The Runt domain (Runx) is a highly conserved 128 amino acid sequence motif that defines a metazoan family of sequence-specific DNA binding proteins required for the ontogeny of each of the animal species in which it has been functionally studied, as well as for the regulation of somatic stem cells and development of the lineages to which they give rise [14]. Runx genes facilitate developmental coordination of cell proliferation and differentiation [1], integrating the transduction of multiple signalling pathways [2] by nucleating the assembly of signal-responsive cis-regulatory modules [5]. Runx genes have only been found in animals [6, 7], suggesting that they may have evolved in concert with metazoan systems for developmental signalling.

All heretofore known Runx genes encode proteins that bear at their C-terminus a WRPY sequence motif (or a close variant thereof), which functions to recruit the Groucho/TLE corepressor to the cis-regulatory system [812]. Runx-WRPY mediated recruitment of Groucho is relatively weak and controlled by cis-regulatory sequence context [12, 13]. Depending on such context, Runx proteins can also function as Groucho-independent repressors, as well as activators [8, 14].

The purpose of this study was to extend our previous investigation of the evolution of Runx genes [6] by analyzing and comparing several new Runx gene sequences collected from recently sequenced genomes of lophotrochozoans and basally branching metazoans (see Additional File 1 for detailed methods). Although cnidarian and sponge Runx genes were described in a recent report [7], that study left open the question of whether the sponge Runx proteins bear a C-terminal Groucho recruitment motif. To address that question we examined Runx-encoding genomic and cDNA sequences from two sponges (Amphimedon queenslandica and Oscarella carmela), and compared these to Runx sequences collected from a phylogenetically broad sampling of other metazoan genomes, including that of the placozoan Trichoplax adhaerens [15].

Runx is a metazoan synapomorphy that has undergone independent duplications in a subset of triploblast lineages

Figure 1 depicts several representative examples of previously known [6, 7] or newly revealed (Table 1) Runx genes from across metazoan phylogeny, clustered according to the phylogenetic topology obtained by Sperling et al. [16]. As recently shown by Sullivan et al. [7], Runx-encoding sequences extend to the base of the metazoan family tree, with single orthologues encoded in the genome of the haplosclerid demosponge A. queenslandica and in expressed sequence tags from the homoscleromorph sponge O. carmela. Similarly, the anthozoan cnidarian Nematostella vectensis and the placozoan Trichoplax adherens each have a single Runx gene, as do several triploblast species, including the lancelet Branchiostoma floridae and the sea squirt Ciona intestinalis among deuterostomes; the nematode Caenorhabditis elegans among ecdysozoans; and the polychaete Capitella sp.I and the mollusk Lottia gigantea among lophotrochozoans. In contrast, vertebrates, sea urchins (Strongylocentrotus purpuratus), dipteran insects (Drosophila melanogaster), clitellate annelids (Helobdella robusta), and planarians (Schmidtea mediterranea) each have two or more Runx genes.

Figure 1
figure 1

Schematic structure of Runx genes from the major metazoan clades. Scale models of Runx genes described previously [6, 7] from mouse (Mus musculus, Mm), sea squirt (Ciona intestinalis, Ci), fruit fly (Drosophila melanogaster, Dm), nematode worm (Caenorhabditis elegans, Ce), and sea anemone (Nematostella vectensis, Nv) are shown in comparison to new models obtained from various recent genome projects (Table 1). The latter include Runx genes from lancelet (Branchiostoma floridae, Bf), sea urchin (Strongylocentrotus purpuratus, Sp, corrected; the arrow points to an intron that was previously missed [6, 21]), leech (Helobdella robusta, Hr), polychaete (Capitella sp. I, CspI), snail (Lottia gigantea) planarian (Schmidtea mediterranea, Sm), placozoan (Trichoplax adherens, Ta), and demosponge (Amphimedon queesnslandica, Amq). The Runt domain is shaded grey and black, with the black box denoting the highly conserved exon encoding its C-terminal end. The C-terminal WRPY Groucho-recruitment motif is shaded Red. A hypothetical model of the homoscleromorph sponge Runx gene (Oscarella carmela, Osc) is shown; although as yet there is no genomic sequence from which exon-intron structure of this gene can be inferred (as indicated by question marks), the predicted exonic coding sequences containing the Runt domain and C-terminal LWRPY are represented in assembled ESTs.

Table 1 Sources of sequences used in this analysis

Comparison of the gene architectures suggests that the primordial Runx gene contained three introns, the first of which interrupts the coding sequence of the Runt domain (found in every representative except for the insect runt orthologues), the second of which lies at the C-terminal end of the Runt domain (found in all of the representatives except two, HrRunx2 and LgRunx, both from lophotrochozoans), and the third lying between the two exons that encode the poorly conserved C-terminal sequence of the protein (missing in three of the insect genes and one of the leech genes; Fig. 1). This basic four-exon architecture is displayed by the demosponge, placozoan and anthozoan Runx genes, and among the known triploblast Runx genes, by the two sea urchin paralogues, the single lancelet orthologue, and the two planarian paralogues. Except for the additional intron within the sequence that encodes the N-terminal half of the Runt domain in all the vertebrate paralogues (Fig. 1), the basal architecture is conserved in vertebrate Runx3, which supports previous propositions for that gene being the most ancient of the vertebrate paralogues [17]. The additional N-terminal intron in Runx3, which is also found in each of the other vertebrate Runx paralogues, is also found in the C. intestinalis orthologue (but not in the cephalochordate B. floridae), consistent with recent phylogenies that place cephalochordates basal to {urochordates+vertebrates} in the chordate lineage [18].

