Evolution & Development最新文献

Lambeosaurine hadrosaurids exhibited extreme modifications to the skull, where the premaxillae, nasals, and prefrontals were modified to form their iconic supracranial crests. This morphology contrasts with their sister group, Hadrosaurinae, which possessed the plesiomorphic arrangement of bones. Although studies have discussed differences between lambeosaurine and hadrosaurine skull morphology and ontogeny, there is little information detailing suture modifications through ontogeny and evolution. Suture morphology is of particular interest due to its correlation with the mechanical loading of the skull in extant vertebrates. We quantify and contrast the morphology of calvarial sutures in iguanodontians and ontogenetic series of Corythosaurus and Gryposaurus to test whether the evolution of lambeosaurine crests impacted the mechanical loading of the skull. We found that suture interdigitation (SI) increases through ontogeny in hadrosaurids, although this increase is more extreme in Corythosaurus than Gryposaurus, and overall suture complexity (i.e., overall shape) remained constant. Lambeosaurines also have higher SI than other iguanodontians, even in crestless juveniles, suggesting that increased sinuosity is unrelated to the structural support of the crest. Hadrosaurines and basal iguanodontians did not differ. Similarly, lambeosaurines have more complexly shaped sutures than hadrosaurines and basal iguanodontians, while the latter two groups do not differ. Taken together, these results suggest that lambeosaurine calvarial sutures are more interdigitated than other iguanodontians, and although suture sinuosity increased through ontogeny, the suture shape remained constant. These ontogenetic and evolutionary patterns suggest that increased suture complexity in lambeosaurines coincided with crest evolution, and corresponding modifications to their facial skeleton altered the distribution of stress while feeding.

Novel phenotypes can come about through a variety of mechanisms including standing genetic variation from a founding population. Cave animals are an excellent system in which to study the evolution of novel phenotypes such as loss of pigmentation and eyes. Asellus aquaticus is a freshwater isopod crustacean found in Europe and has both a surface and a cave ecomorph which vary in multiple phenotypic traits. An orange eye phenotype was previously revealed by F₂ crosses and backcrosses to the cave parent within two examined Slovenian cave populations. Complete loss of pigmentation, both in eye and body, is epistatic to the orange eye phenotype and therefore the orange eye phenotype is hidden within the cave populations. Our goal was to investigate the origin of the orange eye alleles within the Slovenian cave populations by examining A. aquaticus individuals from Slovenian and Romanian surface populations and Asellus aquaticus infernus individuals from a Romanian cave population. We found orange eye individuals present in lab raised surface populations of A. aquaticus from both Slovenia and Romania. Using a mapping approach with crosses between individuals of two surface populations, we found that the region known to be responsible for the orange eye phenotype within the two previously examined Slovenian cave populations was also responsible within both the Slovenian and the Romanian surface populations. Complementation crosses between orange eye Slovenian and orange eye Romanian surface individuals suggest that the same gene is responsible for the orange eye phenotype in both surface populations. Additionally, we observed a low frequency phenotype of eye loss in crosses generated between the two surface populations and also in the Romanian surface population. Finally, in a cave population from Romania, A. aquaticus infernus, we found that the same region is also responsible for the orange eye phenotype as the Slovenian cave populations and the Slovenian and Romanian surface populations. Therefore, we present evidence that variation present in the cave populations could originate from standing variation present in the surface populations and/or transgressive hybridization of different surface phylogenetic lineages rather than de novo mutations.

The cichlid fishes comprise the largest extant vertebrate family and are the quintessential example of rapid “explosive” adaptive radiations and phenotypic diversification. Despite low genetic divergence, East African cichlids harbor a spectacular intra- and interspecific morphological diversity, including the hyper-variable, neural crest (NC)-derived traits such as coloration and craniofacial skeleton. Although the genetic and developmental basis of these phenotypes has been investigated, understanding of when, and specifically how early, in ontogeny species-specific differences emerge, remains limited. Since adult traits often originate during embryonic development, the processes of embryogenesis could serve as a potential source of species-specific variation. Consequently, we designed a staging system by which we compare the features of embryogenesis between three Malawi cichlid species—Astatotilapia calliptera, Tropheops sp. ‘mauve’ and Rhamphochromis sp. “chilingali”—representing a wide spectrum of variation in pigmentation and craniofacial morphologies. Our results showed fundamental differences in multiple aspects of embryogenesis that could underlie interspecific divergence in adult adaptive traits. First, we identified variation in the somite number and signatures of temporal variation, or heterochrony, in the rates of somite formation. The heterochrony was also evident within and between species throughout ontogeny, up to the juvenile stages. Finally, the identified interspecific differences in the development of pigmentation and craniofacial cartilages, present at the earliest stages of their overt formation, provide compelling evidence that the species-specific trajectories begin divergence during early embryogenesis, potentially during somitogenesis and NC development. Altogether, our results expand our understanding of fundamental cichlid biology and provide new insights into the developmental origins of vertebrate morphological diversity.

