The initiation knot is a signaling center required for molar tooth development.

Signaling centers, or organizers, regulate many aspects of embryonic morphogenesis. In the mammalian molar tooth, reiterative signaling in specialized centers called enamel knots (EKs) determines tooth patterning. Preceding the primary EK, transient epithelial thickening appears, the significance of which remains debated. Using tissue confocal fluorescence imaging with laser ablation experiments, we show that this transient thickening is an earlier signaling center, the molar initiation knot (IK), that is required for the progression of tooth development. IK cell dynamics demonstrate the hallmarks of a signaling center: cell cycle exit, condensation and eventual silencing through apoptosis. IK initiation and maturation are defined by the juxtaposition of cells with high Wnt activity to Shh-expressing non-proliferating cells, the combination of which drives the growth of the tooth bud, leading to the formation of the primary EK as an independent cell cluster. Overall, the whole development of the tooth, from initiation to patterning, is driven by the iterative use of signaling centers.

Polygenic and major-locus contributions to sexual maturation timing in Atlantic salmon.

Sexual maturation timing is a life-history trait central to the balance between mortality and reproduction. Maturation may be triggered when an underlying compound trait, called liability, exceeds a threshold. In many different species and especially fishes, this liability is approximated by growth and body condition. However, environmental vs. genetic contributions either directly or via growth and body condition to maturation timing remain unclear. Uncertainty exists also because the maturation process can reverse this causality and itself affect growth and body condition. In addition, disentangling the contributions of polygenic and major loci can be important. In many fishes, males mature before females, enabling the study of associations between male maturation and maturation-unbiased female liability traits. Using 40 Atlantic salmon families, longitudinal common-garden experimentation, and quantitative genetic analyses, we disentangled environmental from polygenic and major locus (vgll3) effects on male maturation, and sex-specific growth and condition. We detected polygenic heritabilities for maturation, growth, and body condition, and vgll3 effects on maturation and body condition but not on growth. Longitudinal patterns for sex-specific phenotypic liability, and for genetic variances and correlations between sexes suggested that early growth and condition indeed positively affected maturation initiation. However, towards spawning time, causality appeared reversed for males whereby maturation affected growth negatively and condition positively via both the environmental and genetic effects. Altogether, the results indicate that growth and condition are useful traits to study liability for maturation initiation, but only until maturation alters their expression, and that vgll3 contributes to maturation initiation via condition.

The origin and loss of periodic patterning in the turtle shell.

The origin of the turtle shell over 200 million years ago greatly modified the amniote body plan, and the morphological plasticity of the shell has promoted the adaptive radiation of turtles. The shell, comprising a dorsal carapace and a ventral plastron, is a layered structure formed by basal endochondral axial skeletal elements (ribs, vertebrae) and plates of bone, which are overlain by keratinous ectodermal scutes. Studies of turtle development have mostly focused on the bones of the shell; however, the genetic regulation of the epidermal scutes has not been investigated. Here, we show that scutes develop from an array of patterned placodes and that these placodes are absent from a soft-shelled turtle in which scutes were lost secondarily. Experimentally inhibiting Shh, Bmp or Fgf signaling results in the disruption of the placodal pattern. Finally, a computational model is used to show how two coupled reaction-diffusion systems reproduce both natural and abnormal variation in turtle scutes. Taken together, these placodal signaling centers are likely to represent developmental modules that are responsible for the evolution of scutes in turtles, and the regulation of these centers has allowed for the diversification of the turtle shell.

The integumental appendages of the turtle shell: an evo-devo perspective.

The turtle shell is composed of dorsal armor (carapace) and ventral armor (plastron) covered by a keratinized epithelium. There are two epithelial appendages of the turtle shell: scutes (large epidermal shields separated by furrows and forming a unique mosaic) and tubercles (numerous small epidermal bumps located on the carapaces of some species). In our perspective, we take a synthetic, comparative approach to consider the homology and evolution of these integumental appendages. Scutes have been more intensively studied, as they are autapomorphic for turtles and can be diagnostic taxonomically. Their pattern of tessellation is stable phylogenetically, but labile in the individual. We discuss the history of developmental investigations of these structures and hypotheses of evolutionary and anomalous variation. In our estimation, the scutes of the turtle shell are an evolutionary novelty, whereas the tubercles found on the shells of some turtles are homologous to reptilian scales.

The dawn of chelonian research: turtles between comparative anatomy and embryology in the 19th century.

Many evo-devo studies of the turtle's shell draw hypotheses and support from historical sources. The groundbreaking works of Cuvier, Geoffroy St. Hilaire, Carus, Rathke, Owen, and others are being revived in modern research, and their centuries-old understanding of the turtle's shell reconsidered. In the works of these eminent biologists of the 19th century, comparative anatomy and embryology of turtle morphology set the stage for future studies in developmental biology, histology, and paleontology. Given the impact that these works still make on modern research, it is important to develop a thorough appreciation of previous authors, regarding how they arrived at their conclusions (i.e., what counted as evidence?), whether there was debate amongst these authors about shell development (i.e., what counted as an adequate explanation?), and even why these men, some of the most powerful and influential thinkers and anatomists of their day, were concerned with turtles. By tracing and exposing the context and content of turtle shell studies in history, our aim is to inform modern debates about the evolution and development of the turtle's shell.