To confirm and extend previous analyses of Runx family relations [6, 7], we used our expanded Runx sequence dataset to calculate trees by Bayesian, distance neighbor-joining (NJ), and maximum likelihood (ML) methods. The three trees have slightly different topologies; the Bayesian tree is shown in Figure 2A. All three analyses confidently support the branch separating the two sponge Runx genes from eumetazoan genes. Additionally, the protostome and chordate clades are recovered in all three trees but the positions of cnidarian, placozoan, and echinoderm genes differ between analyses. While only the NJ tree places echinoderms correctly inside a deuterostome clade, this clade also erroneously includes cnidarian and placozoan genes. Bayesian and ML analyses correctly place the latter two genes at the base of the bilaterian clade but wrongly group echinoderm genes with protostome genes. Relationships within the protostomes are unclear and none of the three analyses separates these genes into lophotrochozoan and ecdysozoan clades. This may be due to long-branch attraction between the Runx genes from S. mediterranea, H. robusta, and C. elegans. Thus, these genes were removed in a second set of analyses (Fig. 2B), where a lophotrochozoan clade and a clade comprising the four D. melanogaster genes are recovered in all three trees. These analyses suggest that there was only one Runx gene in the lineage between the metazoan and the lophotrochozoan-ecdysozoan last common ancestors. Hence, the multiple Runx genes present in some of the animals in this study are most likely the products of independent duplications within each of the lineages [6] (Fig. 1, colored boxes; note that a second sea urchin Runx gene, SpRunt-2, was recently found to be encoded in the sea urchin genome [19, 20], in contradiction to several previous reports [1, 6, 7, 21]).

Figure 2
figure 2

Bayesian trees of Runx sequences. In a first analysis (A), all the genes from Figure 1 were included and, in a second analysis (B), long-branched taxa (Runx genes from S. mediterranea, H. robusta, and C. elegans) were excluded from the dataset. The trees were calculated using a multiple sequence alignment of amino acid sequences corresponding to the Runt domain of each species. Percentages of bootstrap support greater than or equal to 50% are indicated above the node for the distance analysis (Phylip 3.6; 1000 replicates) and below the node for the maximum likelihood analysis (Phylip 3.6; 100 replicates). An asterisk under the node indicates a Bayesian posterior probability greater than or equal to 95%. Abbreviations as in Figure 1.

Previous reports have noted the absence of any Runx homologues in sequenced genomes of unicellular organisms [6, 7], including the choanoflagellate M. brevicolis [22], a member of the Holozoa taxon that is most closely related to Metazoa. We confirmed the absence of a Runx sequence motif in the M. brevicolis genome using tBLASTn searches. Thus, the Runt domain appears to have evolved in concert with complex multicellularity in the animal clade. Furthermore, unlike many other metazoan-specific transcription factor classes [23], the Runx gene did not duplicate in early animals, or even within some of the bilaterian lineages.

AmqRunx lacks a Groucho recruitment motif

As reported previously [7], Runx genes are found in both the haplosclerid demosponge A. queenslandica and the homoscleromorph sponge O. carmela. Although genome sequence is not yet available for the latter, a sequence encoding a Runx protein was recovered from an assembly of available ESTs. The predicted OscRunx protein terminates with the amino acid sequence WRPY (Fig. 3) [see Additional File 2], the C-terminal Groucho-recruitment motif found encoded in all heretofore known Runx genes (Fig. 1). Note that there are vertebrate splice variants that lack a C-terminal WRPY [2426], and that one each of the two leech and two planarian paralogues do not appear to terminate in WRPY (Fig. 1) [see Additional File 2]. Thus, some contexts have functional requirements for Runx protein isoforms lacking a C-terminal WRPY. Nevertheless, all of the eumetazoan species depicted in Fig. 1 (as well as the homoscleromorph sponge) encode at least one Runx protein that terminates in WRPY or a close variant thereof.

Figure 3
figure 3

Alignment of AmqRunx and OscRunx amino acid sequences. Identities are marked with asterisks, whereas conservative changes are marked with dots. The Runt domains are highlighted in grey and black, for reference to the scheme depicted in Figure 1. The arrows indicate predicted intron positions with respect to the coding sequence of AmqRunx. Proline residues in the C-terminal domains are highlighted in green, whereas serines and threonines are highlighted in yellow. The C-terminal WRPY motif in OscRunx is highlighted in red.

