The spotted ratfish, native to the northeastern Pacific Ocean and particularly abundant around Puget Sound, offers a rare window into evolutionary history. Unlike sharks, rays, and skates, which bear dermal denticles or tooth-like scales covering much of their bodies, the spotted ratfish is notably “naked,” lacking these structures with the exception of those on its pelvic claspers and this newly documented tenaculum. The tenaculum itself is a small, white, peanut-shaped appendage between the eyes during rest, which becomes erect and barbed with rows of formidable teeth in mature males.

The evolutionary implications of these teeth extend far beyond mere anatomical curiosity. Teeth are the products of a complex developmental process regulated by a conserved genetic toolkit shared among vertebrates. What the researchers uncovered, through detailed micro-CT scans, tissue analyses, and gene expression studies, is the presence of dental lamina—the critical band of tissue responsible for tooth development—beyond the confines of the oral cavity. Traditionally, it was believed this structure only existed inside the jaws, making the presence of dental lamina on the tenaculum a revolutionary observation.

From a developmental perspective, these findings suggest that a cluster of tooth-forming cells migrated from the oral region to the forehead during embryogenesis. In male spotted ratfish, the tenaculum initiates as a small protrusion which grows and mineralizes, eventually erupting through the skin and sprouting true teeth. In females, this process halts early and the tenaculum remains underdeveloped, highlighting a clear case of sexual dimorphism linked to reproductive behaviors.

Functionally, the toothed tenaculum serves an essential role in mating. Because sharks, ray-like fish, and chimaeras lack limbs, males have evolved diverse grasping structures to maintain contact with females during the physically demanding reproductive act underwater. The tenaculum’s hooked teeth latch firmly onto the female’s pectoral fin, complementing the function of the pelvic claspers, which inseminate the female. This adaptation ensures successful copulation despite the challenges posed by aquatic environments.

The bridge between fossil evidence and modern species strengthens the evolutionary narrative. Fossilized remains of ancient relatives to the spotted ratfish exhibit similar toothed structures on their tenacula, indicating that this trait has deep historical roots extending millions of years back. Paleontological data, coupled with modern genetic and anatomical studies, elucidate the repurposing of an ancient developmental program for teeth, which has been coopted to form this novel head appendage critical in reproduction.

Moreover, this discovery reveals a complex regulatory shift. Unlike teeth inside the jaw, which follow a well-characterized developmental timeline, the tenaculum’s teeth develop in concert with the pelvic claspers and are independent of the overall body size of the fish. This indicates that gene networks governing these novel teeth have been rewired through evolution, demonstrating remarkable plasticity in vertebrate morphogenesis.

Beyond the anatomical and evolutionary insights, the genetic analyses conducted were particularly illuminating. The researchers found gene expression profiles in the tenaculum that were congruent with those active in oral tooth development, but distinct from those regulating dermal denticles. This genetic evidence decisively confirms that the tenaculum’s tooth structures are homologous to classical vertebrate teeth, rather than mere residual denticles or modified scales.

This work invites a paradigm shift in how scientists conceptualize dental diversity across vertebrates. Sharks, long the model organisms for studying teeth due to their multiple rows and continuous tooth replacement, represent only a narrow spectrum of the broader evolutionary landscape. As Dr. Karly Cohen from the University of Washington’s Friday Harbor Labs remarks, chimaeras, often overlooked, reveal a more dynamic history of tooth placement and developmental flexibility. This could mean that many other species may harbor overlooked or cryptic tooth structures outside the oral cavity.

Technically, this study leveraged state-of-the-art imaging and molecular methodologies to track tooth development. High-resolution micro-CT scans allowed for precise visualization of the mineralized teeth erupting from the tenaculum, while histological sections verified the presence of dental lamina and tooth-forming germs. Complementary transcriptomic analyses provided the genetic blueprint underscoring these novel teeth’s formation, establishing a clear molecular signature aligned with traditional oral teeth.

The implications for evolutionary developmental biology are profound. Tooth development is controlled by a deeply conserved set of genes and signaling pathways, yet this discovery reveals that these genetic tools can be deployed in unconventional locations, enabling the evolution of new, functionally significant traits. This flexibility may explain the morphological innovations observed throughout vertebrate evolutionary history and motivates further search for atypical dentitions that could reshape our understanding of vertebrate anatomy.

In summary, the identification of teeth on the tenaculum of the spotted ratfish not only defies established biological dogma but also uncovers a new dimension in vertebrate developmental biology. Far from an evolutionary anomaly, these oral-adjacent toothed claspers are evolutionary relics offering a vivid glimpse into the creative potential of nature’s genetic and developmental frameworks. This discovery heralds a new chapter in the study of tooth evolution, function, and diversity, with potentially transformative implications for paleontology, developmental biology, and evolutionary theory.

