Comparative analysis of squamate brains unveils multi-level variation in cerebellar architecture associated with locomotor specialization

Ecomorphological studies evaluating the impact of environmental and biological factors on the brain have so far focused on morphology or size measurements, and the ecological relevance of potential multi-level variations in brain architecture remains unclear in vertebrates. Here, we exploit the extraordinary ecomorphological diversity of squamates to assess brain phenotypic diversification with respect to locomotor specialization, by integrating single-cell distribution and transcriptomic data along with geometric morphometric, phylogenetic, and volumetric analysis of high-definition 3D models. We reveal significant changes in cerebellar shape and size as well as alternative spatial layouts of cortical neurons and dynamic gene expression that all correlate with locomotor behaviours. These findings show that locomotor mode is a strong predictor of cerebellar structure and pattern, suggesting that major behavioural transitions in squamates are evolutionarily correlated with mosaic brain changes. Furthermore, our study amplifies the concept of 'cerebrotype', initially proposed for vertebrate brain proportions, towards additional shape characters.Peer reviewe

The alternative regenerative strategy of bearded dragon unveils the key processes underlying vertebrate tooth renewal

Deep understanding of tooth regeneration is hampered by the lack of lifelong replacing oral dentition in most conventional models. Here, we show that the bearded dragon, one of the rare vertebrate species with both polyphyodont and monophyodont teeth, constitutes a key model for filling this gap, allowing direct comparison of extreme dentition types. Our developmental and high-throughput transcriptomic data of microdissected dental cells unveils the critical importance of successional dental lamina patterning, in addition to maintenance, for vertebrate tooth renewal. This patterning process happens at various levels, including directional growth but also gene expression levels, dynamics, and regionalization, and involves a large number of yet uncharacterized dental genes. Furthermore, the alternative renewal mechanism of bearded dragon dentition, with dual location of slow-cycling cells, demonstrates the importance of cell migration and functional specialization of putative epithelial stem/progenitor niches in tissue regeneration, while expanding the diversity of dental replacement strategies in vertebrates.Peer reviewe

Ontogeny of Hemidactylus (Gekkota, Squamata) with emphasis on the limbs

Squamate reptiles constitute a major component of the world's terrestrial vertebrate diversity, encompassing many morphotypes related to ecological specialization. Specifically, Gekkota, the sister clade to most other squamates, have highly specialized autopodia, which have been linked to their ecological plasticity. In this study, a developmental staging table of the gecko Hemidactylus, housed at the Museum fur Naturkunde, is established. Twelve post-ovipositional stages are erected, monitoring morphological embryological transitions in eye, ear, nose, heart, limbs, pharyngeal arches, and skin structures. Ecomorphological specializations in the limbs include multiple paraphalanges, hypothesized to aid in supporting the strong muscles, that are situated adjacent to metacarpal and phalangeal heads. Furthermore, some phalanges are highly reduced in manual digits III and IV and pedal digits III, IV, and V. Development, composition, and growth of limb elements is characterized in detail via mu CT, histochemistry, and bone histological analysis. Using known life history data from two individuals, we found an average lamellar bone accretion rate in the humeral diaphysis comparable to that of varanids. Various adult individuals also showed moderate to extensive remodeling features in their long bone cortices, indicating that these animals experience a highly dynamic bone homeostasis during their growth, similar to some other medium-sized to large squamates. This study of in-ovo development of the gecko Hemidactylus and its ecomorphological specializations in the adult autopodia, enlarges our knowledge of morphological trait evolution and of limb diversity within the vertebrate phylum.Peer reviewe

The developmental origins of heterodonty and acrodonty as revealed by reptile dentitions

Despite the exceptional diversity and central role of dentitions in vertebrate evolution, many aspects of tooth characters remain unknown. Here, we exploit the large array of dental phenotypes in acrodontan lizards, including EDA mutants showing the first vertebrate example of positional transformation in tooth identity, to assess the developmental origins and evolutionary patterning of tooth types and heterodonty. We reveal that pleurodont versus acrodont dentition can be determined by a simple mechanism, where modulation of tooth size through EDA signaling has major consequences on dental formula, thereby providing a new flexible tooth patterning model. Furthermore, such implication of morphoregulation in tooth evolution allows predicting the dental patterns characterizing extant and fossil lepidosaurian taxa at large scale. Together, the origins and diversification of tooth types, long a focus of multiple research fields, can now be approached through evo-devo approaches, highlighting the importance of underexplored dental features for illuminating major evolutionary patterns.Peer reviewe

