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The left-right body axis, along with the dorso-ventral and antero-posterior axis, is certainly very important, but at the same time the most neglected body axis of bilaterally symmetrical animals. The asymmetrical distribution of visceral organs along this body axis is vital for many animals. In vertebrates, this asymmetry becomes established at early embryonic stages. An essential role in this process is played by the organizer, which is responsible for the correct establishment of the left-right axis. Among vertebrates, organizers of left-right asymmetry are found in different parts of the embryonic body, and with few exceptions, they are composed of cells with motile cilia, which create an unidirectional leftward flow of extracellular fluids in the organizer. The flow is probably sensed by the cells that surround the left side of the organizer. These cells respond to the aforementioned mechanical stimulus by triggering the Nodal signaling pathway. This signaling cascade results in the left-sided expression of the Pitx2 gene, which specifies the left side of the embryonic body. It appears, that the role of Nodal signaling pathway in determining left-right asymmetry is not only present in all vertebrates, but also is probably ancestral to bilaterally symmetrical animals. Another mechanism ensuring...

Left-right asymmetry determines the orientation of visceral organs during gastrulation in the mouse embryo. The asymmetry originates in ventral node which is located on the anterior end of primitive streak. Cells of the ventral node possess motile nodal cilia. These cilia rotate and generate leftward flow of extra-embryonic fluid. The leftward flow initiates asymmetric expression of Nodal signalling pathways in nodal cells. Abnormal nodal flow and the interruption of asymmetric gene expression cause reversal arrangement of visceral organs, called situs inversus. The mechanosensing and chemosensing models are the main theories addressing the question of how the leftward nodal flow is sensed by nodal cilia. The mechanosensing model, also referred to as 'two cilia model', is based on two types of cilia, motile and immotile. Motile cilia generate nodal flow while immotile cilia act as mechanosensors with polycystin cation channels. According to the chemosensing model, morphogens are secreted and form gradient that induces asymmetric gene expression in the node. It is still unclear which model is the right one, but it is possible that the final model is combination of both.

Five new species belonging to the astome ciliates, living in the digestive tract of Oligochaeta worms belonging to the genus Alma from Cameroon, have been described. The techniques used are: vital staining, staining of the nucleus with Diamidino Phenyl Indol (DAPI), scanning electron microscopy and silver staining method (Fernandez Galiano, 1976, 1994). This work confirms the presence of the genus Paracoelophrya and Dicoelophrya in the digestive track of the oligochaete Alma from Gabon and Cameroon; it helps to understand the general taxonomy of this Metaracoelophryinae subfamily. Moreover, the homogeneity of this group is confirmed and the phylogenetic relationship inside the Hoplitophryida order need more studies to be solved.

International audience; Five new species belonging to the astome ciliates, living in the digestive tract of Oligochaeta worms belonging to the genus Alma from Cameroon, have been described. The techniques used are: vital staining, staining of the nucleus with Diamidino Phenyl Indol (DAPI), scanning electron microscopy and silver staining method (Fernandez Galiano, 1976, 1994). This work confirms the presence of the genus Paracoelophrya and Dicoelophrya in the digestive track of the oligochaete Alma from Gabon and Cameroon; it helps to understand the general taxonomy of this Metaracoelophryinae subfamily. Moreover, the homogeneity of this group is confirmed and the phylogenetic relationship inside the Hoplitophryida order need more studies to be solved.

Background The hydrogenosomes of the anaerobic ciliate Nyctotherus ovalis show how mitochondria can evolve into hydrogenosomes because they possess a mitochondrial genome and parts of an electron-transport chain on the one hand, and a hydrogenase on the other hand. The hydrogenase permits direct reoxidation of NADH because it consists of a [FeFe] hydrogenase module that is fused to two modules, which are homologous to the 24 kDa and the 51 kDa subunits of a mitochondrial complex I. Results The [FeFe] hydrogenase belongs to a clade of hydrogenases that are different from well-known eukaryotic hydrogenases. The 24 kDa and the 51 kDa modules are most closely related to homologous modules that function in bacterial [NiFe] hydrogenases. Paralogous, mitochondrial 24 kDa and 51 kDa modules function in the mitochondrial complex I in N. ovalis. The different hydrogenase modules have been fused to form a polyprotein that is targeted into the hydrogenosome. Conclusion The hydrogenase and their associated modules have most likely been acquired by independent lateral gene transfer from different sources. This scenario for a concerted lateral gene transfer is in agreement with the evolution of the hydrogenosome from a genuine ciliate mitochondrion by evolutionary tinkering.

