Precambrian palaeontology in the light of molecular phylogeny – an example: the radiation of the green algae

International audienceThe problem of the antiquity of the radiation of the green algae (phylum Viridiplantae) has been hotly debated and is still controversial today. A method combining Precambrian paleontology and molecular phylogeny is applied to shed light on this topic. As a critical method, molecular phylogeny is essential for avoiding taxonomic mistakes. As a heuristic method, it helps us to discern to what extent the presence of such and such clade is likely at such and such time, and it may even suggest the attribution of some fossil to a clade whose taxonomic position will be distinctly defined even though it has no previously known representative. Some well characterized Precambrian fossils of green algae are Palaeastrum and Proterocladus at Svanbergfjellet (ca. 750 Ma), Tasmanites and Pterospermella at Thule (ca. 1200 Ma), Spiromorpha at Ruyang (ca. 1200 Ma) and Leiosphaeridia crassa at Roper (ca. 1450 Ma). The position of these fossils in the taxonomy and the phylogeny of the Viriplantae is discussed. The conclusions are that the Chlorophyceae and the Ulvophyceae were separated long before 750 Ma, that the Chlorophyta and the Streptophyta were separated long before 1200 Ma and that the last common ancestor of the Viridiplantae and the Rhodophyta was possibly two billion years old

A revised linear ozone photochemistry parameterization for use in transport and general circulation models: multi-annual simulations

International audienceThis article describes the validation of a linear parameterization of the ozone photochemistry for use in upper tropospheric and stratospheric studies. The present work extends a previously developed scheme by improving the 2-D model used to derive the coefficients of the parameterization. The chemical reaction rates are updated from a compilation that includes recent laboratory work. Furthermore, the polar ozone destruction due to heterogeneous reactions at the surface of the polar stratospheric clouds is taken into account as a function of the stratospheric temperature and the total chlorine content. Two versions of the parameterization are tested. The first one only requires the solution of a continuity equation for the time evolution of the ozone mixing ratio, the second one uses one additional equation for a cold tracer. The parameterization has been introduced into the chemical transport model MOCAGE. The model is integrated with wind and temperature fields from the ECMWF operational analyses over the period 2000–2004. Overall, the results from the two versions show a very good agreement between the modelled ozone distribution and the Total Ozone Mapping Spectrometer (TOMS) satellite data and the "in-situ" vertical soundings. During the course of the integration the model does not show any drift and the biases are generally small, of the order of 10%. The model also reproduces fairly well the polar ozone variability, notably the formation of "ozone holes" in the Southern Hemisphere with amplitudes and a seasonal evolution that follow the dynamics and time evolution of the polar vortex. The introduction of the cold tracer further improves the model simulation by allowing additional ozone destruction inside air masses exported from the high to the mid-latitudes, and by maintaining low ozone content inside the polar vortex of the Southern Hemisphere over longer periods in spring time. It is concluded that for the study of climate scenarios or the assimilation of ozone data, the present parameterization gives a valuable alternative to the introduction of detailed and computationally costly chemical schemes into general circulation models

Les algues vertes (phylum Viridiplantae) sont-elles vieilles de deux milliards d'années ?

Dans son livre, Life on a young planet, A.H. KNOLL prétend que les plus anciens fossiles bien attestés d'algues vertes remontent à 750 Ma. Cependant, selon le livre de B. TEYSSEDRE, La vie invisible, certains d'entre eux sont beaucoup plus vieux. L'article qui suit recourt à une méthode qui combine la paléontologie et la phylogénie moléculaire pour une recherche sur les fossiles précambriens de quelques "acritarches" et d'un clade primitif d'algues vertes, les Pyramimonadales. Un assemblage paraphylétique d'algues vertes unicellulaires, appelé "Prasinophyceae", est représenté vers 1200 Ma dans le Supergroupe de Thulé, au Groenland, par plusieurs morphotypes des Pyramimonadales, qui sont monophylétiques, en particulier Tasmanites et Pterospermella apparentés à des algues encore vivantes aujourd'hui. Ces deux genres, parmi d'autres, ont eu probablement des précurseurs vers 1450 / 1550 Ma. Certains acritarches qui pourraient représenter des Pyramimonadales produisant des "phycomes" qui s'ouvraient par une fissure lors de leur déhiscence ont été confusément rassemblés dans le pseudo-taxon polyphylétique "Leiosphaeridia". Ils étaient peut-être déjà présents à Chuanlinggou, en Chine, vers 1730 Ma. Beaucoup d'acritarches de Russie que TIMOFEEV a obtenus par macération dans l'acide fluorhydrique et qui ont été datés entre 1800 et 2000 Ma représentent probablement des Chlorophytes unicellulaires capables de synthétiser des algaenanes ou autres biopolymères résistants à l'acétolyse. Les Prasinophyceae qui vivent encore de nos jours sont incontestablement des algues vertes (Viridiplantae). Par conséquent, s'il est vrai que les fossiles de Prasinophyceae remontent certainement à 1200 Ma, probablement à 1500 Ma et peut-être même à 1730 Ma, il faut conclure que l'ancêtre des algues vertes (Chlorophyta et Streptophyta) s'était probablement déjà séparé de l'ancêtre des algues rouges (Rhodophyta) à une date proche de 2000 Ma.In his book, Life on a young planet, A.H. KNOLL states that the first documented fossils of green algae date back 750 Ma. However, according to B. TEYSSÈDRE's book, La vie invisible, they are much older. Using a method which combines paleontology and molecular phylogeny, this paper is an inquiry into the Precambrian fossils of some "acritarchs" and of a primitive clade of green algae, the Pyramimonadales. A paraphyletic group of unicellular green algae, named "Prasinophyceae", is represented at Thule (Greenland) ca. 1200 Ma by several morphotypes of the monophyletic Pyramimonadales, including Tasmanites and Pterospermella that are akin to algae still living today. These two, and others, probably had forerunners going back 1450 / 1550 Ma. Some acritarchs that may represent Pyramimonadales producing "phycomas" which split open for dehiscence were confusingly included in the polyphyletic pseudo-taxon "Leiosphaeridia" and are possibly already present at Chuanlinggou, China, ca. 1730 Ma. Many acritarchs that TIMOFEEV obtained by acid maceration of Russian samples dated between 1800 and 2000 Ma were probably unicellular Chlorophyta which synthesized algaenans or other biopolymers resistant to acetolysis. Living Prasinophyceae are undoubtedly green algae (Viridiplantae). Thus, if Prasinophyceae fossils go back certainly to 1200 Ma, probably to 1500 Ma and possibly to 1730 Ma, then the ancestor of green algae (Chlorophyta and Streptophyta) probably separated from the ancestor of red algae (Rhodophyta) as early as 2000 Ma

