![]() In all other species including turtles, lepidosaurs and crocodiles, the closest relatives of birds, no such elongated structure is observed. However, while the latter is shared by all amniotes, the presence of a primitive streak is actually restricted to mammals and avians. Most studies aimed at understanding the relationships between amniote- and amphibian-type gastrulation patterns have therefore been focussed on the emergence of these characteristics. Another difference is the presence of an extraembryonic cell population (extra-embryonic ectoderm in the mouse and area opaca in the chick), which encircles embryonic territories and has no equivalent in amphibians. An obvious difference is that in avians as in mammals, mesendoderm internalization takes place by ingression through an elongated posterior structure, the primitive streak, while in most amphibians it involves involution cell movements at the level of a round-shaped blastopore. During gastrulation, these two species share a number of unique features, never found in amphibians, despite the substantial variations observed in this taxon –. Our current knowledge of this process in amniotes mainly relies on studies conducted in two model organisms, the mouse and the chick. We have used this strategy in order to gain insight into the emergence of the amniote-like gastrulation pattern and better understand its link with the gastrulation patterns observed in amphibians, the sister-group of amniotes. Such comparisons at moderate evolutionary scale must help to infer the ancestral state of a given taxon, which can then be used for comparisons with more distantly related species. Such a problem is not easily resolved in the absence of extant transition forms but one way to alleviate the difficulty can be to first assess the generality of the mechanisms characterized in a given model organism within a taxon of relatively closely related species. Developmental genetics, which mainly focus on phenotypes associated to a very limited number of often dramatic genetic changes such as gene inactivation or ectopic mis-expressions, are unlikely to reconstruct this succession of events. ![]() The major obstacle to these approaches is that each model organism has diverged from ancestral patterns by an accumulation of taxa- or even species-specific changes. However, at the macroevolutionary scale, attempts to reconstruct an evolutionary pathway through comparisons between selected, often distantly related, model organisms or through analyses of mutant phenotypes interpreted as atavisms, often remain hazardous. At the microevolutionary scale, they have also paved the way for accurate identifications of genetic modifications responsible for behavioral or morphological diversifications, thus enlightening the genetic architecture underlying these evolutionary processes. These studies have deeply impacted our understanding of the unity of metazoans, by showing that as diverse organisms as the mouse, Drosophila and even Hydra use a relatively small set of related regulatory modules, repeatedly co-opted and adapted to different cellular contexts, to build their body plan. The biphasic Brachyury expression pattern is also consistent with recent models of emergence of bilateral symmetry, which raises the question of its evolutionary significance.Īnalyses focused on a very limited number of model organisms have led to major advances in our understanding of the molecular mechanisms controlling key developmental processes. Systematic comparisons with tetrapod model organisms lead to new insights into the relationships of the blastopore/blastoporal plate system shared by all reptiles, with the blastopore of amphibians and the primitive streak of birds and mammals. Analysis of Brachyury expression also highlights the presence of two distinct phases, less easily recognizable in model organisms and respectively characterized by an early ring-shaped and a later bilateral symmetrical territory. ![]() ![]() The study of Brachyury, Lim1, Otx2 and Otx5 expression patterns points to a highly conserved dynamic of expression with amniote model organisms and makes it possible to identify the site of mesoderm internalization, which is a long-standing issue in reptiles. In order to test this hypothesis and gain insight into the emergence of the unique characteristics of amniotes during gastrulation, we have performed the first molecular characterization of the gastrula in a reptile, the turtle Emys orbicularis. Due to the presence of a blastopore as in amphibians, the turtle has been suggested to exemplify a transition form from an amphibian- to an avian-type gastrulation pattern.
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