Cooperation‐mediated plasticity in dispersal and colonization

Kin selection theory predicts that costly cooperative behaviors evolve most readily when directed toward kin. Dispersal plays a controversial role in the evolution of cooperation: dispersal decreases local population relatedness and thus opposes the evolution of cooperation, but limited dispersal increases kin competition and can negate the benefits of cooperation. Theoretical work has suggested that plasticity of dispersal, where individuals can adjust their dispersal decisions according to the social context, might help resolve this paradox and promote the evolution of cooperation. Here, we experimentally tested the hypothesis that conditional dispersal decisions are mediated by a cooperative strategy: we quantified the density‐dependent dispersal decisions and subsequent colonization efficiency from single cells or groups of cells among six genetic strains of the unicellular Tetrahymena thermophila that differ in their aggregation level (high, medium, and low), a behavior associated with cooperation strategy. We found that the plastic reaction norms of dispersal rate relative to density differed according to aggregation level: highly aggregative genotypes showed negative density‐dependent dispersal, whereas low‐aggregation genotypes showed maximum dispersal rates at intermediate density, and medium‐aggregation genotypes showed density‐independent dispersal with intermediate dispersal rate. Dispersers from highly aggregative genotypes had specialized long‐distance dispersal phenotypes, contrary to low‐aggregation genotypes; medium‐aggregation genotypes showing intermediate dispersal phenotype. Moreover, highly aggregation genotypes showed evidence for beneficial kin‐cooperation during dispersal. Our experimental results should help to resolve the evolutionary conflict between cooperation and dispersal: cooperative individuals are expected to avoid kin‐competition by dispersing long distances, but maintain the benefits of cooperation by dispersing in small groups.     

Ciguatoxins and brevetoxins: dissection of the neurobiological actions]

This review focuses on the neurobiological actions of ciguatoxins and brevetoxins which are phycotoxins produced respectively by the dinoflagellates Gambierdiscus toxicus and Ptychodiscus brevis. These actions are illustrated in particular by the effects of the toxins on myelinated nerve fibres and on skeletal neuromuscular junctions of vertebrates. Ciguatoxins and brevetoxins, through different vectors, are responsible for human intoxications characterized mainly by neurological disturbances. The molecular target of these families of lipid-soluble cyclic polyethers is the voltage-gated sodium channel, a fundamental transmembrane protein involved in cellular excitability. The different toxins share a common binding site (the receptor-site 5) located on the alpha sub-unit of this neuronal transmembrane protein. Electrophysiological studies of the mode of action of ciguatoxins and brevetoxins identify these toxins as specific sodium channel activators. Indeed, during the action of these phycotoxins, sodium channels remain permanently opened, at the resting membrane potential, which produces a continuous entry of sodium ions in most excitable cells. Such a sodium entry has various consequences on sodium-dependent physiological mechanisms, consisting in a membrane depolarization which, in turn, causes spontaneous and/or repetitive action potential discharges and thereby increases membrane excitability. These neuronal discharges may be transient or continuous according to the preparation and the toxin tested. The increase in membrane excitability during the action of ciguatoxins and brevetoxins is responsible for the different effects exerted by these toxins on various chemical synapses and secretory cells. Another consequence of the continuous entry of sodium ions into cells was revealed using confocal laser scanning microscopy and vital staining of plasma membranes with the fluorescent dye FM1-43. These techniques made feasible the dynamic study of morphological alterations produced by ciguatoxins and brevetoxins on various cellular preparations in situ. Thus, it has been possible to bring to the fore that these phycotoxins cause a marked increase in the volume of nodes of Ranvier of myelinated nerve fibres, motor nerve terminals innervating skeletal muscle and perisynaptic non-myelinating Schwann cell somata. This increase could be reversed by hyperosmotic external solutions and completely prevented by the blockade of voltage-gated sodium channels. The mechanisms involved in the increase in cellular volume, during the action of ciguatoxins and brevetoxins, are discussed.     

Crystal structure of the oxidised and reduced acidic cytochrome c3from Desulfovibrio africanus.