Les algues vertes (phylum Viridiplantae), sont-elles vieilles de deux milliards d'années ?

Dans son livre, Life on a young planet, A.H. KNOLL prétend que les plus anciens fossiles bien attestés d'algues vertes remontent à 750 Ma. Cependant, selon le livre de B. TEYSSEDRE, La vie invisible, certains d'entre eux sont beaucoup plus vieux. L'article qui suit recourt à une méthode qui combine la paléontologie et la phylogénie moléculaire pour une recherche sur les fossiles précambriens de quelques "acritarches" et d'un clade primitif d'algues vertes, les Pyramimonadales. Un assemblage paraphylétique d'algues vertes unicellulaires, appelé "Prasinophyceae", est représenté vers 1200 Ma dans le Supergroupe de Thulé, au Groenland, par plusieurs morphotypes des Pyramimonadales, qui sont monophylétiques, en particulier Tasmanites et Pterospermella apparentés à des algues encore vivantes aujourd'hui. Ces deux genres, parmi d'autres, ont eu probablement des précurseurs vers 1450 / 1550 Ma. Certains acritarches qui pourraient représenter des Pyramimonadales produisant des "phycomes" qui s'ouvraient par une fissure lors de leur déhiscence ont été confusément rassemblés dans le pseudo-taxon polyphylétique "Leiosphaeridia". Ils étaient peut-être déjà présents à Chuanlinggou, en Chine, vers 1730 Ma. Beaucoup d'acritarches de Russie que TIMOFEEV a obtenus par macération dans l'acide fluorhydrique et qui ont été datés entre 1800 et 2000 Ma représentent probablement des Chlorophytes unicellulaires capables de synthétiser des algaenanes ou autres biopolymères résistants à l'acétolyse. Les Prasinophyceae qui vivent encore de nos jours sont incontestablement des algues vertes (Viridiplantae). Par conséquent, s'il est vrai que les fossiles de Prasinophyceae remontent certainement à 1200 Ma, probablement à 1500 Ma et peut-être même à 1730 Ma, il faut conclure que l'ancêtre des algues vertes (Chlorophyta et Streptophyta) s'était probablement déjà séparé de l'ancêtre des algues rouges (Rhodophyta) à une date proche de 2000 Ma.In his book, Life on a young planet, A.H. KNOLL states that the first documented fossils of green algae date back 750 Ma. However, according to B. TEYSSÈDRE's book, La vie invisible, they are much older. Using a method which combines paleontology and molecular phylogeny, this paper is an inquiry into the Precambrian fossils of some "acritarchs" and of a primitive clade of green algae, the Pyramimonadales. A paraphyletic group of unicellular green algae, named "Prasinophyceae", is represented at Thule (Greenland) ca. 1200 Ma by several morphotypes of the monophyletic Pyramimonadales, including Tasmanites and Pterospermella that are akin to algae still living today. These two, and others, probably had forerunners going back 1450 / 1550 Ma. Some acritarchs that may represent Pyramimonadales producing "phycomas" which split open for dehiscence were confusingly included in the polyphyletic pseudo-taxon "Leiosphaeridia" and are possibly already present at Chuanlinggou, China, ca. 1730 Ma. Many acritarchs that TIMOFEEV obtained by acid maceration of Russian samples dated between 1800 and 2000 Ma were probably unicellular Chlorophyta which synthesized algaenans or other biopolymers resistant to acetolysis. Living Prasinophyceae are undoubtedly green algae (Viridiplantae). Thus, if Prasinophyceae fossils go back certainly to 1200 Ma, probably to 1500 Ma and possibly to 1730 Ma, then the ancestor of green algae (Chlorophyta and Streptophyta) probably separated from the ancestor of red algae (Rhodophyta) as early as 2000 Ma

