On the Unique Perspective of Paleontology in the Study of Developmental Evolution and Biases

The growing interest and major advances of the last decades in evolutionary developmental biology (EvoDevo) have led to the recognition of the incompleteness of the Modern Synthesis of evolutionary theory. Here we discuss how paleontology makes significant contributions to integrate evolution and development. First, extinct organisms often inform us about developmental processes by showing a combination of features unrecorded in living species. We illustrate this point using the vertebrate fossil record and studies relating bone ossification to life history traits. Second, we discuss exceptionally preserved fossils that document rare ontogenetic sequences and illustrate this case with the patterns of heterochrony observed in Cambrian crustacean larvae preserved three-dimensionally. Third, most fossils potentially document the evolutionary patterns of allometry and modularity, as well as some of the (paleo)ecological factors that had influenced them. The temporal persistence of adaptive patterns in rodent evolution serves to address the importance of ecological constraints in evolution. Fourth, we discuss how the macroevolutionary patterns observed in the tetrapod limb, in the mammal molar proportions, and in the molluscan shell provide independent tests of the validity of morphogenetic models proposed on living species. Reciprocally, these macroevolutionary patterns often act as a source of inspiration to investigate the underlying rules of development, because, at the end, they are the patterns that the neo-Darwinian theory was unable to account fo

On the evolution of morphogenetic models: mechano-chemical interactions and an integrated view of cell differentiation, growth, pattern formation and morphogenesis

htmlabstractIn the 1950s, embryology was conceptualized as four relatively independent problems: cell differentiation, growth, pattern formation and morphogenesis. The mechanisms underlying the first three traditionally have been viewed as being chemical in nature, whereas those underlying morphogenesis have usually been discussed in terms of mechanics. Often, morphogenesis and its mechanical processes have been regarded as subordinate to chemical ones. However, a growing body of evidence indicates that the biomechanics of cells and tissues affect in striking ways those phenomena often thought of as mainly under the control of cell-cell signalling. This accumulation of data has led to a revival of the mechano-transduction concept in particular, and of complexity in general, causing us now to consider whether we should retain the traditional conceptualization of development. The researchers’ semantic preferences for the terms ‘patterning’, ‘pattern formation’ or ‘morphogenesis’ can be used to describe three main ‘schools of thought’ which emerged in the late 1970s. In the ‘molecular school’, the term patterning is deeply tied to the positional information concept. In the ‘chemical school’, the term ‘pattern formation’ regularly implies reaction-diffusion models. In the ‘mechanical school’, the term ‘morphogenesis’ is more frequently used in relation to mechanical instabilities. Major differences among these three schools pertain to the concept of self-organization, and models can be classified as morphostatic or morphodynamic. Various examples illustrate the distorted picture that arises from the distinction among differentiation, growth, pattern formation and morphogenesis, based on the idea that the underlying mechanisms are respectively chemical or mechanical. Emerging quantitative approaches integrate the concepts and methods of complex sciences and emphasize the interplay between hierarchical levels of organization via mechano-chemical interactions. They draw upon recent improvements in mathematical and numerical morphogenetic models and upon considerable progress in collecting new quantitative data. This review highlights a variety of such models, which exhibit important advances, such as hybrid, stochastic and multiscale simulations

Learning from beautiful monsters: phylogenetic and morphogenetic implications of left-right asymmetry in ammonoid shells.

