Prenatal development of Crocodylus niloticus niloticus Laurenti, 1768

Ontogeny of the nasolacrimal apparatus and nasal sensory systems of the American alligator (Alligator mississippiensis)

The nasolacrimal apparatus (NLA) is a feature common to many sauropsid amniotes. It consists of an orbital Harderian gland (HG)whose secretions drain into the nasal cavity, in the vicinity of the vomeronasal organ (VNO), an accessory olfactory organ derived from the olfactory epithelium, and a connecting nasolacrimal duct (NLD). Though not all features are present in all posthatchling sauropsids (i.e., no VNO in crocodilomorphs), it is not clear if this system either never existed or failed to develop during the embryonic stages. The purpose of this study is to histologically describe the ontogeny of the NLA and the main olfactory organ in Alligator mississippiensis. Alligator specimens, from embryonic stage 9 to hatchling, were serially histologically sectioned, stained, photographed, and segmented into different tissues using Abobe Photoshop and then reconstructed using Amira for 3D analysis and quantitative nasal epithelial distribution. Though there was no evidence of a VNO, the rest of the NLA was present. The development of the NLA could be subdivided into four phases: (1) inception of NLD, (2) establishment of orbitonasal connections of NLD, (3) bone development, and (4) nasal cavity growth. Glands mature during this last phase and the nasal region rapidly grows, rotates, and is displaced anteriorly. The gradual proportional increase in nonolfactory epithelial distribution during ontogeny is consistent with the literature. Alligator embryonic nasal and NLD growth differs from that of mammals and squamates. The NLD is connected to the anterior third of the nasal region during its initial attachment, but as anterior nasal growth exceeds posterior growth, it is gradually displaced into the posterior third of the nasal region by hatching. It is unknown whether this is a derived archosaur condition or just another example of the morphological variation seen within sauropsid amniotes.

Morphological research on amniote eggs and embryos: An introduction and historical retrospective

Evolution of the terrestrial egg of amniotes (reptiles, birds, and mammals) is often considered to be one of the most significant events in vertebrate history. Presence of an eggshell, fetal membranes, and a sizeable yolk allowed this egg to develop on land and hatch out well-developed, terrestrial offspring. For centuries, morphologically-based studies have provided valuable information about the eggs of amniotes and the embryos that develop from them. This review explores the history of such investigations, as a contribution to this special issue of Journal of Morphology, titled Developmental Morphology and Evolution of Amniote Eggs and Embryos. Anatomically-based investigations are surveyed from the ancient Greeks through the Scientific Revolution, followed by the 19th and early 20th centuries, with a focus on major findings of historical figures who have contributed significantly to our knowledge. Recent research on various aspects of amniote eggs is summarized, including gastrulation, egg shape and eggshell morphology, eggs of Mesozoic dinosaurs, sauropsid yolk sacs, squamate placentation, embryogenesis, and the phylotypic phase of embryonic development. As documented in this review, studies on amniote eggs and embryos have relied heavily on morphological approaches in order to answer functional and evolutionary questions.

How do Crocodylian embryos process yolk? Morphological evidence from the American alligator, Alligator mississippiensis

Recent studies have demonstrated a mechanism of embryonic yolk processing in lizards, snakes and turtles that differs markedly from that of birds. In the avian pattern, cells that line the inside of the yolk sac take up products of yolk digestion and deliver nutrients into the vitelline circulation. In contrast, in squamates and turtles, proliferating endodermal cells invade and fill the yolk sac cavity, forming elongated strands of yolk-filled cells that surround small blood vessels. This arrangement provides a means by which yolk material becomes cellularized, digested, and transported for embryonic use. Ultrastructural observations on late-stage Alligator mississippiensis eggs reveal elongated, vascular strands of endodermal cells within the yolk sac cavity. The strands of cells are intermixed with free yolk spheres and clumps of yolk-filled endodermal cells, features that reflect early phases in the yolk-processing pattern. These observations indicate that yolk processing in Alligator is more like the pattern of other reptiles than that of birds.

Development of the chondrocranium of two caiman species, Caiman latirostris and Caiman yacare

Little is known about the embryonic development and variation of the chondrocranium in Crocodylia and there are no works on any Caiman species. Due to the importance of cranial features in the systematics of this clade, investigating the development of the skull in embryonic stages is essential. In this study, we present for the first time the development of the cartilaginous skull of two extant Caiman species. Anatomical descriptions of the embryonic chondrocranium of Caiman latirostris and Caiman yacare were made, paying special attention to their inter- and intraspecific variation. For this purpose, pre-hatching ontogenetic cranial series of these two caiman species were prepared with a double staining and diaphanization technique. The main differences with other crocodylian species were observed in the palatoquadrate, and interspecific variation within the genus was recorded in the hyobranchial apparatus and larynx. Some characters may be distinctive of Caiman (posterior and ventral surface of the otic process of the palatoquadrate articulated with the dorsal process of the columella auris, and otic process articulated with the lateral wall of the auditory capsule), Alligatoridae (presence of an epiphanial foramen) or C. latirostris and C. yacare (Corpus hyoidei with different number and position of foramina and different shapes of its anterior contour and anterior and posterior notch, different degrees of broadening of the distal end of the Cornu branchiale I, and presence/absence of a notch in the posteroventral surface of the cricoid). Homologies of the elements belonging to the hyobranchial apparatus could not be confirmed. As in other tetrapods the trachea consists of incomplete cartilaginous rings. Morphological changes and dissimilarities found in this study are useful as a context to start studying phylogenetic constraints. Moreover, in a heterochronic context, variations may be involved.