Subject of Research: Animals

Article Title: Teeth outside the jaw: Evolution and development of the toothed head clasper in chimaeras

News Publication Date: 4-Sep-2025

Web References:
http://dx.doi.org/10.1073/pnas.2508054122

Image Credits: Ray Troll

Keywords:
Marine biology, Morphology, Natural history, Aquatic animals, Fish, Developmental biology, Aquatic ecology

Horses, long celebrated for their exceptional speed and endurance, boast remarkable physiological characteristics that enable them to perform as outstanding endurance runners. What sets them apart is their ability to efficiently take in, transport, and utilize oxygen, a trait that has been widely recognized as superior to that of elite human athletes. The maximal oxygen consumption (VO₂max) of horses is astonishingly more than twice that of the best human competitors, a testament to their extraordinary capabilities.

While the concentration of mitochondria in horse skeletal muscle is a crucial component that enhances energy production, it comes with a drawback. The increased mitochondrial activity leads to the production of reactive oxygen species (ROS), which can result in damaging tissue effects and cellular dysfunction. Understanding how horses have evolved mechanisms to manage the oxidative stress associated with their remarkable mitochondrial activity has been elusive until now.

To fill this knowledge gap, researchers led by Gianni Castiglione undertook an extensive evolutionary analysis of the KEAP1 gene, a critical regulator of redox balance and mitochondrial function, across 196 mammalian species. KEAP1 has emerged as a vital target in exercise science and has been implicated in various human health issues, including lung cancer and chronic obstructive pulmonary disease (COPD). The team’s comprehensive study reveals that modern horses, along with donkeys and zebras, have developed a unique adaptive feature involving a premature stop codon (UGA) in the KEAP1 gene.

This premature stop codon is not a detrimental error that truncates the protein as might typically be expected. Instead, the research reveals that horses have evolved a remarkable mechanism by which this stop codon is recoded efficiently into a cysteine amino acid (C15). This critical adaptation enhances the functionality of the KEAP1 protein, fundamentally altering its regulatory capacity. By mitigating the repression of NRF2—a protein essential for combating oxidative stress—this mutation leads to pronounced increases in mitochondrial respiration and ATP production.

The implications of this single-point genetic mutation are profound. Horses benefit from improved energy production while maintaining a careful balance with oxidative stress management. This stands in contrast to other mammals, where increased NRF2 activity can lead to detrimental effects. Thus, horses have managed to evolve a finely tuned adaptation that enhances their athletic performance, providing insights into the intricate relationship between genetics, energy metabolism, and oxidative stress.

The findings from Castiglione and colleagues open new avenues for understanding how specific genetic adaptations can contribute to extraordinary physical capabilities. The identification of the KEAP1 mutation and its functional consequences not only sheds light on equine physiology but also poses fascinating questions regarding its potential relevance to human health and performance.

This research underscores the significance of continued exploration into the genetic basis of endurance and athleticism in various species. The molecular mechanisms governing energy production and oxidative stress management could inform future approaches in sports science and medicine, paving the way for innovative strategies to enhance performance and mitigate diseases linked to oxidative stress in humans.

Moreover, the study emphasizes the evolutionary adaptability of vertebrates, showcasing how processes previously thought to be confined to viral adaptations can also be realized in mammals. The concept of recoding a de novo stop codon challenges established paradigms of genetic mutation and adaptation, suggesting a rich and complex evolutionary landscape waiting to be explored.

As the scientific community delves deeper into the implications of this research, it bears the potential to influence various fields, including genetics, exercise physiology, and even therapeutic strategies for diseases characterized by mitochondrial dysfunction and oxidative stress. The genetic blueprint unveiled in horses serves as a reminder of the intricate connections between evolution, adaptation, and performance that are ubiquitous in nature.

The revelation of the KEAP1 mutation as a significant factor underlying horse endurance adds a new layer to our understanding of genetic innovation in the animal kingdom. It demonstrates how a seemingly minor genetic alteration can yield substantial advantages, illustrating the delicate balance between evolution, environmental pressures, and physiological demands.

As we look to the future, the question remains—what other secrets might be hidden within the genomes of animals that enable extraordinary feats? This research serves as a catalyst for further investigations into the genetic determinants of endurance, providing hope for advancements in both animal performance and human health.

In conclusion, the study of the KEAP1 gene mutation in horses not only reveals a fascinating evolutionary adaptation but also suggests important implications for our understanding of athleticism across species. As researchers continue to uncover the complexities of genetic variations and their functional outcomes, we move closer to unlocking the full potential of genetic engineering and biotechnology in enhancing performance and treating diseases.

Subject of Research: Adaptation in horses’ endurance through KEAP1 gene mutation
Article Title: Running a genetic stop sign accelerates oxygen metabolism and energy production in horses
News Publication Date: 28-Mar-2025
Web References: DOI
References: N/A
Image Credits: N/A

Keywords: KEAP1 gene, endurance, horses, oxidative stress, energy metabolism, genetics, evolution, mitochondrial function, NRF2, adaptation, athletic performance.