Biology subjects

Fraser, G. J.

Publications and source records attributed to Fraser, G. J..

5 recordsLinked to original sources

bioRxiv2025

Teeth Outside the Jaw: Evolution and Development of the Toothed Head Clasper in Chimaeras

Chimaeras (Holocephali) are an understudied group of mostly deep-ocean cartilaginous fishes (Chondrichthyes) with unique characteristics that distinguish them from their distant relatives, sharks, skates, and rays. Unlike sharks, chimaeras lack scales and do not have serially replacing rows of serrated teeth crowned with enameloid. Instead, they possess a fused dentition of dentine toothplates. Additionally, male chimaeras develop an articulated cartilaginous facial appendage, the tenaculum, which is covered in an arcade of tooth-like structures. These seeming extraoral teeth remain poorly understood, and their evolutionary origin is unclear. We investigate the development of the tenaculum and its teeth throughout the ontogeny of the Spotted Ratfish, Hydrolagus colliei, to assess homology and convergence between this novel craniofacial feature and oral jaws. Our study aims to: (1) describe the development of the tenaculum, (2) assess tenaculum tooth development in comparison to oral teeth and denticles, and (3) characterize the genes and tissues responsible for tenaculum tooth emergence. We found that juvenile male chimaeras develop a full tenaculum before tooth development is complete and that only mature males possess a fully toothed tenaculum. These extraoral teeth emerge from within the tenaculum rather than from the surrounding epithelium. We integrate our developmental data with fossil evidence of the tenacula dentition from the Carboniferous holocephalan Helodus simplex. Our findings show that the tenaculum is closely associated with the upper jaw and that tenacula dentition resembles separate shark-like oral tooth whorls more than modified dermal denticles. Significance StatementThe development and evolutionary history of extraoral dentition in vertebrates remain largely unexplored. This study investigates the ontogeny of the male tenaculum, a unique feature of chimaeras, revealing a tooth development pathway similar to the oral dentition in sharks. By integrating fossil data and molecular techniques, we hypothesize that tenaculum teeth are homologous to oral teeth rather than modified skin denticles, providing key insights into the plasticity of odontogenesis and craniofacial diversity in vertebrates.

developmental biology↗

bioRxiv2025

Tooth development in frogs: Implications for the re-evolution of lost mandibular teeth and the origin of a morphological innovation

Teeth have been a prominent feature of most vertebrates for 400 million years, and the core regulatory network underlying embryonic tooth formation is deeply conserved from fishes to amniotes. In frogs, however, odontogenesis is delayed, occurring instead during the postembryonic metamorphosis. Nearly all adult frogs have teeth on the upper jaw and lack lower jaw dentition, but a single species re-evolved mandibular teeth. Developmental-genetic mechanisms that underlie tooth formation in frogs are poorly understood, including whether an ancestral program is partially retained in the lower jaw that could facilitate the evolutionary reappearance of lost mandibular teeth. Using a developmental series of an unconventional model species, we assessed 1) if the gene network underlying odontogenic competence is conserved in the late-forming teeth of frogs; 2) if unique keratinized mouthparts, which function as an alternative feeding tool in anuran larvae, impede tooth induction; and 3) if transient tooth rudiments form in the anuran mandible. The tooth development network is conserved in the frog upper jaw, which displays dental expression patterns comparable to those of other vertebrates. There is, however, no evidence of tooth development in the mandible. Teeth emerge before keratinized mouthparts degenerate, but their location may be spatially constrained by keratin, and gene expression patterns of keratinized mouthparts and teeth overlap. We hypothesize that the novel mouthparts of tadpoles did not arise de novo but instead originated by partially co-opting the developmental program that typically mediates true tooth development.

evolutionary biology↗

bioRxiv2022

Teeth outside the mouth: the evolution and development of shark denticles

Vertebrate skin appendages are incredibly diverse. This diversity, which includes structures such as scales, feathers, and hair, likely evolved from a shared anatomical placode, suggesting broad conservation of the early development of these organs. Some of the earliest known skin appendages are dentine and enamel-rich tooth-like structures, collectively known as odontodes. These appendages evolved over 450 million years ago. Elasmobranchs (sharks, skates, and rays) have retained these ancient skin appendages in the form of both dermal denticles (scales) and oral teeth. Despite our knowledge of denticle function in adult sharks, our understanding of their development and morphogenesis is less advanced. Even though denticles in sharks appear structurally similar to oral teeth, there has been limited data directly comparing the molecular development of these distinct elements. Here, we chart the development of denticles in the embryonic small-spotted catshark (Scyliorhinus canicula) and characterise the expression of conserved genes known to mediate dental development. We find that shark denticle emergence shares a vast gene expression signature with developing teeth. However, denticles have restricted regenerative potential, as they lack a sox2+ stem cell niche associated with the maintenance of a dental lamina, an essential requirement for continuous tooth replacement. We compare developing denticles to other skin appendages, including both sensory skin appendages and avian feathers. This reveals that denticles are not only tooth-like in structure, but that they also share an ancient developmental gene set that is likely common to all epidermal appendages.

developmental biology↗

bioRxiv2021

An ancient dental signalling centre supports homology of an enamel knot in sharks

Development of tooth cusps is regulated by the enamel knot signalling centre. Fgf signalling regulates differential proliferation between the enamel knot and adjacent dental epithelia during tooth development, leading to formation of the dental cusp. The presence of an enamel knot in non-mammalian vertebrates is debated given differences in signalling. Here we show the conservation and restriction of fgf10 and fgf3 to the sites of future dental cusps in the shark (Scyliorhinus canicula), whilst also highlighting striking differences between the shark and mouse. We reveal shifts in tooth size, shape and cusp number following small molecule perturbations of canonical Wnt signalling. Resulting tooth phenotypes mirror observed effects in mammals, where canonical Wnt has been implicated as an upstream regulator of enamel knot signalling. In silico modelling of shark dental morphogenesis demonstrates how subtle changes in activatory and inhibitory signals can alter tooth shape, resembling phenotypes observed following experimental Wnt perturbation. Our results support the functional conservation of an enamel knot-like signalling centre throughout vertebrates and suggest that varied tooth types from sharks to mammals follow a similar developmental bauplan. Lineage-specific differences in signalling are not sufficient in refuting homology of this signalling centre, which is likely older than teeth themselves.

developmental biology↗

bioRxiv2018

Evolution and developmental diversity of skin spines in pufferfish

Teleost fishes develop a huge variety of skin ornaments. How these diverse skin structures develop in fishes is unknown. The teleost fish order Tetraodontiformes includes some of the most unusual fishes such as the ocean sunfish, triggerfish and pufferfish, and they all can develop a vast assortment of scale derivatives that cover their bodies. Pufferfish have some of the most extreme scale derivatives, dermal spines, which are erected during their characteristic puffing behavior. Here we show that pufferfish spines develop through conserved gene interactions essential for other vertebrate skin appendage formation, like hair and feathers. However, pufferfish spines form without EDA (ectodysplasin), an essential molecule for the development of most vertebrate skin appendages. Modifying signaling pathways lead to loss or reduction of spine coverage in pufferfish, suggesting a mechanism for skin appendage diversification. We suggest that pufferfish skin spines evolved from a basic teleost scale-type through derived gene network modification in Tetraodontiformes.

developmental biology↗