Recent insights into the morphological diversity in the amniote primary and secondary palates

The assembly of the upper jaw is a pivotal moment in the embryonic development of amniotes. The upper jaw forms from the fusion of the maxillary, medial nasal, and lateral nasal prominences, resulting in an intact upper lip/beak and nasal cavities; together called the primary palate. This process of fusion requires a balance of proper facial prominence shape and positioning to avoid craniofacial clefting, whilst still accommodating the vast phenotypic diversity of adult amniotes. As such, variation in craniofacial ontogeny is not tolerated beyond certain bounds. For clarity, we discuss primary palatogenesis of amniotes into in two categories, according to whether the nasal and oral cavities remain connected throughout ontogeny or not. The transient separation of these cavities occurs in mammals and crocodilians, while remaining connected in birds, turtles and squamates. In the latter group, the craniofacial prominences fuse around a persistent choanal groove that connects the nasal and oral cavities. Subsequently, all lineages except for turtles, develop a secondary palate that ultimately completely or partially separates oral and nasal cavities. Here, we review the shared, early developmental events and highlight the points at which development diverges in both primary and secondary palate formation.

Diversity in primary palate ontogeny of amniotes revealed with 3D imaging

The amniote primary palate encompasses the upper lip and the nasal cavities. During embryonic development, the primary palate forms from the fusion of the maxillary, medial nasal and lateral nasal prominences. In mammals, as the primary palate fuses, the nasal and oral cavities become completely separated. Subsequently, the tissue demarcating the future internal nares (choanae) thins and becomes the bucconasal membrane, which eventually ruptures and allows for the essential connection of the oral and nasal cavities to form. In reptiles (including birds), the other major amniote group, primary palate ontogeny is poorly studied with respect to prominence fusion, especially the formation of a bucconasal membrane. Using 3D optical projection tomography, we found that the prominences that initiate primary palate formation are similar between mammals and crocodilians but distinct from turtles and lizards, which are in turn similar to each other. Chickens are distinct from all non-avian lineages and instead resemble human embryos in this aspect. The majority of reptiles maintain a communication between the oral and nasal cavities via the choanae during primary palate formation. However, crocodiles appear to have a transient separation between the oral and nasal cavities. Furthermore, the three lizard species examined here, exhibit temporary closure of their external nares via fusion of the lateral nasal prominences with the frontonasal mass, subsequently reopening them just before hatching. The mechanism of the persistent choanal opening was examined in chicken embryos. The mesenchyme posterior/dorsal to the choana had a significant decline in proliferation index, whereas the mesenchyme of the facial processes remained high. This differential proliferation allows the choana to form a channel between the oral and nasal cavities as the facial prominences grow and fuse around it. Our data show that primary palate ontogeny has been modified extensively to support the array of morphological diversity that has evolved among amniotes.

Embryonic development of the monitor lizard, Varanus indicus

Describing the stages of normal development of Varanus indicus, the present paper provides the first developmental data on Varanidae. The incubation period is relatively long (180 days at 28°C) and without any diapause. The development is rather slow during the first 50 days, after which a considerable acceleration can be observed. The stage of accelerated growth terminates at app. 100 days when all essential specificities of adult organisation (prolonged narial region with vomeronasal organ, eyes, claws, large heart and robust body and limbs) are established. The remaining period of the embryonic development is characterized by continuation of the respective trends, i.e., enlarging body, prolongation of rostrum, enlarging teeth and claws, keratinisation of claws and scales etc. In short, the second half of the embryonic development of Varanus is devoted to refining the structures supporting its adaptations for active predation.

Reptilian-transcriptome v1.0, a glimpse in the brain transcriptome of five divergent Sauropsida lineages and the phylogenetic position of turtles

Background

Reptiles are largely under-represented in comparative genomics despite the fact that they are substantially more diverse in many respects than mammals. Given the high divergence of reptiles from classical model species, next-generation sequencing of their transcriptomes is an approach of choice for gene identification and annotation.

Results

Here, we use 454 technology to sequence the brain transcriptome of four divergent reptilian and one reference avian species: the Nile crocodile, the corn snake, the bearded dragon, the red-eared turtle, and the chicken. Using an in-house pipeline for recursive similarity searches of >3,000,000 reads against multiple databases from 7 reference vertebrates, we compile a reptilian comparative transcriptomics dataset, with homology assignment for 20,000 to 31,000 transcripts per species and a cumulated non-redundant sequence length of 248.6 Mbases. Our approach identifies the majority (87%) of chicken brain transcripts and about 50% of de novo assembled reptilian transcripts. In addition to 57,502 microsatellite loci, we identify thousands of SNP and indel polymorphisms for population genetic and linkage analyses. We also build very large multiple alignments for Sauropsida and mammals (two million residues per species) and perform extensive phylogenetic analyses suggesting that turtles are not basal living reptiles but are rather associated with Archosaurians, hence, potentially answering a long-standing question in the phylogeny of Amniotes.

Conclusions

The reptilian transcriptome (freely available at http://www.reptilian-transcriptomes.org) should prove a useful new resource as reptiles are becoming important new models for comparative genomics, ecology, and evolutionary developmental genetics.