Evolution of vertebrate gill covers via shifts in an ancient Pou3f3 enhancer

Abstract

15 Whereas the gill chambers of extant jawless vertebrates (lampreys and hagfish) open directly into 16 the environment, jawed vertebrates evolved skeletal appendages that promote the unidirectional flow 17 of oxygenated water over the gills. A major anatomical difference between the two jawed vertebrate 18 lineages is the presence of a single large gill cover in bony fishes versus separate covers for each gill 19 chamber in cartilaginous fishes. Here we find that these divergent gill cover patterns correlate with 20 the pharyngeal arch expression of Pou3f3 orthologs. We identify a Pou3f3 arch enhancer that is 21 deeply conserved from cartilaginous fish through humans but undetectable in lampreys, with minor 22 sequence differences in the bony versus cartilaginous fish enhancers driving the corresponding single 23 versus multiple gill arch expression patterns. In zebrafish, loss of Pou3f3 gene function disrupts gill 24 cover formation, and forced expression of Pou3f3b in the gill arches generates ectopic skeletal 25 elements resembling the multiple gill cover pattern of cartilaginous fishes. Emergence of this Pou3f3 26

Foxc1 establishes enhancer accessibility for craniofacial cartilage differentiation

“…In zebrafish Foxc1 (foxc1a-/-; foxc1b-/-) mutants, cartilages of the upper/dorsal face fail to develop (Xu et al, 2018). In order to isolate the dorsal arch CNCC precursors affected in Foxc1 mutants, we used a pou3f3b:Gal4; UAS:nlsGFP (pou3f3b>GFP) dorsal CNCC transgenic line (Barske et al, 2020) The decreased accessibility of cartilage-specific elements in dorsal CNCCs at 48 hpf supports previous studies that dorsal chondrocytes develop later than other chondrocytes in the zebrafish face (Barske et al, 2016;Schilling and Kimmel, 1997).…”

“…The Institutional Animal Care and Use Committee of the University of Southern California approved all experiments on zebrafish (Danio rerio) (Protocol #10885). Existing mutant and transgenic lines used in this study include foxc1a el542 and foxc1b el620 (Xu et al, 2018); Tg(sox10:Dsred) el110 and Tg(fli1a:EGFP) y1 (Askary et al, 2017); Tg(col2a1aBAC:GFP) el483 (Paul et al, 2016); and Tg(UAS:nlsGFP;α-crystallin:Cerulean) el609 and pou3f3b (Barske et al, 2020) . For enhancer transgenic lines, we synthesized accessible elements with flanking attB4 and attB1 sequences using IDT gBlocks and cloned these into pDONR-P4-P1R using the Gateway Tol2kit (Invitrogen) to create p5E enhancer constructs (Kwan et al, 2007).…”

Migratory patterns and evolutionary plasticity of cranial neural crest cells in ray-finned fishes

“…However, this may be due to little-studied embryonic development of different groups of cartilaginous fishes and a lack of focus on the details of CNC migration. Nevertheless, it is possible to reveal the developmental potential of the hyoid domain for the morphogenesis of the respiratory structure in the chimera, where gill filaments and especially opercular flap form on the outside of the hyoid arch (Didier et al, 1998;Barske et al, 2020) and thus could perform the same function as the gar opercular flap. This hyoid opercular flap remains in early embryogenesis of amniotes (Richardson et al, 2012), suggesting that it was present in the common ancestor.…”

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

“…Endothelin-1 (edn1) and bone morphogenetic protein 4 (bmp4) signalling from ventral mandibular arch epithelium and mesoderm promotes ventral expression of dlx5/6, hand2 and msx1/2 and imparts lower jaw identity (Ozeki et al, 2004, Miller et al, 2003; Clouthier et al, 1998; Beverdam et al, 2002; Depew et al, 2002; Yanagisawa et al, 2003; Alexander et al, 2011; Zuniga et al, 2011). Conversely, notch signalling (Barske et al, 2016; Zuniga et al, 2010) and six1 expression (Tavares et al, 2017) repress transcription of edn1 , and promote transcription of the upper jaw marker pou3f3 (Jeong et al, 2008; Barske et al, 2020), thereby imparting upper jaw identity. Finally, the jaw joint is specified at the interface of these upper and lower jaw gene expression domains, with the presumptive joint marked by the expression of bapx1/nkx3.2 (Miller et al, 2003; Lukas and Olsson, 2018) and gdf5 (Miller et al, 2003), and flanked by expression of the pro-chondrogenic (and joint-repressing) transcription factor barx1 (Nichols et al, 2013).…”