Contrôle de la sécrétion stéréomique dans les stolons d'un Crinoïde nouveau du Dévonien inferieur d'Algérie

The discovery of numerous distal stem fragments of Eurax marhoumensis n. sp. brings new data on the formation of the stolons and the control of stereomic secretion. The presence of primary fixations in the dististele demonstrates the composite nature of stolonifcrous scgmcnts. The creeping zone is formed by gradual incorporation of the lower columnals of the mesistele into the stolon. Stereom growth rings observed on weathered facets or transverse sections are markers of the morphofunctional evolution of the columnals during different growth stages. The fixation organs are true stereomic outgrowths without axial canal or Segmentation; they grow from a single columnal and remain in connexion with it. The morphology of stolons results partly from the topography of the substrate and partly from control by stereomic secretion which influences the columnal thickness. The introduction of wedge-shaped columnals in curves of the stem and on the development of fixation devices. Epizoan attachment (bryozoans, corals, crinoids) induces different stereomic reaction depending on the growth stage of the crinoid column. The stereom of the host may proliferate as a sheet, abortive ossicles or shapeless stercomic masses which tend to limit the growth of the epizoans and incorporate them in the column stercom. □Crinoid, column, morphology, stereomic secretion, Devonian, Algeria. Le découverte de nombreux fragments pédonculaires distaux d'Eurax marhoumensis n. sp. apporte des données nouvelles sur le processus de la formation des stolons et sur le contrôle de la sécrétion stéréomique. L'existence de fixations primaires dans la dististéle démontre la nature composite des segments stoloniaux. Les zones rampantes se forment par incorporation progressive des columnales de la partie inférieure du pédoncule dans les stolons. Les cernes de croissance du stéréome observées sur les facettes altérées ou sur les sections constituent des témoins de leur évolution morphofonctionnelle au cours des différents stades de la croissance. Les organes de fixation correspondent à de véritables expansions stéréomiques dépourvues de canal axial et de segmentation; ils bourgeonnent àpartir d'une seule columnale avec laquelle ils rcstent en conncxion. La morphologie des stolons résultc en partie de la topographie du substratum et du contrôle de la sécrétion stéréomique qui se traduit par I'ajustement de l'epaisseur des columnales, l'introduction d'articles cunéiformes dans les courbures et le développement des organes de fixation. L'implantation d'épizoaires (Bryozoaires, Coraux, Crinoïdes) provoque des réactions stéréomiques différentes suivant le stade de croissance du pédoncule du Crinoïde. Le stéréome de I'hôte peut proliférer sous formc d'une lame, d'articles abortifs ou d'amas difformes qui tendent a limiter le développement de I'épizoaire et à l'incorporer dans le pédoncule. □Crinoïde, pédoncule, morphologie. Sécrétion stéréomique, Dévonien, Algérie.     

Epistasis and frequency dependence influence the fitness of an adaptive mutation in a diversifying lineage

The opportunity for a mutation to invade a population can dramatically vary depending on the context in which this mutation occurs. Such context dependence is difficult to document as it requires the ability to measure how a mutation affects phenotypes and fitness and to manipulate the context in which the mutation occurs. We identified a mutation in a gene encoding a global regulator in one of two ecotypes that diverged from a common ancestor during 1200 generations of experimental evolution. We replaced the ancestral allele by the mutant allele, and vice versa, in several clones isolated during the time course of the evolution experiment, and compared the phenotype and fitness of clones isogenic except for the focal mutation. We show that the fitness and phenotype of the mutation are strongly affected by epistatic interactions between genes in the same genome, as well as by frequency dependent selection resulting from biotic interactions between individuals in the same population. We conclude that amongst the replicate population in which it spread, the mutation we identified is only adaptive when occurring in specific genomes and competing with specific individuals. This study thus demonstrates that the opportunity for an adaptive mutation to spread in an evolutionary lineage can only be understood in the light of its genomic and competitive environments.     

Soil [N] modulates soil C cycling in CO2‐fumigated tree stands: a meta‐analysis

Under elevated atmospheric CO₂ concentrations, soil carbon (C) inputs are typically enhanced, suggesting larger soil C sequestration potential. However, soil C losses also increase and progressive nitrogen (N) limitation to plant growth may reduce the CO₂ effect on soil C inputs with time. We compiled a data set from 131 manipulation experiments, and used meta‐analysis to test the hypotheses that: (1) elevated atmospheric CO₂ stimulates soil C inputs more than C losses, resulting in increasing soil C stocks; and (2) that these responses are modulated by N. Our results confirm that elevated CO₂ induces a C allocation shift towards below‐ground biomass compartments. However, the increased soil C inputs were offset by increased heterotrophic respiration (Rh), such that soil C content was not affected by elevated CO₂. Soil N concentration strongly interacted with CO₂ fumigation: the effect of elevated CO₂ on fine root biomass and –production and on microbial activity increased with increasing soil N concentration, while the effect on soil C content decreased with increasing soil N concentration. These results suggest that both plant growth and microbial activity responses to elevated CO₂ are modulated by N availability, and that it is essential to account for soil N concentration in C cycling analyses.     

Quantifying the Origin of Woodcock Wintering in France

Abstract: Migratory birds wintering at the same location are usually coming from populations of different origins, in variable proportions. Using data from individuals banded on their wintering grounds and recovered on their breeding areas, we show that such proportions are estimable given that 1) all breeding populations are identified, and 2) the wintering population can be stratified into ≥1 more site than the number of breeding populations. We applied this technique to woodcock (Scolopax rusticola) banded while wintering in France (N = 35,000) and recovered in other European countries (N = 520). We estimated that the proportion of eastern woodcock among those wintering in France varied spatially, ranging from 70% in northwest France to nearly 100% in southeast France, and increased substantially over the last 15 years. Overall, the method appears powerful to quantify spatial variation of the composition of a population receiving individuals from various origins.     

Devenir de la putrescine-1,4-14C chez Glycine max

Fate of Putrescine-1,4-¹⁴C in Glycine max Putrescine-1,4-¹⁴C was supplied to young decotylized Glycine max (L.) Merr. cv. Chippewa plants growing under aseptic conditions on a liquid medium with nitrogen supplied either as ammonium chloride or nitrates. Whatever the source of nitrogen the diamine was quickly transformed to γ-aminobutyric acid, succinic acid and malic acid; only a very minor part was utilized for the synthesis of polyamines. In the presence of ammonium chloride the putrescine catabolism may be slower than in the presence of nitrates. The results are explained by a weak isotopic dilution or by a diamine oxidase activity lower in “ammonium” plants than in “nitrate” plants; the two causes might co-exist. The possibility of a diamine compartmentation has to be considered.