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 bapx1 and gdf5 and pro-chondrogenic transcription factors barx1 and gsc, and the dorsalising transcription factor 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).…”

hox gene expression predicts tetrapod-like axial regionalization in the skate, Leucoraja erinacea

“…Gene Cloning, mRNA In Situ Hybridization, and Vibratome Sectioning. We performed gene cloning and whole-mount mRNA ISH as described in Hirschberger et al (52) for selected genes within the hox5 to 11 paralogy groups at S18, S20, S22, and S25, covering the onset to termination of somitogenesis. To precisely determine expression boundaries in selected embryos in which precise anterior expression limits were difficult to count in whole mount (i.e., hoxa11), we embedded post-ISH embryos in 15% (weight/volume) gelatin (as described in ref.…”

The skate spiracular organ develops from a unique neurogenic placode, distinct from lateral line placodes

“…In situ hybridisation was performed in wholemount and on sections according to the protocol of O'Neill et al (2012), with modifications according to Gillis et al (2012). Wholemount in situ hybridisation was performed as described by Hirschberger et al (2021). All sequence data are accessible through NCBI under the following accession numbers: Atoh1 (OP429207), Eya4 (JQ425114.1), Pax2 (OP429214), Phox2b (OP429208), Pou4f1 (OP429209), Six1 (OP429210), Sox2 (OP429211), Sox3 (OP429212), Tbx3 (OP429213).…”