Molecular Characterization of the Gastrula in the Turtle Emys orbicularis: An Evolutionary Perspective on Gastrulation

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. 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. 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. 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. 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. The biphasic Brachyury expression pattern is also consistent with recent models of emergence of bilateral symmetry, which raises the question of its evolutionary significance

The Lesser Known Challenge of Climate Change: Thermal Variance and Sex-Reversal in Vertebrates with Temperature-Dependent Sex Determination

Climate change is expected to disrupt biological systems. Particularly susceptible are species with temperature-dependent sex determination (TSD), as in many reptiles. While the potentially devastating effect of rising mean temperatures on sex ratios in TSD species is appreciated, the consequences of increased thermal variance predicted to accompany climate change remain obscure. Surprisingly, no study has tested if the effect of thermal variance around high-temperatures (which are particularly relevant given climate change predictions) has the same or opposite effects as around lower temperatures. Here we show that sex ratios of the painted turtle (Chrysemys picta) were reversed as fluctuations increased around low and high unisexual mean-temperatures. Unexpectedly, the developmental and sexual responses around female-producing temperatures were decoupled in a more complex manner than around male-producing values. Our novel observations are not fully explained by existing ecological models of development and sex determination, and provide strong evidence that thermal fluctuations are critical for shaping the biological outcomes of climate change

Sequencing three crocodilian genomes to illuminate the evolution of archosaurs and amniotes

The International Crocodilian Genomes Working Group (ICGWG) will sequence and assemble the American alligator (Alligator mississippiensis), saltwater crocodile (Crocodylus porosus) and Indian gharial (Gavialis gangeticus) genomes. The status of these projects and our planned analyses are described

Contribution \ue0 l\u27\ue9tude de la formation du cloaque chez l\u27orvet (Anguis, Fragilis L.)

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Heterochrony in limb evolution: developmental mechanisms and natural selection

The tetrapod limb provides several examples of heterochrony - changes in the timing of developmental events. These include species differences in the sequence of skeletal chondrogenesis, in gene transcription in the developing limbs, and in the relative time at which forelimb and hind limb buds develop. Here, we examine (i) phylogenetic trends in limb heterochrony; (ii) changes in developmental mechanisms that may lead to heterochrony; and (iii) the possible role that heterochrony plays in generating adaptive traits. We analyze the published literature and present preliminary data on turtle (Emys orbicularis) and bat (Rousettus amplexicaudatus) limb development. Teleosts, marsupials, and some urodeles show extreme timing differences between forelimb (or pectoral fin) and hind limb (or pelvic fin) development; this heterochrony may, in some cases, be adaptive. Published data on limb chondrogenesis reveal sequence elements that are strongly conserved (possibly owing to constraints); and others that vary between higher taxa (for unknown reasons). We find little evidence that chondrogenic sequences are modified by selection for limb functional traits. There are a few examples of developmental mechanisms that may be modified under heterochrony to produce adaptive changes in the limb (e.g. some cases of hyperphalangy or limb reduction). In conclusion, numerous examples of limb heterochrony have been recorded. However, few cases are obviously adaptive. Indeed, current data and methodologies make it difficult to identify the developmental changes, or selective pressures, that may underlie limb heterochrony. More integrative studies, including studies of heterochrony within populations, are needed to assess the role of timing shifts in limb evolution