Background The horizontal transfer of expressed genes from Bacteria into Ciliates which live in close contact with each other in the rumen (the foregut of ruminants) was studied using ciliate Expressed Sequence Tags (ESTs). More than 4000 ESTs were sequenced from representatives of the two major groups of rumen Cilates: the order Entodiniomorphida (Entodinium simplex, Entodinium caudatum, Eudiplodinium maggii, Metadinium medium, Diploplastron affine, Polyplastron multivesiculatum and Epidinium ecaudatum) and the order Vestibuliferida, previously called Holotricha (Isotricha prostoma, Isotricha intestinalis and Dasytricha ruminantium). Results A comparison of the sequences with the completely sequenced genomes of Eukaryotes and Prokaryotes, followed by large-scale construction and analysis of phylogenies, identified 148 ciliate genes that specifically cluster with genes from the Bacteria and Archaea. The phylogenetic clustering with bacterial genes, coupled with the absence of close relatives of these genes in the Ciliate Tetrahymena thermophila, indicates that they have been acquired via Horizontal Gene Transfer (HGT) after the colonization of the gut by the rumen Ciliates. Conclusion Among the HGT candidates, we found an over-representation (>75%) of genes involved in metabolism, specifically in the catabolism of complex carbohydrates, a rich food source in the rumen. We propose that the acquisition of these genes has greatly facilitated the Ciliates' colonization of the rumen providing evidence for the role of HGT in the adaptation to new niches.

Cilia and flagella are hair-like appendages of eukaryotic cells. They are actively bending structures that exhibit regular beat patterns and thereby play an important role in many different circumstances where motion on a cellular level is required. Most dramatic is the effect of nodal cilia whose vortical motion leads to a fluid flow that is directly responsible for establishing the left-right axis during embryological development in many vertebrate species, but examples range from the propulsion of single cells, such as the swimming of sperm, to the transport of mucus along epithelial cells, e.g. in the ciliated trachea. Cilia and flagella contain an evolutionary highly conserved structure called the axoneme, whose characteristic architecture is based on a cylindrical arrangement of elastic filaments (microtubules). In the presence of a chemical fuel (ATP), molecular motors (dynein) exert shear forces between neighbouring microtubules, leading to a bending of the axoneme through structural constraints. We address the following two questions: How can these organelles generate regular oscillatory beat patterns in the absence of a biochemical signal regulating the activity of the force generating elements? And how can the beat patterns be so different for apparently very similar structures? We present a theoretical description of the axonemal structure as an actively bending elastic cylinder, and show that in such a system bending waves emerge from a non-oscillatory state via a dynamic instability. The corresponding beat patterns are solutions to a set of coupled partial differential equations presented herein.

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In January 1993, a 24 hour survey based on an hourly sampling was conducted in a station (1 m depth) located in the Ebrie lagoon, Cote d'lvoire, in order to determine short-term changes in seston quality and quantity and relate them to physical conditions. Water samples were collected just below the surface and at 0.9m depth to assess the seston distribution in the water column. Located in an estuarine environment, the study site was submitted to periodic effect of tides with low currents. Under certain conditions, sediment resuspension by wind-induced waves was observed in the shallow parts of this ecosystem. From the analysis of hydrodynamic processes (tide, wind and sedimentation rates), four basic situations were distinguished in the course of the study: a period with no wind and high sedimentation rates, a period characterized by wind-induced resuspension, followed by a period of sedimentation and finallly, a period without wind during the second flood tide. The particle size spectra were similar throughout the study, the distributions being unimodal (mode of 4 μm diameter). The finer particles (

Les ciliés et amibes ont été étudiés dans le réservoir Lobo (Etat de Sao Paulo, Brésil) de septembre 1984 à octobre 1985. Avec le propos d'étudier l'abondance et la composition des protozoaires, des échantillonnages bimensuels ont été effectués à deux stations; en surface à la station 1 dans la région profonde et peu influencée par les rives, et à la station 2, dans une région moins profonde du réservoir, où on peut signaler un très grand nombre de macrophytes aquatiques . La chlorophylle a, l'oxygène dissous, le pH et la température de l'eau, ont été également mesurés. Nous n'avons pas mis en évidence de relation significative entre la distribution saisonnière des protozoaires et celles observée pour les paramètres écologiques mesurés. On a signalé une période de forte densité de février à mai avec un nombre moyen de 3,98 x 10 "puissance" 3 protozoaires 1-1 pour la station et 1 et 5,09 x 10 "puissance" 3 pour la station 2. Dans les autres mois la densité a été peu élevée. Le nombre moyen d'organismes dans la période de haute densité fut 4 fois plus grand que celui obtenu dans la période de densité peu élevée. La plus haute densité de protozoaires planctoniques fut observée à la station avec 9,87 x 10 "puissance" 3 organismes 1- "puissance" 1. Les ciliés les plus abondants furent #Colpoda$ sp., #Halteria grandinella$, #Mesodinium pulex$ et #Strombidinopsis setigera$ (Résumé d'auteur)

Huit espèces de Trichonides réparties en deux genres ont été recensées sur les poissons du réseau lagunaire côtier du Bénin. Les relations hôtes-protozoaires d'une part et protozoaires-protozoaires d'autre part ont été définies avec mise en évidence de relations d'indifférence spécifique entre les poissons et les protozoaires et d'une exclusion mutuelle entre deux espèces de Trichodines. Les tolérances à la salinité sont définies pour chaque espèce de Trichodinidae. (Résumé d'auteur)