13 pagesInternational audienceBackground: Many pathologies that modify the shell geometry and ornamentation of ammonoids are known fromthe fossil record. Since they may reflect the developmental response of the organism to a perturbation (usually asublethal injury), their study is essential for exploring the developmental mechanisms of these extinct animals.Ammonoid pathologies are also useful to assess the value of some morphological characters used in taxonomy, aswell as to improve phylogenetic reconstructions and evolutionary scenarios.Results: We report on the discovery of an enigmatic pathological middle Toarcian (Lower Jurassic) ammonoidspecimen from southern France, characterized by a pronounced left-right asymmetry in both ornamentation andsuture lines. For each side independently, the taxonomic interpretations of ornamentation and suture lines arecongruent, suggesting a Hildoceras semipolitum species assignment for the left side and a Brodieia primaria speciesassignment for the right side. The former exhibits a lateral groove whereas the second displays sinuous ribs. Thisspecimen, together with the few analogous cases reported in the literature, lead us to erect a new forma-typepathology herein called “forma janusa” for specimens displaying a left-right asymmetry in the absence of any clearevidence of injury or parasitism, whereby the two sides match with the regular morphology of two distinct, knownspecies.Conclusions: Since “forma janusa” specimens reflect the underlying developmental plasticity of the ammonoidtaxa, we hypothesize that such specimens may also indicate unsuspected phylogenetic closeness between the twodisplayed taxa and may even reveal a direct ancestor-descendant relationship. This hypothesis is not, as yet,contradicted by the stratigraphical data at hand: in all studied cases the two distinct taxa correspond tocontemporaneous or sub-contemporaneous taxa. More generally, the newly described specimen suggests that ahitherto unidentified developmental link may exist between sinuous ribs and lateral grooves. Overall, werecommend an integrative approach for revisiting aberrant individuals that illustrate the intricate links among shellmorphogenesis, developmental plasticity and phylogeny

A simple smiFISH pipeline to quantify mRNA at the single-cell level in 3D

Summary: Techniques allowing the precise quantification of mRNA at the cellular level are essential for understanding biological processes. Here, we present a semi-automated smiFISH (single-molecule inexpensive FISH) pipeline enabling quantification of mRNA in a small number of cells (∼40) in fixed whole mount tissue. We describe steps for sample preparation, hybridization, image acquisition, cell segmentation, and mRNA quantification. Although the protocol was developed in Drosophila, it can be optimized for use in other organisms.For complete details on the use and execution of this protocol, please refer to Guan et al.1 : Publisher’s note: Undertaking any experimental protocol requires adherence to local institutional guidelines for laboratory safety and ethics

Post-transcriptional regulation of transcription factor codes in immature neurons drives neuronal diversity

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The post-transcriptional regulation of TFs in immature motoneurons shapes the axon-muscle connectome

SUMMARY Temporal factors expressed sequentially in neural stem cells, such as RNA binding proteins (RBPs) or transcription factors (TFs), are key elements in the generation of neuronal diversity. The molecular mechanism underlying how the temporal identity of stem cells is decoded into their progeny to generate neuronal diversity is largely unknown. Here, we used genetic and new computational tools to study with precision the unique fates of the progeny of a stem cell producing 29 morphologically distinct leg motoneurons (MNs) in Drosophila . We identified 40 TFs expressed in this MN lineage, 15 of which are expressed in a combinatorial manner in immature MNs just before their morphological differentiation. By following TF expression patterns at an earlier developmental stages, we discovered 19 combinatorial codes of TFs that were progressively established in immature MNs as a function of their birth order. The comparison of the RNA and protein expression profiles of 6 TFs revealed that post-transcriptional regulation plays an essential role in shaping these TF codes. We found that the two known RBPs, Imp and Syp, expressed sequentially in neuronal stem cells, are upstream regulators of the TF codes. Both RBPs are key players in the construction of axon-muscle connectome through the post-transcriptional regulation of 5 of the 6 TFs examined. By deciphering the function of Imp in the immature MNs with respect to the stem cell of the same lineage, we propose a model where RBPs shape the morphological fates of MNs through post-transcriptional regulation of TF codes in immature MNs. Taken together, our study reveals that immature MNs are plastic cells that have the potential to acquire many morphological fates. The molecular basis of MN plasticity originates in the broad expression of different TF mRNA, that are post-transcriptionally shaped into TF codes by Imp and Syp, and potentially by other RBPs that remain to be discovered, to determine their morphological fates