Skull Development, Ossification Pattern, and Adult Shape in the Emerging Lizard Model Organism Pogona vitticeps: A Comparative Analysis With Other Squamates

The rise of the Evo-Devo field and the development of multidisciplinary research tools at various levels of biological organization have led to a growing interest in researching for new non-model organisms. Squamates (lizards and snakes) are particularly important for understanding fundamental questions about the evolution of vertebrates because of their high diversity and evolutionary innovations and adaptations that portrait a striking body plan change that reached its extreme in snakes. Yet, little is known about the intricate connection between phenotype and genotype in squamates, partly due to limited developmental knowledge and incomplete characterization of embryonic development. Surprisingly, squamate models have received limited attention in comparative developmental studies, and only a few species examined so far can be considered as representative and appropriate model organism for mechanistic Evo-Devo studies. Fortunately, the agamid lizard Pogona vitticeps (central bearded dragon) is one of the most popular, domesticated reptile species with both a well-established history in captivity and key advantages for research, thus forming an ideal laboratory model system and justifying his recent use in reptile biology research. We first report here the complete post-oviposition embryonic development for P. vitticeps based on standardized staging systems and external morphological characters previously defined for squamates. Whereas the overall morphological development follows the general trends observed in other squamates, our comparisons indicate major differences in the developmental sequence of several tissues, including early craniofacial characters. Detailed analysis of both embryonic skull development and adult skull shape, using a comparative approach integrating CT-scans and gene expression studies in P. vitticeps as well as comparative embryology and 3D geometric morphometrics in a large dataset of lizards and snakes, highlights the extreme adult skull shape of P. vitticeps and further indicates that heterochrony has played a key role in the early development and ossification of squamate skull bones. Such detailed studies of embryonic character development, craniofacial patterning, and bone formation are essential for the establishment of well-selected squamate species as Evo-Devo model organisms. We expect that P. vitticeps will continue to emerge as a new attractive model organism for understanding developmental and molecular processes underlying tissue formation, morphology, and evolution.Peer reviewe

The ecological origins of snakes as revealed by skull evolution

The ecological origin of snakes remains amongst the most controversial topics in evolution, with three competing hypotheses: fossorial; marine; or terrestrial. Here we use a geometric morphometric approach integrating ecological, phylogenetic, paleontological, and developmental data for building models of skull shape and size evolution and developmental rate changes in squamates. Our large-scale data reveal that whereas the most recent common ancestor of crown snakes had a small skull with a shape undeniably adapted for fossoriality, all snakes plus their sister group derive from a surface-terrestrial form with non-fossorial behavior, thus redirecting the debate toward an underexplored evolutionary scenario. Our comprehensive heterochrony analyses further indicate that snakes later evolved novel craniofacial specializations through global acceleration of skull development. These results highlight the importance of the interplay between natural selection and developmental processes in snake origin and diversification, leading first to invasion of a new habitat and then to subsequent ecological radiations.Peer reviewe

Multiple evolutionary origins and losses of tooth complexity in squamates

Teeth act as tools for acquiring and processing food, thus holding a prominent role in vertebrate evolution. In mammals, dental-dietary adaptations rely on tooth complexity variations controlled by cusp number and pattern. Complexity increase through cusp addition has dominated the diversification of mammals. However, studies of Mammalia alone cannot reveal patterns of tooth complexity conserved throughout vertebrate evolution. Here, we use morphometric and phylogenetic comparative methods across fossil and extant squamates to show they also repeatedly evolved increasingly complex teeth, but with more flexibility than mammals. Since the Late Jurassic, multiple-cusped teeth evolved over 20 times independently from a single-cusped common ancestor. Squamates frequently lost cusps and evolved varied multiple-cusped morphologies at heterogeneous rates. Tooth complexity evolved in correlation with changes in plant consumption, resulting in several major increases in speciation. Complex teeth played a critical role in vertebrate evolution outside Mammalia, with squamates exemplifying a more labile system of dental-dietary evolution.Peer reviewe

Déterminisme de la structure sociale chez le bar juvénile Dicentrarchus labrax en conditions d'auto-nourrissage (approches neuro-éthologique et physiologique)

