Origins of neurogenesis, a cnidarian view.

New perspectives on the origin of neurogenesis emerged with the identification of genes encoding post-synaptic proteins as well as many "neurogenic" regulators as the NK, Six, Pax, bHLH proteins in the Demosponge genome, a species that might differentiate sensory cells but no neurons. However, poriferans seem to miss some key regulators of the neurogenic circuitry as the Hox/paraHox and Otx-like gene families. Moreover as a general feature, many gene families encoding evolutionarily-conserved signaling proteins and transcription factors were submitted to a wave of gene duplication in the last common eumetazoan ancestor, after Porifera divergence. In contrast gene duplications in the last common bilaterian ancestor, Urbilateria, are limited, except for the bHLH Atonal-class. Hence Cnidaria share with Bilateria a large number of genetic tools. The expression and functional analyses currently available suggest a neurogenic function for numerous orthologs in developing or adult cnidarians where neurogenesis takes place continuously. As an example, in the Hydra polyp, the Clytia medusa and the Acropora coral, the Gsx/cnox2/Anthox-2 ParaHox gene likely supports neurogenesis. Also neurons and nematocytes (mechanosensory cells) share in hydrozoans a common stem cell and several regulatory genes indicating that they can be considered as sister cells. Performed in anthozoan and medusozoan species, these studies should tell us more about the way(s) evolution hazards achieved the transition from epithelial to neuronal cell fate, and about the robustness of the genetic circuitry that allowed neuromuscular transmission to arise and be maintained across evolution.

Arthropod-like expression patterns of engrailed and wingless in the annelid Platynereis dumerilii suggest a role in segment formation.

The origin of animal segmentation, the periodic repetition of anatomical structures along the anteroposterior axis, is a long-standing issue that has been recently revived by comparative developmental genetics. In particular, a similar extensive morphological segmentation (or metamerism) is commonly recognized in annelids and arthropods. Mostly based on this supposedly homologous segmentation, these phyla have been united for a long time into the clade Articulata. However, recent phylogenetic analysis dismissed the Articulata and thus challenged the segmentation homology hypothesis. Here, we report the expression patterns of genes orthologous to the arthropod segmentation genes engrailed and wingless in the annelid Platynereis dumerilii. In Platynereis, engrailed and wingless are expressed in continuous ectodermal stripes on either side of the segmental boundary before, during, and after its formation; this expression pattern suggests that these genes are involved in segment formation. The striking similarities of engrailed and wingless expressions in Platynereis and arthropods may be due to evolutionary convergence or common heritage. In agreement with similarities in segment ontogeny and morphological organization in arthropods and annelids, we interpret our results as molecular evidence of a segmented ancestor of protostomes.

Hydra, a niche for cell and developmental plasticity.

The silencing of genes whose expression is restricted to specific cell types and/or specific regeneration stages opens avenues to decipher the molecular control of the cellular plasticity underlying head regeneration in hydra. In this review, we highlight recent studies that identified genes involved in the immediate cytoprotective function played by gland cells after amputation; the early dedifferentiation of digestive cells into blastema-like cells during head regeneration, and the early late proliferation of neuronal progenitors required for head patterning. Hence, developmental plasticity in hydra relies on spatially restricted and timely orchestrated cellular modifications, where the functions played by stem cells remain to be characterized.

Silencing of the hydra serine protease inhibitor Kazal1 gene mimics the human SPINK1 pancreatic phenotype.

In hydra, the endodermal epithelial cells carry out the digestive function together with the gland cells that produce zymogens and express the evolutionarily conserved gene Kazal1. To assess the hydra Kazal1 function, we silenced gene expression through double-stranded RNA feeding. A progressive Kazal1 silencing affected homeostatic conditions as evidenced by the low budding rate and the induced animal death. Concomitantly, a dramatic disorganization followed by a massive death of gland cells was observed, whereas the cytoplasm of digestive cells became highly vacuolated. The presence of mitochondria and late endosomes within those vacuoles assigned them as autophagosomes. The enhanced Kazal1 expression in regenerating tips was strongly diminished in Kazal1(-) hydra, and the amputation stress led to an immediate disorganization of the gland cells, vacuolization of the digestive cells and death after prolonged silencing. This first cellular phenotype resulting from a gene knock-down in cnidarians suggests that the Kazal1 serine-protease-inhibitor activity is required to prevent excessive autophagy in intact hydra and to exert a cytoprotective function to survive the amputation stress. Interestingly, these functions parallel the pancreatic autophagy phenotype observed upon mutation within the Kazal domain of the SPINK1 and SPINK3 genes in human and mice, respectively.