A comparative examination of odontogenic gene expression in both toothed and toothless amniotes.

A well-known tenet of murine tooth development is that BMP4 and FGF8 antagonistically initiate odontogenesis, but whether this tenet is conserved across amniotes is largely unexplored. Moreover, changes in BMP4-signaling have previously been implicated in evolutionary tooth loss in Aves. Here we demonstrate that Bmp4, Msx1, and Msx2 expression is limited proximally in the red-eared slider turtle (Trachemys scripta) mandible at stages equivalent to those at which odontogenesis is initiated in mice, a similar finding to previously reported results in chicks. To address whether the limited domains in the turtle and the chicken indicate an evolutionary molecular parallelism, or whether the domains simply constitute an ancestral phenotype, we assessed gene expression in a toothed reptile (the American alligator, Alligator mississippiensis) and a toothed non-placental mammal (the gray short-tailed opossum, Monodelphis domestica). We demonstrate that the Bmp4 domain is limited proximally in M. domestica and that the Fgf8 domain is limited distally in A. mississippiensis just preceding odontogenesis. Additionally, we show that Msx1 and Msx2 expression patterns in these species differ from those found in mice. Our data suggest that a limited Bmp4 domain does not necessarily correlate with edentulism, and reveal that the initiation of odontogenesis in non-murine amniotes is more complex than previously imagined. Our data also suggest a partially conserved odontogenic program in T. scripta, as indicated by conserved Pitx2, Pax9, and Barx1 expression patterns and by the presence of a Shh-expressing palatal epithelium, which we hypothesize may represent potential dental rudiments based on the Testudinata fossil record.

Turing's turtles all the way down: A conserved role of EDAR in the carapacial ridge suggests a deep homology of prepatterns across ectodermal appendages.

The scutes of the turtle shell are epidermal shields that begin their formation during the early stages of shell development. Like other skin appendages, turtle scutes are hypothesized to be patterned by reaction-diffusion systems. We have previously established ex vivo and in silico systems to study these mechanisms experimentally and have further shown that mathematical models can explain the dynamics of the induction of turtle scute primordia and the generation of final scute architecture. Using these foundations, we expand our current knowledge and test the roles of ectodysplasin and activin signaling in the development of turtle scutes. We find that these molecules play important roles in the prepatterning of scute primordia along the carapacial ridge and show that blocking Edar signaling may lead to a complete loss of marginal scute primordia. We show that it is possible to reproduce these observations using simple mathematical modeling, thereby suggesting a stabilizing role for ectodysplasin within the reaction-diffusion mechanisms. Finally, we argue that our findings further entrench turtle scutes within a class of developmental systems composed of hierarchically nested reaction-diffusion mechanisms, which is conserved across ectodermal organs.

Manipulation of Developmental Function in Turtles with Notes on Alligators.

Reptiles have great taxonomic diversity that is reflected in their morphology, ecology, physiology, modes of reproduction, and development. Interest in comparative and evolutionary developmental biology makes protocols for the study of reptile embryos invaluable resources. The relatively large size, seasonal breeding, and long gestation times of turtles epitomize the challenges faced by the developmental biologist. We describe protocols for the preparation of turtle embryos for ex ovo culture, electroporation, in situ hybridization, and microcomputed tomography. Because these protocols have been adapted and optimized from methods used for frog, chick, and mouse embryos, it is likely that they could be used for other reptilian species. Notes are included for alligator embryos where appropriate.

Correction to: Manipulation of Developmental Function in Turtles with Notes on Alligators.

The author added a sentence to this chapter. The text has been added to the chapter opening page.

Patterning of the turtle shell.

Interest in the origin and evolution of the turtle shell has resulted in a most unlikely clade becoming an important research group for investigating morphological diversity in developmental biology. Many turtles generate a two-component shell that nearly surrounds the body in a bony exoskeleton. The ectoderm covering the shell produces epidermal scutes that form a phylogenetically stable pattern. In some lineages, the bones of the shell and their ectodermal covering become reduced or lost, and this is generally associated with different ecological habits. The similarity and diversity of turtles allows research into how changes in development create evolutionary novelty, interacting modules, and adaptive physiology and anatomy.

Environmental Causation of Turtle Scute Anomalies in ovo and in silico.

The turtle shell is often described as an evolutionary novelty that facilitated the radiation of the clade Testudines. The scutes, or keratinous plates, of the turtle shell are hypothesized to be patterned by reaction-diffusion dynamics, and this property of their development provides explanatory power to mechanisms of anomalous variation. A mathematical model of scute development predicts that anomalous variation in the phylogenetically stable pattern of scutes is achieved by environmental influence on the developmental program. We test this prediction with data on patterns of scute variation from natural nests and controlled incubation of sea turtle eggs in Florida and Western Australia. We find that high temperatures are sufficient to produce anomalous patterns in turtle scutes, and that this correlation is even stronger when conditions are dry. Furthermore, we find that the patterns of variation are not random; greater anomalous variation is found in the midline vertebral scutes and during a critical period of turtle development.