Ce travail avait pour objectif de déterminer, par une approche multidisciplinaire, la structure sociale émergente au sein de groupes de bars juvéniles Dicentrarchus labrax en conditions d'auto-nourrissage. Les résultats montrent que seuls quelques individus, les manipulateurs principaux, sont particulièrement actifs vis-à-vis du distributeur à la demande, alors que le reste du groupe reste peu, voire totalement inactif. Une telle structuration sociale semble pérenne et particulièrement robuste. Les manipulateurs principaux ne présentent aucun bénéfice en termes de prise alimentaire et de croissance, ils ne démontrent pas non plus de comportement agressif pour défendre l'accès au levier ou à l'aliment. Néanmoins, l'activité du manipulateur principal semble ajustée sur les besoins du groupe qui bénéficie de sa production. Ce modèle social ressemble à de nombreux égards au modèle de producteur / profiteur , dans lequel les profiteurs usurpent la nourriture trouvée par le (ou les) producteur(s). Si aucun bénéfice apparent n'est décelable pour les manipulateurs principaux, en revanche, ces individus présentent une bonne efficacité alimentaire. Il reste à déterminer si cette différence d'efficacité alimentaire est une conséquence de leur fonction ou une cause qui les prédisposerait à devenir les manipulateurs principaux. Nos résultats démontrent également une corrélation négative entre les niveaux physiologiques de stress (5-HIAA/5-HT), expression ARNm POMC, cortisol plasmatique) et le statut de demande alimentaire des individus. Ces différences de stress peuvent s'expliquer par les différences inter-individuelles dans le contrôle du système de nourrissage et la possibilité d'anticiper ou non l'heure et la fréquence des repas. Enfin, ces travaux montrent que les canaux de communication visuels et chimiques sont la base de systèmes de communication chez le bar, et que ce dernier dispose de capacités cognitives relativement élaborées pour la transmission sociale et pour la reconnaissance conspécifiques familiers (individus du même groupe social). Cette capacité de reconnaissance module les réponses comportementales mais également l'expression neurale du neuropeptide arginine vasotocine (AVT) au niveau de l'aire préoptique de l'hypothalamus. Au-delà de la contribution de ces travaux dans la connaissance du comportement social du bar juvénile, nos résultats sont susceptibles d'intéresser les pisciculteurs. Il contribue à la mise en évidence d'indicateurs éthologiques et physiologiques dans le cadre de l'amélioration du bien être des poissons au sein des élevagesBy a multidisciplinary approach, this PhD work aimed to determine the social structure underlying the individual differences in self-feeding behaviour within juvenile sea bass Dicentrarchus labrax groups. The results show that a few individuals, the high-triggering fish, within the group strongly operate the self-feeder, whereas the remaining individuals are composed of low- and null-triggering fish. Such triggering structure exhibits a high constancy and seems to be particulary robust. However, the high-triggering fish do not show direct benefits, neither in feed intake nor in growth rate. In addition, they do not exhibit agonistic behaviour for the food ressource or the trigger defence. Moreover, the activity of the high-triggering fish seems to be adjusted on the whole group food needs rather than on their own needs. Such social model looks like the producer / scrounger social model, in which the scroungers are individuals usurping the ressources found by producers. No visible benefits are noticed for the high-triggerers, however, they appear to be most feed efficient fish in the group. Then a question remains, whether such food efficiency differences are the consequence of their triggering function or the cause which lead one individual to achieve the high-triggering status. Our results also show a negative correlation between physiological stress levels (5-HIAA /5-HT, POMC mRNA expression, plasma cortisol) and the individual triggering behaviour. We assume that these differences may be linked to the individual differences in the control of the time and the frequency of the meal distribution, which permits to the high-triggering fish an advantage to anticipate the meal time. Finally, the last studies show that the visual and chemical sensory cues are the basis of conspecific communication in juvenile sea bass. In addition, this fish species exhibits cognitive abilities for social transmission and recognition of familiar conspecifies (based on a past experience). Such social recognition modulates behavioural responses, as well as, the neural expression of the neuropeptide arginine vosotocin (AVT) in the preoptic area of the hypothalamus. On one hand, this wotrk contributes to improve scientific knowledge on the social behaviour of sea bass ; on the other hand, our results may have a real interest for fish farmers to identify ethological and physiological indicators of sea bass welfare in aquacultureST ETIENNE-BU Sciences (422182103) / SudocSudocFranceF