Gill developmental program in the teleost mandibular arch

Nkx2.7 is a conserved regulator of craniofacial development

Craniofacial malformations arise from developmental defects in the head, face, and neck with phenotypes such as 22q11.2 deletion syndrome illustrating a developmental link between cardiovascular and craniofacial morphogenesis. NKX2-5 is a key cardiac transcription factor associated with congenital heart disease and mouse models of Nkx2-5 deficiency highlight roles in cardiac development. In zebrafish, nkx2.5 and nkx2.7 are paralogues in the NK4 family expressed in cardiomyocytes and pharyngeal arches. Despite shared cellular origins of cardiac and craniofacial tissues, the function of NK4 factors in head and neck patterning has not been elucidated. Molecular evolutionary analysis of NK4 genes shows that nkx2.5 and nkx2.7 are ohnologs resulting from whole genome duplication events. Nkx2.7 serves as a previously unappreciated regulator of branchiomeric muscle and cartilage formation for which nkx2.5 cannot fully compensate. Mechanistically, our results highlight that Nkx2.7 patterns the cranial neural crest and functions upstream of Endothelin1 to inhibit Notch signals. Together, our studies shed light on an evolutionarily conserved Nkx transcription factor with unique functions in vertebrate craniofacial development, advancing our understanding of congenital head and neck deformities.

Repurposing of a gill gene regulatory program for outer-ear evolution

How new structures emerge during evolution has long fascinated biologists. An example is how the diminutive bones of the mammalian middle ear arose from ancestral fish jawbones1. By contrast, the evolutionary origin of the outer ear, another mammalian innovation, remains a mystery, partly because it is supported by non-mineralized elastic cartilage, which is rarely recovered in fossils. Whether the outer ear arose de novo or through the reuse of ancestral developmental programs has remained unknown. Here we show that the outer ear shares gene regulatory programs with the gills of fishes and amphibians for both its initial outgrowth and the later development of the elastic cartilage. Comparative single-nucleus multiomics of the human outer ear and zebrafish gills reveals conserved gene expression and putative enhancers enriched for common transcription factor binding motifs. This is reflected by the transgenic activity of human outer-ear enhancers in gills, and of fish gill enhancers in the outer ear. Furthermore, single-cell multiomics of the cartilaginous book gills of horseshoe crabs reveals a developmental program shared with the distal-less homeobox (DLX)-mediated gill program of vertebrates, with a book-gill distal-less enhancer driving expression in zebrafish gills. We propose that elements of an invertebrate gill program were reutilized in vertebrates to generate first gills and then the outer ear.

Structure and function of the larval teleost fish gill

The fish gill is a multifunctional organ that is important in multiple physiological processes such as gas transfer, ionoregulation, and chemoreception. This characteristic organ of fishes has received much attention, yet an often-overlooked point is that larval fishes in most cases do not have a fully developed gill, and thus larval gills do not function identically as adult gills. In addition, large changes associated with gas exchange and ionoregulation happen in gills during the larval phase, leading to the oxygen and ionoregulatory hypotheses examining the environmental constraint that resulted in the evolution of gills. This review thus focuses exclusively on the larval fish gill of teleosts, summarizing the development of teleost larval fish gills and its function in gas transfer, ionoregulation, and chemoreception, and comparing and contrasting it to adult gills where applicable, while providing some insight into the oxygen vs ionoregulatory hypotheses debate.

The paradise fish, an advanced animal model for behavioral genetics and evolutionary developmental biology

Paradise fish (Macropodus opercularis) is an air-breathing freshwater fish species with a signature labyrinth organ capable of extracting oxygen from the air that helps these fish to survive in hypoxic environments. The appearance of this evolutionary innovation in anabantoids resulted in a rewired circulatory system, but also in the emergence of species-specific behaviors, such as territorial display, courtship and parental care in the case of the paradise fish. Early zoologists were intrigued by the structure and function of the labyrinth apparatus and a series of detailed descriptive histological studies at the beginning of the 20th century revealed the ontogenesis and function of this specialized system. A few decades later, these fish became the subject of numerous ethological studies, and detailed ethograms of their behavior were constructed. These latter studies also demonstrated a strong genetic component underlying their behavior, but due to lack of adequate molecular tools, the fine genetic dissection of the behavior was not possible at the time. The technological breakthroughs that transformed developmental biology and behavioral genetics in the past decades, however, give us now a unique opportunity to revisit these old questions. Building on the classic descriptive studies, the new methodologies will allow us to follow the development of the labyrinth apparatus at a cellular resolution, reveal the genes involved in this process and also the genetic architecture behind the complex behaviors that we can observe in this species.

Pre-mandibular pharyngeal pouches in early non-teleost fish embryos

The vertebrate pharynx is a key embryonic structure with crucial importance for the metameric organization of the head and face. The pharynx is primarily built upon progressive formation of paired pharyngeal pouches that typically develop in post-oral (mandibular, hyoid and branchial) domains. However, in the early embryos of non-teleost fishes, we have previously identified pharyngeal pouch-like outpocketings also in the pre-oral domain of the cranial endoderm. This pre-oral gut (POG) forms by early pouching of the primitive gut cavity, followed by the sequential formation of typical (post-oral) pharyngeal pouches. Here, we tested the pharyngeal nature of the POG by analysing expression patterns of selected core pharyngeal regulatory network genes in bichir and sturgeon embryos. Our comparison revealed generally shared expression patterns, including Shh, Pax9, Tbx1, Eya1, Six1, Ripply3 or Fgf8, between early POG and post-oral pharyngeal pouches. POG thus shares pharyngeal pouch-like morphogenesis and a gene expression profile with pharyngeal pouches and can be regarded as a pre-mandibular pharyngeal pouch. We further suggest that pre-mandibular pharyngeal pouches represent a plesiomorphic vertebrate trait inherited from our ancestor's pharyngeal metameric organization, which is incorporated in the early formation of the pre-chordal plate of vertebrate embryos.

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

The pseudobranch is a gill-like epithelial elaboration that sits behind the jaw of most fishes. This structure was classically regarded as a vestige of the ancestral gill arch-like condition of the gnathostome jaw. However, more recently, hypotheses of jaw evolution by transformation of a gill arch have been challenged, and the pseudobranch has alternatively been considered a specialised derivative of the second (hyoid) pharyngeal arch. Here, we demonstrate in the skate (Leucoraja erinacea) that the pseudobranch does, in fact, derive from the mandibular arch, and that it shares gene expression features and cell types with gills. We also show that the skate mandibular arch pseudobranch is supported by a spiracular cartilage that is patterned by a shh-expressing epithelial signalling centre. This closely parallels the condition seen in the gill arches, where cartilaginous appendages called branchial rays, which support the respiratory lamellae of the gills, are patterned by a shh-expressing gill arch epithelial ridge. Together with similar discoveries in zebrafish, our findings support serial homology of the pseudobranch and gills, and an ancestral origin of gill arch-like anatomical features from the gnathostome mandibular arch.

Summary: The skate pseudobranch is a gill serial homologue and reveals the ancestral gill arch-like nature of the jawed vertebrate mandibular arch.