Unique among sulphate-reducing bacteria, Desulfovibrio africanus has two periplasmic tetraheme cytochromes c3, one with an acidic isoelectric point which exhibits an unusually low reactivity towards hydrogenase, and another with a basic isoelectric point which shows the usual cytochrome c3reactivity. The crystal structure of the oxidised acidic cytochrome c3of Desulfovibrio africanus (Dva.a) was solved by the multiple anomalous diffraction (MAD) method and refined to 1.6 A resolution. Its structure clearly belongs to the same family as the other known cytochromes c3, but with weak parentage with those of the Desulfovibrio genus and slightly closer to the cytochromes c3of Desulfomicrobium norvegicum. In Dva.a, one edge of heme I is completely exposed to the solvent and surrounded by a negatively charged protein surface. Heme I thus seems to play an important role in electron exchange, in addition to heme III or heme IV which are the electron exchange ports in the other cytochromes c3. The function of Dva.a and the nature of its redox partners in the cell are thus very likely different.By alignment of the seven known 3D structures including Dva.a, it is shown that the structure which is most conserved in all cytochromes c3is the four-heme cluster itself. There is no conserved continuous protein structure which could explain the remarkable invariance of the four-heme cluster. On the contrary, the proximity of the heme edges is such that they interact directly by hydrophobic and van der Waals contacts. This direct interaction, which always involves a pyrrole CA-CB side-chain and its bound protein cysteine Sgammaatom, is probably the main origin of the four-heme cluster stability. The same kind of interaction is found in the chaining of the hemes in other multihemic redox proteins.The crystal structure of reduced Dva. a was solved at 1.9 A resolution. The comparison of the oxidised and reduced structures reveals changes in the positions of water molecules and polar residues which probably result from changes in the protonation state of amino acids and heme propionates. Water molecules are found closer to the hemes and to the iron atoms in the reduced than in the oxidised state. A global movement of a chain fragment in the vicinity of hemes III and IV is observed which result very likely from the electrostatic reorganization of the polypeptide chain induced by reduction.     

Factors that influence bidirectional long-tract homozygosis due to double-strand break repair in Candida albicans

Genomic rearrangements have been associated with the acquisition of adaptive phenotypes, allowing organisms to efficiently generate new favorable genetic combinations. The diploid genome of Candida albicans is highly plastic, displaying numerous genomic rearrangements that are often the by-product of the repair of DNA breaks. For example, DNA double-strand breaks (DSB) repair using homologous-recombination pathways are a major source of loss-of-heterozygosity (LOH), observed ubiquitously in both clinical and laboratory strains of C. albicans. Mechanisms such as break-induced replication (BIR) or mitotic crossover (MCO) can result in long tracts of LOH, spanning hundreds of kilobases until the telomere. Analysis of I-SceI-induced BIR/MCO tracts in C. albicans revealed that the homozygosis tracts can ascend several kilobases toward the centromere, displaying homozygosis from the break site toward the centromere. We sought to investigate the molecular mechanisms that could contribute to this phenotype by characterizing a series of C. albicans DNA repair mutants, including pol32^-/-, msh2^-/-, mph1^-/-, and mus81^-/-. The impact of deleting these genes on genome stability revealed functional differences between Saccharomyces cerevisiae (a model DNA repair organism) and C. albicans. In addition, we demonstrated that ascending LOH tracts toward the centromere are associated with intrinsic features of BIR and potentially involve the mismatch repair pathway which acts upon natural heterozygous positions. Overall, this mechanistic approach to study LOH deepens our limited characterization of DNA repair pathways in C. albicans and brings forth the notion that centromere proximal alleles from DNA break sites are not guarded from undergoing LOH.     

Terrigenous sediment-dominated reef platform infilling: an unexpected precursor to reef island formation and a test of the reef platform size–island age model in the Pacific

Low-lying coral reef islands are considered highly vulnerable to climate change, necessitating an improved understanding of when and why they form, and how the timing of formation varies within and among regions. Several testable models have been proposed that explain inter-regional variability as a function of sea-level history and, more recently, a reef platform size model has been proposed from the Maldives (central Indian Ocean) to explain intra-regional (intra-atoll) variability. Here we present chronostratigraphic data from Pipon Island, northern Great Barrier Reef (GBR), enabling us to test the applicability of existing regional island evolution models, and the platform size control hypothesis in a Pacific context. We show that reef platform infilling occurred rapidly (~4–5 mm yr⁻¹) under a “bucket-fill” type scenario. Unusually, this infilling was dominated by terrigenous sedimentation, with platform filling and subsequent reef flat formation complete by ~5000 calibrated years BP (cal BP). Reef flat exposure as sea levels slowly fell post highstand facilitated a shift towards intertidal and subaerial-dominated sedimentation. Our data suggest, however, a lag of ~1500 yr before island initiation (at ~3200 cal BP), i.e. later than that reported from smaller and more evolutionarily mature reef platforms in the region. Our data thus support: (1) the hypothesis that platform size acts to influence the timing of platform filling and subsequent island development at intra-regional scales; and (2) the hypothesis that the low wooded islands of the northern GBR conform to a model of island formation above an elevated reef flat under falling sea levels.

