Phylogenetic and local genomic synteny analysis of Noggin genes of elasmobranchs
We searched for homologues of noggin genes in Chondrichthyes in available genomic databases for Callorhinchus milii, as a representative of Holocephali, the sister branch to elasmobranchs, as well as for representatives of all evolutionary branches of elasmobranchs: rays (small-tooth sawfish Pristis pectinata), skates (thorny skate Amblyraja radiata) and sharks (smaller spotted catshark Scyliorhinus canicula, whale shark Rhincodon typus, great white shark Carcharodon carcharias, zebra shark Stegostoma fasciatum, white-spotted bamboo shark Chiloscyllium plagiosum).
Phylogenetic analysis shows that Noggin proteins of Chondrichthyes generally cluster with homologues of other gnathostomes, although in all cases, they tend to form subgroups on the branches of individual Noggin branches.
Among the analysed representative chondrichthyans, noggin1 was found only in the genome of the elephant shark (C. milii) and in a reduced form of a pseudogene in the great white shark C. carcharias genome. The noggin1 pseudogene of C. carcharias is located on chromosome 22 and has stop codons in frame (JAGDEE010000072.1:65581332-65581823).
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The same gene is present in the database with Gene ID: 121293691 as a two-exon version. This splicing variant theoretically makes it possible to obtain a shortened Noggin1 protein without stop codons; however, all genes of the Noggin family described to date in vertebrates are single-exon genes. The absence of functional Noggin1 in C. archarias is indirectly confirmed by the fact that we were unable to detect noggin1 cDNA fragments in the EST databases of this shark species.
The Noggin1 of C. milii and the translated pseudogene of C. carcharias cluster closer to Noggin1 of gnathostomes but not very confidently and instead form a mini-subgroup within the Noggin1 clade.
Noggin2 was found in all cartilaginous fishes examined, and the Noggin2 cluster was quite confidently allocated to the Noggin2 branch of gnathostomes.
Noggin4 was found in C. milii, R. typus, S. canicula, C. plagiosum, and S. fasciatum. At the same time, Noggin4 was not found in the genomes of C. carcharias, A. radiata, and P. pectinata. The noggin4 genes of gnathostomes are very confidently clustered on a common branch, on which the chondrichthyan genes tend to move closer together, but with varying degrees of reliability in different tree algorithms.
In general, we can conclude from phylogenetic analysis that of the noggin genes described in gnathostomes, all three paralogues are present only in the most basally divergent branch - Holocephali. In the elasmobranchs examined, only noggin2 is stably present. Noggin4 has disappeared in some of the species examined, and surprisingly, noggin1 is absent in all elasmobranch genomes analysed (except for the pseudogene preserved in C. carcharias).
To supplement the results of phylogenetic analysis and confirm the disappearance of noggin1 in elasmobranchs, an analysis of local genomic synteny of noggins in this clade and other gnathostomes was carried out. The main objectives of this analysis were to confirm, using an independent criterion, the identified orthology of the noggin genes of chondrichthyans and other gnathostomes, as well as to confirm the absence of noggin1 homologues in the vicinity of characteristic neighbouring genes.
The analysis showed that one of the characteristic neighbouring genes for the noggin genes is the ankfn1 gene (which encodes Ankyrin repeat and fibronectin type-III domain-containing protein 1), which was previously noted as a neighbour of the noggin1/2 genes 30. In this case, it is important that this gene was also found in the vicinity of the chondrichthyan noggin4 gene (Figure 2, dotted lines). This reflects the unity of origin of all three gnathostome noggins. In addition to chondrichthyan noggins, the ankfn1 gene is found in the vicinity of noggin4 in birds (Figure 2).
To establish the orthology of noggins in different groups of vertebrates, it is important to identify unique neighbouring genes for each of the noggin orthologues. The screening showed that such genes for noggin2 are netrin-3 and tedc2 (which encode tubulin epsilon and delta complex protein 2) and for noggin4 are gcat (glycine C-acetyltransferase), galr2b (galanin receptor 2b-like) and mei1 (meiosis inhibitor protein 1). These neighbouring genes are found in the vicinity of the Noggin genes in all chondrichthyans considered and are also present in representatives of other groups of gnathostomes. Thus, based on a set of neighbouring genes, one can confidently identify the orthologous identity of the noggin gene in question. Additional confirmation of the disappearance of the noggin4 genes in C. carcharias, A. radiata and P. pectinata is their absence in the syntenic region of the genome - between the galr2b and gcat genes, where it is located in all other representatives of chondrichthyans and other representatives of gnathostomes (Figure 2).
