and loss events could explain the adar family distribution observed in sponges, ctenophores and eumetazoans, depending on whether sponges or ctenophores are the earliest-branching metazoan lineage. filled and blank shapes represent gene or zb domain gain and loss events, respectively. in panel v, the arrow represents the possible conversion of an adar1-like sequence to an adad-like architecture via domain loss. domain architecture of the adar1-like genes adar1-like genes were identified in a diverse set of metazoans, and are present in a variety of domain conformations. human and other vertebrate adar genes encode two zb, three dsrb, and one ad domain, while the sea urchin strongylocentrotus purpuratus genome encodes a protein equipped with three zb, one dsrb and one ad domain. the nematostella vectensis adar protein possesses two zb, one dsrb and one ad domain. all adar1-like proteins identified in the other studied non-deuterostome eumetazoan taxa encode one copy each of the zb, dsrb and ad domains. interestingly, a diversity of domain architectures are encoded amongst the adar1-like genes and transcripts of sponges. in a. queenslandica, the adar1-like gene encodes three zb, one dsrb and one ad domain, identical to the architecture of the s. purpuratus adar while the o. carmela gene encodes the vertebrate-like domain complement of two zb, three dsrb and one ad domain; the unpublished s. ciliatum genome encodes an adar1-like protein with two zb, one dsrb and one ad domain. we also identified adar1-like transcripts from ephydatia muelleri and corticium candelabrum. these sequences both possess one dsrb and one ad domain, and the e. muelleri sequence contains one zb domain while the c. candelabrum sequence has two. the diversity of adar1-like architectures present in modern sponges complicates the resolution of the ancestral adar1-like form. however, a combination of one zb, one dsrb and one ad domain remains the most parsimonious ancestral conformation; this form is seen within the sponge lineage and in all analysed non-deuterostome eumetazoan species except n. vectensis. adar1-like domain diversification has occurred in the sponge lineage, perhaps indicative of molecular tinkering allowing the testing and retaining in various species of different adar1-like domain architecture combinations. it is currently unknown whether similar levels of interspecies diversity exist in other phyla or classes. origin of the metazoan adar protein family adat genes are present throughout eukaryotes and are responsible for the deamination of adenosine into inosine for trna functionality. although ad and dsrb domains evolved prior to eukaryotic cladogenesis, the first evidence of these domains coming together to form an ancestral adar exists in the lineage leading to the crown metazoa. this is likely to have occurred when a duplicated adat gene was coupled to a gene or part of a gene encoding one – or possibly more – dsrb domains, via domain shuffling. it appears most plausible that the first adar had one copy each of a dsrb and ad domain and thus was adad-like. this new gene then duplicated and incorporated a second dsrb domain, forming an adar2-like gene. the formation of the adar1-like gene involved the incorporation of one or more zb domains into either an adad- or adar2-like gene. it is not clear which of these two family members was the original acceptor for the zb domain, however, the combination of a single zb and dsrb domain together in a number of species suggests the former is more likely. the adar suite was thus in place early in metazoan history. minor alterations, namely gene loss and duplication events, have occurred in some animal lineages, but dramatic expansion and diversification events do not characterise the evolutionary history of the adar family.  the ancestral role of the adars is currently unknown. indeed, the biochemical functionality of basal metazoan adar protein family members in a-to-i editing remains to be tested experimentally. the existence of a diversified gene family in the earliest branching lineages of animals, but not in their close unicellular holozoan and fungal relatives, is consistent with this gene family being an animal-specific innovation. the evolution of metazoan multicellularity and complexity was accompanied by a wide range of genomic innovations. the origin and expansion of the adar gene family prior to the diversification of crown metazoans is similar to other regulatory gene families, including micrornas and piwirnas, and many transcription factor and signalling pathway families. the maintenance of the adar gene family in most modern phyla suggests that rna editing was and remains an essential part of the metazoan regulatory toolkit. 
to reconstruct the origin of this protein family, we identified and categorised adar family members encoded in the genomes and/or transcriptomes of early-branching metazoan and closely related non-metazoan taxa, including thirteen sponge and ten ctenophore species. depending on the relationship of sponges and ctenophores to the rest of the metazoa, this is consistent with either adar being lost in ctenophores, as it has been in multiple metazoan lineages, or being an innovation that evolved after ctenophores diverged from the rest of the animal kingdom. the modern vertebrate adar family is comprised of adad, adar and adar each of which has a conserved domain architecture. a-to-i editingadenosine deaminase acting on rna evolutionrna editingdomain evolutionspongectenophoremetazoadomain architectureissue-copyright-statement© the author 2015 the adar family appears to be a metazoan innovation, with all family members in place in the earliest phyletic branches of the crown metazoa. the presence of adars in sponges and ctenophores is consistent with a-to-i editing being a post-transcriptional regulatory mechanism that was used by the last common ancestor to all living animals and subsequently has been preserved in most modern lineages.
 adar proteins convert adenosine into inosine in double-stranded rnas and have been shown to increase gene product diversity in a number of bilaterians, particularly mammals and flies. this enzyme family appears to have evolved from an adat ancestor, via the addition of a double-stranded rna binding domain. the modern vertebrate adar family is comprised of adad, adar and adar each of which has a conserved domain architecture. to reconstruct the origin of this protein family, we identified and categorised adar family members encoded in the genomes and/or transcriptomes of early-branching metazoan and closely related non-metazoan taxa, including thirteen sponge and ten ctenophore species.  we demonstrate that the adar protein family is a metazoan innovation, with the three adar subtypes being present in representatives of the earliest phyletic lineages of animals – sponges and ctenophores – but not in other closely related choanoflagellate and filasterean holozoans. adar is missing from all ctenophore genomes and transcriptomes surveyed. depending on the relationship of sponges and ctenophores to the rest of the metazoa, this is consistent with either adar being lost in ctenophores, as it has been in multiple metazoan lineages, or being an innovation that evolved after ctenophores diverged from the rest of the animal kingdom. the presence of z-dna binding domains in some sponge adars indicates an ancestral adar included this domain and it has been lost in multiple animal lineages.  the adar family appears to be a metazoan innovation, with all family members in place in the earliest phyletic branches of the crown metazoa. the presence of adars in sponges and ctenophores is consistent with a-to-i editing being a post-transcriptional regulatory mechanism that was used by the last common ancestor to all living animals and subsequently has been preserved in most modern lineages. electronic supplementary material the online version of this article contains supplementary material, which is available to authorized users. keywords a-to-i editingadenosine deaminase acting on rna evolutionrna editingdomain evolutionspongectenophoremetazoadomain architectureissue-copyright-statement© the author 2015 