     

Spatial genetic structure of <Emphasis Type="Italic">Lissotriton helveticus L</Emphasis>. following the restoration of a forest ponds network

Preserving amphibian genetic diversity through ecological restoration and conservation actions is a major challenge since their populations are declining worldwide. We studied the genetic diversity and spatial genetic structure of the palmate newt (Lissotriton helveticus) 2 years after the restoration of a pond network in northwestern France with the aim of reconstructing fine-scale genetic structure and patterns of colonization. We sampled newts from 29 forest ponds including both restored and non-degraded reference ponds, and genotyped 391 individuals at 12 microsatellite loci. We used two Bayesian clustering methods to spatially delineate genetic clusters, and we also detected potential recent migrants within the network. All ponds showed low levels of observed heterozygosity (Ho?=?0.534) and a mean F _IS of 0.251, possibly indicating a Wahlund or bottleneck effect. Pairwise F _ST suggested limited evidence of genetic differentiation among ponds. Within the pond network, we identified 3 to 4 genetic clusters. Combined with the detection of migrants, the results suggest an increase in gene flow within the restored pond network and that a high number of migrants came from the reference ponds. Our findings indicate an unexpected high dispersal ability for this small-bodied species. Overall, the absence of population structure represents a positive beginning for the restoration project. It also emphasizes the importance of spatial design in restoring a pond network and that such genetic data and methods should be used to monitor amphibians in restored habitats.     

A new photobioreactor for continuous microalgal production in hatcheries based on external-loop airlift and swirling flow

This study deals with the scale of a new photobioreactor for continuous microalgal production in hatcheries. The combination of the state-of-art with the constraints inherent to hatcheries has turned the design into a closed, artificially illuminated and external-loop airlift configuration based on a succession of elementary modules, each one being composed of two transparent vertical interconnected columns. The liquid circulation is ensured pneumatically (air injections) with respect to a swirling motion (tangential inlets). A single module of the whole photobioreactor was built-up to scale its geometry (diameter and length) and to optimize its design (air sparger, tangential inlets). The volumetric productivities were predicted by modeling radiative transfer and growth of Isochrysis affinis galbana (clone Tahiti). The hydrodynamics of the liquid phase was modeled in terms of global flow behavior (circulation and mixing times, Péclet number) and of swirling motion decay along the column (Particle Image Velocimetry). The aeration performances were determined by overall volumetric mass transfer measurements. Continuous cultures of Isochrysis affinis galbana (clone Tahiti) were run in two geometrical configurations, generating either an axial or a swirling flow. Lastly, the definitive options of design are presented as well as a 120-L prototype, currently implemented in a French mollusk hatchery and commercialized.     

Kinetic modeling of light limitation and sulfur deprivation effects in the induction of hydrogen production with Chlamydomonas reinhardtii: Part I. Model development and parameter identification

Chlamydomonas reinhardtii is a green microalga capable of turning its metabolism towards H2 production under specific conditions. However this H2 production, narrowly linked to the photosynthetic process, results from complex metabolic reactions highly dependent on the environmental conditions of the cells. A kinetic model has been developed to relate culture evolution from standard photosynthetic growth to H2 producing cells. It represents transition in sulfur-deprived conditions, known to lead to H2 production in Chlamydomonas reinhardtii, and the two main processes then induced which are an over-accumulation of intracellular starch and a progressive reduction of PSII activity for anoxia achievement. Because these phenomena are directly linked to the photosynthetic growth, two kinetic models were associated, the first (one) introducing light dependency (Haldane type model associated to a radiative light transfer model), the second (one) making growth a function of available sulfur amount under extracellular and intracellular forms (Droop formulation). The model parameters identification was realized from experimental data obtained with especially designed experiments and a sensitivity analysis of the model to its parameters was also conducted. Model behavior was finally studied showing interdependency between light transfer conditions, photosynthetic growth, sulfate uptake, photosynthetic activity and O2 release, during transition from oxygenic growth to anoxic H2 production conditions.