The neighbourhood of noggin1 is characterized by the presence of the following genes: in the 5' region, these genes are tmem100 (transmembrane protein 100), mmd (monocyte to macrophage differentiation factor), and pctp (phosphatidylcholine transfer protein); in the 3' region, these genes are C17orf67, dgke (diacylglycerol kinase, epsilon), trim25 (tripartite motif containing 25), scpep1 (serine carboxypeptidase 1) and coil (coilin p80). The genome of any given animal does not necessarily contain the full set of these neighbours, but some of them are always present, and these genes (with the exception of Ankfn1) are unique neighbours of noggin1. The disappearance of noggin1 in chondrichthyans was confirmed by its absence between characteristic neighbouring genes in A. radiata, P. pectinata, S. canicula and C. plagiosum. R. typus and S. fasciatum show disruption of gene arrangement in the potential neighbourhood of noggin1. In the genome of C. carcharias, the noggin1 pseudogene is located in the synteny region and has neighbouring genes characteristic of noggin1.
In C. milii, all three noggin genes have gnathostome-specific neighbouring genes.
Thus, the results of the analysis of genomic synteny confirm the previous idea about the common origin of all three paralogues of the noggin genes of gnathostomes 30. The phylogenetic analysis data were also confirmed, indicating the presence of noggin2 orthologues in all chondrichthyans examined, the disappearance of noggin4 in a number of representatives of the group, and the almost complete disappearance of noggin1 in chondrichthyans (with the exception of the pseudogene preserved in C. carcharias). The fact that the disappearance of noggins is observed only in representatives of elasmobranchs, while in the basally divergent group of chondrichthyan Holocephali, all three gnathostome noggins are present, indicates that the disappearance of noggin1 in elasmobranchs has a secondary nature and may be the result of evolutionary specialization of representatives of this clade.
Spatial expression of noggin2 and noggin4 in grey bamboo shark embryos
Since the absence of noggin1 in elasmobranchs is unique for vertebrates, analysis of the expression pattern of the noggin2 and noggin4 present in this clade is of great interest. In particular, it would be interesting to evaluate whether noggin2, which, according to previous studies, is similar in properties to noggin1 but differs significantly in expression pattern, can spatially compensate for the absence of noggin1 in elasmobranchs.
Analysis of the expression pattern of noggin2 and noggin4 was carried out in embryos of the grey bamboo shark Chiloscyllium griseum by the whole mount in situ hybridization (ISH) method.
At stages 24-28, noggin2 was found to be widely expressed in the brain, with the exception of the anterior part of the hindbrain, the ventral tail, and the heart region (Figure 3 A-H).
Noggin4 at stage 24 was expressed diffusely in the head region, pharyngeal arches and somites of the trunk and tail (Figure 3 I, K). At stages 27–28, widespread diffuse expression continued, with increased levels observed in the pharyngeal arches and gill filaments (Figure 3 J, L).
Thus, noggin2 and noggin4 of C. griseum generally show an expression pattern similar to that of noggin2 and noggin4 orthologues in D. rerio and Xenopus 28, 29, 38.
Shark noggin2 induces complete secondary axes in Xenopus laevis
Since functional experiments on shark embryos in vivo are difficult due to the structural features of their eggs and embryo development, testing of the functional activity of the noggins of chondrichthyan was carried out on amphibian (Xenopus laevis) embryos. The ability of noggin2 and noggin4 of the grey bamboo shark C. griseum to induce secondary body axes in X. laevis embryos was assessed. To do this, synthetic grey bamboo shark noggin2 and noggin4 mRNAs were injected into the equatorial zone of the ventral region of X. laevis embryos at the 8-blastomere stage. As a result, it was found that 50 pg of shark noggin2 mRNA injected into X. laevis embryos led to the disruption of normal development at the neurula stage and the formation of characteristic mushroom-shaped embryos (Figure 4 A, B). Similar phenotypic effects were observed when X. laevis noggin2 mRNA was injected 17. Injections of smaller amounts of noggin2 mRNA (5 pg per embryo) resulted in the induction of additional body axes in 53% of cases (n=250), including complete ones containing forehead structures and eyes in 8% of cases (Figure 4 C–F). In some cases, the formation of full-fledged second heads with absolutely complete anterior cephalic regions and paired eyes was observed (Figure 4 G, H). Similar inductive activity has also been described for X. laevis noggin2.
Shark noggin4 mRNA, similar to X. laevis noggin4 mRNA, did not show the ability to induce the formation of secondary axes in X. laevis (Figure 4 I, J; 33).
The results obtained demonstrate the conserved properties of the shark noggin2 and noggin4 genes.