Precraniate origin of cranial motoneurons.

The craniate head is innervated by cranial sensory and motor neurons. Cranial sensory neurons stem from the neurogenic placodes and neural crest and are seen as evolutionary innovations crucial in fulfilling the feeding and respiratory needs of the craniate "new head." In contrast, cranial motoneurons that are located in the hindbrain and motorize the head have an unclear phylogenetic status. Here we show that these motoneurons are in fact homologous to the motoneurons of the sessile postmetamorphic form of ascidians. The motoneurons of adult Ciona intestinalis, located in the cerebral ganglion and innervating muscles associated with the huge "branchial basket," express the transcription factors CiPhox2 and CiTbx20, whose vertebrate orthologues collectively define cranial motoneurons of the branchiovisceral class. Moreover, Ciona's postmetamorphic motoneurons arise from a hindbrain set aside during larval life and defined as such by its position (caudal to the prosensephalic sensory vesicle) and coexpression of CiPhox2 and CiHox1, whose orthologues collectively mark the vertebrate hindbrain. These data unveil that the postmetamorphic ascidian brain, assumed to be a derived feature, in fact corresponds to the vertebrate hindbrain and push back the evolutionary origin of cranial nerves to before the origin of craniates.

Caudal and even-skipped in the annelid Platynereis dumerilii and the ancestry of posterior growth.

In order to address the question of the conservation of posterior growth mechanisms in bilaterians, we have studied the expression patterns of the orthologues of the genes caudal, even-skipped, and brachyury in the annelid Platynereis dumerilii. Annelids belong to the still poorly studied third large branch of the bilaterians, the lophotrochozoans, and have anatomic and developmental characteristics, such as a segmented body plan, indirect development through a microscopic ciliated larva, and building of the trunk through posterior addition, which are all hypothesized by some authors (including us) to be present already in Urbilateria, the last common ancestor of bilaterians. All three genes are shown to be likely involved in the building of the anteroposterior axis around the slit-like amphistomous blastopore as well as in the patterning of the terminal anus-bearing piece of the body (the pygidium). In addition, caudal and even-skipped are likely involved in the posterior addition of segments. Together with the emerging results on the conservation of segmentation genes, these results reinforce the hypothesis that Urbilateria had a segmented trunk developing through posterior addition.

Hox clusters and bilaterian phylogeny.

A large Hox cluster comprising at least seven genes has evolved by gene duplications in the ancestors of bilaterians. It probably emerged from a mini-cluster of three or four genes that was present before the divergence of cnidarians and bilaterians. The comparison of Hox structural data in bilaterian phyla shows that the genes of the anterior part of the cluster have been more conserved than those of the posterior part. Some specific signature sequences, present in the form of signature residues within the homeodomain or conserved peptides outside the homeodomain, constitute phylogenetic evidence for the monophyly of protostomes and their division into ecdysozoans and lophotrochozoans. These conserved motifs may provide decisive arguments for the phylogenetic position of some enigmatic phyla.

A strategy to retrieve the whole set of protein modules in microbial proteomes.

Protein homology is often limited to long structural segments that we have previously called modules. We describe here a suite of programs used to catalog the whole set of modules present in microbial proteomes. First, the Darwin AllAll program detects homologous segments using thresholds for evolutionary distance and alignment length, and another program classifies these modules. After assembling these homologous modules in families, we further group families which are related by a chain of neighboring unrelated homologous modules. With the automatic analysis of these groups of families sharing homologous modules in independent multimodular proteins, one can split into their component parts many fused modules and/or deduce by logic more distant modules. All detected and inferred modules are reassembled in refined families. These two last steps are made by a unique program. Eventually, the soundness of the data obtained by this experimental approach is checked using independent tests. To illustrate this modular approach, we compared four proteobacterial proteomes (Campylobacter jejuni, Escherichia coli, Haemophilus influenzae, and Helicobacter pylori). It appears that this method might retrieve from present-day proteins many of the modules which can help to trace back ancient events of gene duplication and/or fusion.