Rampant tooth loss across 200 million years of frog evolution

Convergent Evolution Has Led to the Loss of Claw Proteins in Snakes and Worm Lizards

The evolution of cornified skin appendages, such as hair, feathers, and claws, is closely linked to the evolution of proteins that establish the unique mechanical stability of these epithelial structures. We hypothesized that the evolution of the limbless body anatomy of the Florida worm lizard (Rhineura floridana) and the concomitant loss of claws had led to the degeneration of genes with claw-associated functions. To test this hypothesis, we investigated the evolution of three gene families implicated in epithelial cell architecture, namely type I keratins, type II keratins, and genes of the epidermal differentiation complex in R. floridana in comparison with other squamates. We report that the orthologs of mammalian hair and nail keratins have undergone pseudogenization in R. floridana. Likewise, the epidermal differentiation complex genes tentatively named EDYM1 and EDCCs have been lost in R. floridana. The aforementioned genes are conserved in various lizards with claws, but not in snakes. Proteomic analysis of the cornified claws of the bearded dragon (Pogona vitticeps) confirmed that type I and type II hair keratin homologs, EDYM1 and EDCCs, are protein components of claws in squamates. We conclude that the convergent evolution of a limbless body was associated with the convergent loss of claw keratins and differentiation genes in squamates.

Increasing the impact of vertebrate scientific collections through 3D imaging: The openVertebrate (oVert) Thematic Collections Network

The impact of preserved museum specimens is transforming and increasing by three-dimensional (3D) imaging that creates high-fidelity online digital specimens. Through examples from the openVertebrate (oVert) Thematic Collections Network, we describe how we created a digitization community dedicated to the shared vision of making 3D data of specimens available and the impact of these data on a broad audience of scientists, students, teachers, artists, and more. High-fidelity digital 3D models allow people from multiple communities to simultaneously access and use scientific specimens. Based on our multiyear, multi-institution project, we identify significant technological and social hurdles that remain for fully realizing the potential impact of digital 3D specimens.

Unveiling vertebrate development dynamics in frog Xenopus laevis using micro-CT imaging

BACKGROUND

Xenopus laevis, the African clawed frog, is a versatile vertebrate model organism in various biological disciplines, prominently in developmental biology to study body plan reorganization during metamorphosis. However, a notable gap exists in the availability of comprehensive datasets encompassing Xenopus' late developmental stages.

FINDINGS

This study utilized micro-computed tomography (micro-CT), a noninvasive 3-dimensional (3D) imaging technique with micrometer-scale resolution, to explore the developmental dynamics and morphological changes in Xenopus laevis. Our approach involved generating high-resolution images and computed 3D models of developing Xenopus specimens, spanning from premetamorphosis tadpoles to fully mature adults. This dataset enhances our understanding of vertebrate development and supports various analyses. We conducted a careful examination, analyzing body size, shape, and morphological features, focusing on skeletogenesis, teeth, and organs like the brain and gut at different stages. Our analysis yielded valuable insights into 3D morphological changes during Xenopus' development, documenting details previously unrecorded. These datasets hold the solid potential for further morphological and morphometric analyses, including segmentation of hard and soft tissues.

CONCLUSIONS

Our repository of micro-CT scans represents a significant resource that can enhance our understanding of Xenopus' development and the associated morphological changes in the future. The widespread utility of this amphibian species, coupled with the exceptional quality of our scans, which encompass a comprehensive series of developmental stages, opens up extensive opportunities for their broader research application. Moreover, these scans can be used in virtual reality, 3D printing, and educational contexts, further expanding their value and impact.

Using salamanders as model taxa to understand vertebrate feeding constraints during the late Devonian water-to-land transition

The vertebrate water-to-land transition and the rise of tetrapods brought about fundamental changes for the groups undergoing these evolutionary changes (i.e. stem and early tetrapods). These groups were forced to adapt to new conditions, including the distinct physical properties of water and air, requiring fundamental changes in anatomy. Nutrition (or feeding) was one of the prime physiological processes these vertebrates had to successfully adjust to change from aquatic to terrestrial life. The basal gnathostome feeding mode involves either jaw prehension or using water flows to aid in ingestion, transportation and food orientation. Meanwhile, processing was limited primarily to simple chewing bites. However, given their comparatively massive and relatively inflexible hyobranchial system (compared to the more muscular tongue of many tetrapods), it remains fraught with speculation how stem and early tetrapods managed to feed in both media. Here, we explore ontogenetic water-to-land transitions of salamanders as functional analogues to model potential changes in the feeding behaviour of stem and early tetrapods. Our data suggest two scenarios for terrestrial feeding in stem and early tetrapods as well as the presence of complex chewing behaviours, including excursions of the jaw in more than one dimension during early developmental stages. Our results demonstrate that terrestrial feeding may have been possible before flexible tongues evolved. This article is part of the theme issue 'Food processing and nutritional assimilation in animals'.

