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

Abstract

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.

Variations in the proliferative activity of the peripheral retina correlate with postnatal ocular growth in squamate reptiles

“…For embryonic stages, fertilized eggs were incubated on a moistened vermiculite substrate at 29.5°C, and embryos were removed at regular intervals after oviposition to obtain stages covering the whole postovipositional period (about 60 days for P. vitticeps and P. guttatus , 30 days for A. carolinensis ). Embryos were staged on the basis of their external morphology according to developmental tables available for lizards and snakes (Boback, Dichter, & Mistry, ; Ollonen, Da Silva, Mahlow, & Di‐Poï, ). Other adult lizard and snake specimens (21 different species, see Supporting Information) were obtained from private breeders.…”

“…High‐resolution computed tomography (CT)‐scans of early embryonic stages of P. vitticeps , A. carolinensis , and P. guttatus at the oviposition stage were obtained from our previous works (Da Silva et al, ; Ollonen et al, ), while late stages were newly produced at the University of Helsinki or University of Kuopio imaging facilities in Finland using Skyscan 1272 or 1172 microCT, respectively. To visualize eye development, soft tissue of fixed embryos was first stained with 0.6% phosphotungstic acid (PTA) in ethanol, as described before (Metscher, ), before scanning using the following parameters: voltage: 59–70 kV; current: 142–167 μA; voxel size: 3.5–10 μm.…”

Computed tomography analysis of the cranium of Champsosaurus lindoei and implications for the choristoderan neomorphic ossification

“… Alligator mississippiensis – Rowe et al (); Diplometopon zarudnyi – Abo‐Eleneen et al (); Erythrosuchus africanus – Gower (); Euparkeria capensis – Gower & Weber (); Iguana iguana – Lima et al (); Lesothosaurus diagnosticus – Sereno (); Meiolania platyceps – Gaffney (); Nothosaurus sp. – Rieppel (); Palaeoherpeton ( Palaeogyrinus of Romer, ) – Romer (); Pogona vitticeps – Ollonen et al (); Proganochelys quenstedti – Gaffney (); Bhullar & Bever (); Pseudemys texana – Bever (); Seymoria – Romer (); Spinophorosaurus nigerensis – Knoll et al (); Shinisaurus crocodilurus – Bever et al (); Sphenodon punctatus – Romer (); Sobral et al (); Stenonychosaurus inequalis – Currie (); Sternotherus oderatus – Bever (); Trachylepis laevis – Paluh & Bauer (); Tupinambis nigropunctatus – Jollie (); Youngina capensis – Gardner et al (); Varanus exanthematicus – Rieppel & Zaher (); Varanus prisca – Head et al (). …”

Embryonic development of a parthenogenetic vertebrate, the mourning gecko (Lepidodactylus lugubris)

“…Detailed descriptions of embryonic development facilitate effective experimental design and allow for an understanding of the timing of major developmental events. Since the first squamate embryonic staging series was generated almost 115 years ago ( Lacerta agilis ), another 34 complete squamate embryonic staging series have been published (Lima et al; reviewed in References ), thus providing ample comparative material for broad investigations into trends of squamate evolution and development.…”

“…Having sequence information for gene families, such as bone morphogenetic proteins (BMPs), which are critical for myriad functions during embryogenesis (e.g., mesoderm formation and patterning, neural patterning, skeletal development, and so forth; reviewed in Reference ), will allow for the designing and generation of qPCR primers and in situ hybridization probes. Other tools, such as micro‐computed tomography (μCT), have facilitated new view of staging series that can, for example, accurately trace ossification sequence of skeletal elements . Despite what seems to be a renaissance of embryological work, few embryonic staging series of reptiles have utilized soft‐tissue μCT imaging .…”