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 characterize the expression of conserved genes known to mediate dental development. We find that shark denticle development 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.

A jaw joint between the squamosal and dentary is a defining feature of mammals and is referred to as the temporomandibular joint (TMJ) in humans. Driven by changes in dentition and jaw musculature, this new joint evolved early in the mammalian ancestral lineage and permitted the transference of the ancestral jaw joint into the middle ear. The fossil record demonstrates the steps in the cynodont lineage that led to the acquisition of the TMJ, including the expansion of the dentary bone, formation of the coronoid process, and initial contact between the dentary and squamosal. From a developmental perspective, the components of the TMJ form through tissue interactions of muscle and skeletal elements, as well as through interaction between the jaw and the cranial base, with the signals involved in these interactions being both biomechanical and biochemical. In this review, we discuss the development of the TMJ in an evolutionary context. We describe the evolution of the TMJ in the fossil record and the development of the TMJ in embryonic development. We address the formation of key elements of the TMJ and how knowledge from developmental biology can inform our understanding of TMJ evolution.

Covariation among discrete phenotypes can arise due to selection for shared functions, and/or shared genetic and developmental underpinnings. The consequences of such phenotypic integration are far-reaching and can act to either facilitate or limit morphological variation. The vertebrate brain is known to act as an “organizer” of craniofacial development, secreting morphogens that can affect the shape of the growing neurocranium, consistent with roles for pleiotropy in brain–neurocranium covariation. Here, we test this hypothesis in cichlid fishes by first examining the degree of shape integration between the brain and the neurocranium using three-dimensional geometric morphometrics in an F₅ hybrid population, and then genetically mapping trait covariation using quantitative trait loci (QTL) analysis. We observe shape associations between the brain and the neurocranium, a pattern that holds even when we assess associations between the brain and constituent parts of the neurocranium: the rostrum and braincase. We also recover robust genetic signals for both hard- and soft-tissue traits and identify a genomic region where QTL for the brain and braincase overlap, implicating a role for pleiotropy in patterning trait covariation. Fine mapping of the overlapping genomic region identifies a candidate gene, notch1a, which is known to be involved in patterning skeletal and neural tissues during development. Taken together, these data offer a genetic hypothesis for brain–neurocranium covariation, as well as a potential mechanism by which behavioral shifts may simultaneously drive rapid change in neuroanatomy and craniofacial morphology.

Developing organisms are often exposed to fluctuating environments that destabilize tissue-scale processes and induce abnormal phenotypes. This might be common in species that lay eggs in the external environment and with little parental care, such as many reptiles. In turtles, morphological development has provided striking examples of abnormal phenotypic patterns, though the influence of the environment remains unclear. To this end, we compared fluctuating asymmetry, as a proxy for developmental instability, in turtle hatchlings incubated in controlled laboratory and unstable natural conditions. Wild and laboratory hatchlings featured similar proportions of supernumerary scales (scutes) on the dorsal shell (carapace). Such abnormal scutes likely elevated shape asymmetry, which was highest in natural nests. Moreover, we tested the hypothesis that hot and dry environments cause abnormal scute formation by subjecting eggs to a range of hydric and thermal laboratory incubation regimes. Shape asymmetry was similar in hatchlings incubated at five constant temperatures (26–30°C). A hot (30°C) and severely Dry substrate yielded smaller hatchlings but scutes were not overtly affected. Our study suggests that changing nest environments contribute to fluctuating asymmetry in egg-laying reptiles, while clarifying the conditions at which turtle shell development remains buffered from the external environment.

Mammals almost always have seven cervical vertebrae. The strong evolutionary constraint on changes in this number has been broken in sloths and manatees. We have proposed that the extremely low activity and metabolic rates of these species relax the stabilizing selection against changes in the cervical count. Our hypothesis is that strong stabilizing selection in other mammals is largely indirect and due to associated pleiotropic effects, including juvenile cancers. Additional direct selection can occur due to biomechanical problems (thoracic outlet syndrome). Low metabolic and activity rates are thought to diminish these direct and indirect effects. To test this hypothesis within the primates, we have compared the number of cervical vertebrae of three lorisid species with particularly low activity and metabolic rates with those of more active primate species, including with their phylogenetically closest active relatives, the galagids (bushbabies). In support of our hypothesis, we found that 37.6% of the lorisid specimens had an abnormal cervical count, which is a higher percentage than in the other nine primate families, in which the incidence varied from zero to 2.2%. We conclude that our data support the importance of internal selection in constraining evolvability and of a relaxed stabilizing selection for increasing evolvability. Additionally, we discuss that there is no support for a role of the muscularized diaphragm in the evolutionary constraint.