Bat teeth illuminate the diversification of mammalian tooth classes

Tooth classes are an innovation that has contributed to the evolutionary success of mammals. However, our understanding of the mechanisms by which tooth classes diversified remain limited. We use the evolutionary radiation of noctilionoid bats to show how the tooth developmental program evolved during the adaptation to new diet types. Combining morphological, developmental and mathematical modeling approaches, we demonstrate that tooth classes develop through independent developmental cascades that deviate from classical models. We show that the diversification of tooth number and size is driven by jaw growth rate modulation, explaining the rapid gain/loss of teeth in this clade. Finally, we mathematically model the successive appearance of tooth buds, supporting the hypothesis that growth acts as a key driver of the evolution of tooth number and size. Our work reveal how growth, by tinkering with reaction/diffusion processes, drives the diversification of tooth classes and other repeated structure during adaptive radiations.

XROMM Analysis of Feeding Mechanics in Toads: Interactions of the Tongue, Hyoid, and Pectoral Girdle

During feeding in many terrestrial vertebrates, the tongue acts in concert with the hyoid and pectoral girdle. In frogs, these three elements are interconnected by musculature. While the feeding mechanics of the anuran tongue are well-studied, little is known of how the motions of the tongue relate to the movements of the skeleton or how buccal structures move following closure of the mouth. Although features such as the pectoral girdle and hyoid are not externally visible in frogs, their motions can be tracked in X-ray video. We used XROMM (X-ray Reconstruction of Moving Morphology) techniques to track the 3D movements of the tongue, hyoid apparatus, pectoral girdle, skull, and jaw during the feeding cycle of the cane toad, Rhinella marina . We show how the movements of these elements are integrated during tongue protrusion and prey capture, as well as during prey transport, swallowing, and recovery. Our findings suggest that the hyoid apparatus is important both for prey manipulation and swallowing. The tongue consistently stretches posterior to the skull during swallowing, often more than it stretches during protrusion to reach the prey. Feeding kinematics are similar between individuals, and the kinematics of unsuccessful strikes generally resemble those of successful strikes. Our data also provide a new perspective on the potential role of the pectoral girdle, an element with a predominant locomotor function, during feeding events. This work raises new questions about the evolution of feeding in frogs, as well as how the diversity of pectoral and buccal anatomy observed across anurans may influence feeding kinematics.

The abundance, biomass, and distribution of ants on Earth

Knowledge on the distribution and abundance of organisms is fundamental to understanding their roles within ecosystems and their ecological importance for other taxa. Such knowledge is currently lacking for insects, which have long been regarded as the "little things that run the world". Even for ubiquitous insects, such as ants, which are of tremendous ecological significance, there is currently neither a reliable estimate of their total number on Earth nor of their abundance in particular biomes or habitats. We compile data on ground-dwelling and arboreal ants to obtain an empirical estimate of global ant abundance. Our analysis is based on 489 studies, spanning all continents, major biomes, and habitats. We conservatively estimate total abundance of ground-dwelling ants at over 3 × 10¹⁵ and estimate the number of all ants on Earth to be almost 20 × 10¹⁵ individuals. The latter corresponds to a biomass of ∼12 megatons of dry carbon. This exceeds the combined biomass of wild birds and mammals and is equivalent to ∼20% of human biomass. Abundances of ground-dwelling ants are strongly concentrated in tropical and subtropical regions but vary substantially across habitats. The density of leaf-litter ants is highest in forests, while the numbers of actively ground-foraging ants are highest in arid regions. This study highlights the central role ants play in terrestrial ecosystems but also major ecological and geographic gaps in our current knowledge. Our results provide a crucial baseline for exploring environmental drivers of ant-abundance patterns and for tracking the responses of insects to environmental change.

Re-evaluating the morphological evidence for the re-evolution of lost mandibular teeth in frogs

Dollo's law of irreversibility states that once a complex structure is lost, it cannot be regained in the same form. Several putative exceptions to Dollo's law have been identified using phylogenetic comparative methods, but the anatomy and development of these traits are often poorly understood. Gastrotheca guentheri is renowned as the only frog with teeth on the lower jaw. Mandibular teeth were lost in the ancestor of frogs more than 200 million years ago and subsequently regained in G. guentheri. Little is known about the teeth in this species despite being a frequent example of trait “re‐evolution,” leaving open the possibility that it may have mandibular pseudoteeth. We assessed the dental anatomy of G. guentheri using micro‐computed tomography and histology and confirmed the longstanding assumption that true mandibular teeth are present. Remarkably, the mandibular teeth of G. guentheri are nearly identical in gross morphology and development to upper jaw teeth in closely related species. The developmental genetics of tooth formation are unknown in this possibly extinct species. Our results suggest that an ancestral odontogenic pathway has been conserved but suppressed in the lower jaw since the origin of frogs, providing a possible mechanism underlying the re‐evolution of lost mandibular teeth.

Rampant tooth loss across 200 million years of frog evolution

Teeth are present in most clades of vertebrates but have been lost completely several times in actinopterygian fishes and amniotes. Using phenotypic data collected from over 500 genera via micro-computed tomography, we provide the first rigorous assessment of the evolutionary history of dentition across all major lineages of amphibians. We demonstrate that dentition is invariably present in caecilians and salamanders, but teeth have been lost completely more than 20 times in frogs, a much higher occurrence of edentulism than in any other vertebrate group. The repeated loss of teeth in anurans is associated with a specialized diet of small invertebrate prey as well as shortening of the lower jaw, but it is not correlated with a reduction in body size. Frogs provide an unparalleled opportunity for investigating the molecular and developmental mechanisms of convergent tooth loss on a large phylogenetic scale.