A genomic sequence contig from A. queenslandica was predicted to encode a Runx gene with four exons, displaying an architecture very similar to that of the placozoan and cnidarian genes (Fig. 1) [7]. The predicted coding sequence of AmqRunx is 1,566 bp with the Runt domain contained within the first 474 bp. As is typical for Runx proteins, the predicted C-terminal domain of AmqRunx (amino acid residues 159–479) is enriched for proline (12%), serine (16%), and threonine (7%) residues, a PST enrichment similar to that previously reported for the C-terminal domain of NvRunx [7] and that displayed by the C-terminal domain of OscRunx (Fig. 3). Surprisingly however, the C-terminus of AmqRunx does not bear the WRPY motif or any variant thereof (Fig. 3). Furthermore, no open reading frames encoding WRPY were found along the genomic contig in which AmqRunx is found. The A. queenslandica genome does however encode a bona fide Groucho homologue (Additional File 3 and unpublished data), as well as several transcription factors that are predicted to interact with Groucho [12], including a hairy/Hey homologue with a FRPW motif and a number of NK class genes with an engrailed homology 1 (EH-1) motif ([27, 28]; BMD, unpublished data).

The lack of a C-terminal WRPY motif in AmqRunx was verified by expressed sequence data. Based on alignment with genomic DNA, EST sequence 2941805_1 was found to encode the last 115 bp of the AmqRunx coding sequence, the stop codon, and an additional 626 bp of 3' UTR spanning two exons. In order to confirm that this EST was transcribed from AmqRunx, oligonucleotide primers – forward primer in the Runt domain and reverse primer in the EST-encoded 3' UTR region – were used to amplify the sequence both from A. queenslandica adult and embryonic RNA. An amplicon of the correct size and sequence was obtained (Additional File 4), thus confirming the veracity of the AmqRunx gene prediction.

The contig bearing AmqRunx contains sequences predictive of additional genes flanking the Runx gene (Fig. 4), which argues against the possibility that the AmqRunx gene model is missing a C-terminal exon that might produce alternative splice variants. Moreover, the veracity of the contig assembly is further supported by the remarkable fact that a syntenic relationship between Runx and Supt3h, previously reported to exist in vertebrates [29] and which we found also to exist in cnidarians (N. vectensis), lancelets (B. floridae), and polychaetes (Capitella sp. I), is conserved in the demosponge (Fig. 4).

Figure 4
figure 4

Schematic of the 20 kb genomic sequence contig bearing AmqRunx. Predicted exons are shown as black boxes. The syntenic relationship between Runx and Supt3h is conserved between demosponge (A. queenslandica) and mouse (Mus musculus), and is also found (at least) in the genomes of a cnidarian (N. vectensis), a basal chordate (B. floridae), and a teleost (Takifugu rubripes; [29]).

Although homoscleromorph sponges are still commonly grouped with demosponges in the phylum Porifera (Fig. 5A), this classification has been called into question, as has the monophyly of sponges (and hence 'Porifera' as a true phylum) [16]. The fact that AmqRunx lacks a C-terminal WRPY motif is consistent with the more recent proposition that sponges are paraphyletic [16, 30], with calcisponges and homoscleromorphs branching after demosponges along the lineage leading to eumetazoans (Fig. 5B). The conventional scenario, which holds that sponges are monophyletic (Fig. 5A), would require that several characters held in common between eumetazoans and homoscleromorph sponges (i.e., acrosomes, true epithelia, and a C-terminal WRPY motif linked to Runx) be either convergent homoplasies, or metazoan pleisiomorphies that were all lost in the demosponge lineage leading to A. queenslandica. Although it is possible that the loss of multiple characters occurred within the demosponge lineage, it is unlikely that body plan simplification is in itself sufficient to relax the selection pressure for maintaining the Runx-WRPY linkage, as evidenced by its maintenance in placozoans. The more parsimonious scenario is that the C-terminal WRPY motif of Runx proteins, and presumably the consequent recruitment of Groucho to a subset of Runx target cis-regulatory modules, originated in eumetazoan ancestors following their divergence from the sponge lineage leading to A. queenslandica (Fig. 5B). An interesting possibility is that the Runx associated WRPY motif originated in Epitheliozoa {eumetazoans and homoscleromorphs} [16], which would suggest that Runx-WRPY mediated cis-regulatory recruitment of Groucho is functionally linked to the evolution and development of an epithelium. Testing this possibility awaits the sequencing of a calcisponge Runx gene.

Figure 5
figure 5

Scenarios for Runx-WRPY evolution mapped onto alternative metazoan phylogenies. (A) Conventional phylogeny wherein the demosponge Amphimedon queenslandica (Amq) and the homoscleromorph sponge Oscarella carmela (Osc) are both classified as demosponges within the phylum Porifera. This scenario suggests that the WRPY motif was lost in the demosponge sub-lineage leading to Amq. (B) Alternative phylogeny wherein sponges are paraphyletic. In this tree homoscleromorphs are a sister group of eumetazoans within Epitheliozoa [16], which would imply that Runx gained the WRPY motif in the ancestors of the latter group following their divergence from demosponges, either prior to or after divergence from calcisponges.