Conserved and unique transcriptional features of pharyngeal arches in the skate (Leucoraja erinacea) and evolution of the jaw

Abstract

The origin of the jaw is a long-standing problem in vertebrate evolutionary biology. Classical hypotheses of serial homology propose that the upper and lower jaw evolved through modifications of dorsal and ventral gill arch skeletal elements, respectively. If the jaw and gill arches are derived members of a primitive branchial series, we predict that they would share common developmental patterning mechanisms. Using candidate and RNAseq/differential gene expression analyses, we find broad conservation of dorsoventral patterning mechanisms within the developing mandibular, hyoid and gill arches of a cartilaginous fish, the skate (Leucoraja erinacea). Shared features include expression of genes encoding members of the ventralising BMP and endothelin signalling pathways and their effectors, the joint markers nkx3.2 and gdf5 and pro-chondrogenic transcription factor barx1, and the dorsal territory marker pou3f3. Additionally, we find that mesenchymal expression of eya1/six1 is an ancestral feature of the mandibular arch of jawed vertebrates, while differences in notch signalling distinguish the mandibular and gill arches in skate. Comparative transcriptomic analyses of mandibular and gill arch tissues reveal additional genes differentially expressed along the dorsoventral axis of the pharyngeal arches, including scamp5 as a novel marker of the dorsal mandibular arch, as well as distinct transcriptional features of mandibular and gill arch muscle progenitors and developing gill buds. Taken together, our findings reveal conserved patterning mechanisms in the pharyngeal arches of jawed vertebrates, consistent with serial homology of their skeletal derivatives, as well as unique transcriptional features that may underpin distinct jaw and gill arch morphologies.

An evolutionarily conserved cis-regulatory element of Nkx3.2 contributes to early jaw joint morphology in zebrafish

“…The elephant shark JRS1 sequence drove reporter expression in the joint of the zebrafish first (mandibular) pharyngeal arch, consistent with other gnathostomes, but gene expression of nkx3.2 in the related elasmobranch chondrichthyans has been reported in the intermediate domains of all pharyngeal arches, including the hyoid and gill arches (Compagnucci et al, 2013;Hirschberger et al, 2021). If we assume elephant shark nkx3.2 gene expression mirrors that of elasmobranchs and that JRS1 drives nkx3.2 gene expression in all pharyngeal arches, our results would support the conclusion that much of the gene regulatory landscape of osteichthyan mandibular joints is found more broadly in the pharyngeal arches of chondrichthyans (Hirschberger et al, 2021). JRS1 knockout did not phenocopy the striking jaw joint fusion seen in zebrafish nkx3.2 gene knockout mutants (Miyashita et al, 2020;Waldmann et al, 2021), suggesting this enhancer is not essential for nkx3.2 expression at the time points analysed.…”

The pseudobranch of jawed vertebrates is a mandibular arch-derived gill

“…Although the pseudobranch clearly appears to develop from the posterior side of the mandibular arch in the skate, we sought to directly confirm this embryonic origin by cell lineage tracing. We and others have previously reported that foxl2 is expressed in the core mesoderm and presumptive gill-forming epithelium of the pharyngeal arches of cartilaginous fishes (Wotton et al, 2007;Hirschberger et al, 2021). Analysis of foxl2 expression by mRNA in situ hybridisation (ISH) in skate embryos at S24 reveals these expression patterns not only in the developing hyoid and gill arches, but also in the developing mandibular arch (Fig.…”

“…Chromogenic mRNA in situ hybridisation (ISH) for foxl2 (MW457610), gata2 (MZ501824), shh (EF100667) and ptc (EF100663) was performed in wholemount as in Hirschberger et al (2021), and on paraffin sections as in O'Neill et al (2007) with modifications according to Gillis et al (2012). mRNA ISH by chain reaction (HCR) on paraffin sections was carried out according to Choi et al (2018), with modifications as described in Criswell and Gillis (2020).…”

“…Apparent correspondence between the upper and lower jaw and the dorsal (epibranchial) and ventral (ceratobranchial) segments of the gill arch skeleton was recognised over a century ago and has led to various scenarios of jaw evolution by transformation of an anterior member of a hypothetical primitive series of gill-bearing arches (e.g., Gegenbaur, 1878;De Beer, 1937;Jarvik, 1980;Mallatt, 1996). There is now very strong evidence of a shared transcriptional network underlying axial patterning of the mandibular, hyoid and gill arch skeletons of jawed vertebrates (Gillis et al, 2013;Takechi et al, 2013;Hirschberger et al, 2021; reviewed by Medeiros and Crump, 2012), with some features of this network predating the origin of jaws (Cerny et al, 2010;Kuraku et al, 2010; reviewed by Square et al, 2017). These findings are consistent with serial homology of distinct dorsal, ventral, and intermediate domains of jawed vertebrate pharyngeal arches, and of the skeletal elements or tissues that derive, iteratively, from these domains.…”

Mineralized Cartilage and Bone-Like Tissues in Chondrichthyans Offer Potential Insights Into the Evolution and Development of Mineralized Tissues in the Vertebrate Endoskeleton

“…So, both ancestral jawed vertebrates and stem chondrichthyans had endoskeletal bone. What about living chondrichthyans, which are thought to retain the most ancestral vertebrate features ( Criswell and Gillis, 2020 ; Hirschberger et al, 2021 )? Despite all of these phylogenetic data, the chondrichthyan endoskeleton has traditionally been characterized as lacking bone ( Ørvig, 1951 ; Zangerl, 1966 ; Moss, 1977 ; Janvier, 1981 ; Maisey, 1988 ; Clement, 1992 ).…”