Dissection génétique des caractères par analyse de liaison et d'association : aspects méthodologiques et application à la sensibilité à l'ostéochondrose chez les Trotteurs Français

Diverses lésions ostéochondrales peuvent affecter les articulations des jeunes chevaux et réduire leurs futures performances en course. L’objectif de cette thèse est d’identifier les régions du génome, appelées locus à caractère quantitatif (QTL), associées avec des caractères mesurant l’ostéochondrose (OC) enregistrés dans le programme GENEQUIN sur une population de Trotteurs Français. Le génotypage a été réalisé à l’aide de la puce SNP Illumina BeadChip EquineSNP50, qui est dense et permet d’exploiter le déséquilibre de liaison par des analyses d’association. Ces analyses sont sujettes à certains problèmes en présence d’une structure familiale des données. Dans la première partie de la thèse, une comparaison de la puissance et de la robustesse d’un choix restreint de méthodes d’analyses est effectuée. L’originalité de ce travail réside dans la dérivation algébrique des moments des distributions des statistiques de test comparées, donnant ainsi plus de généralité à nos résultats et permettant une meilleure compréhension des différences. Les résultats peuvent notamment servir à l’optimisation du dispositif expérimental. La deuxième partie est consacrée à la cartographie des régions QTL des caractères mesurant l’OC en différents sites articulaires dans une population de 583 Trotteurs Français. Cette étude a permis de mettre en évidence plusieurs régions QTL d’effets moyens et faibles à un niveau significatif mais pas hautement significatif. Nous montrons que l’OC est un caractère polygénique et qu’aucun QTL, ayant un effet à la fois sur l’OC du jarret et l’OC du boulet, n’est détectable dans ce protocole QTL, ce qui infirme l’hypothèse simple d’une cause génétique commune de la sensibilité à cette maladie sur les différents sites anatomiques. Suite à ces travaux, l’identification des gènes candidats et des mutations causales devrait clarifier la physiopathologie moléculaire de l’OC et ainsi permettre de développer des stratégies efficaces pour l’évaluation des risques. Pendant ce temps, les marqueurs peuvent être utilisés dans un contexte de sélection assistée par marqueurs afin d’améliorer la santé et le bien-être du cheval. ABSTRACT : Osteochondral lesions are commonly observed in young horses and may be responsible for reduced performances in racing. The purpose of the PhD thesis was to identify genome regions, called quantitative trait loci (QTL), associated with various traits measuring osteochondrosis (OC) and recorded in the GENEQUIN program in a population of French Trotters horses. Genotyping was performed using the EquineSNP50 Illumina high density chip, which allows to exploit the linkage disequilibrium with genome-wide association studies. These analyses are subject to several problems in presence of family structure. We hence first proposed a comparison of power and robustness of a limited choice of models for this type of analysis. The originality of this work lies in the algebraic derivation of the distribution moments of the test statistics compared, making the outcome of this comparison more general and allowing a better understanding of differences. The results can be used to establish an experimental design. The second part was devoted to the QTL fine mapping of traits that measure OC in different joint sites. This study highlighted several significant QTL with low and medium effects but none of them were highly significant. We showed that OC is a polygenic trait and we were not able to identify QTL affecting both OC on the hock and the fetlock, rejecting the hypothesis of a single genetic determinism of susceptibility to this desease accross anatomical sites. Further studies will now focus on the identification of candidate genes and screening for mutation in an attempt to clarify the molecular physiopathology of OC and develop efficient strategies for risk assessment. Meanwhile, markers could be used in a marker-assisted selection context to improve horse health and welfare

Study of the glass transition in the amorphous interlamellar phase of highly crystallized poly(ethylene terephthalate)

Poly(ethylene terephthalate) (PET) is a semi--crystalline polymer that can be crystallized to different degrees heating from the amorphous state. Even when primary crystallization has been completed, secondary crystallization can take place with further annealing and modify the characteristics of the amorphous interlamellar phase. In this work we study the glass transition of highly crystallized PET and in which way it is modified by secondary crystallization. Amorphous PET samples were annealed for 4 hours at temperatures between 140C and 180C. The secondary crystallization process was monitored by differential scanning calorimetry and the glass transition of the remaining interllamelar amorphous phase was studied by Thermally Stimulated Depolarization Currents measurements. Non--isothermal window polarization is employed to resolve the relaxation in modes with a well--defined relaxation time that are subsequently adjusted to several standard models. Analysis of experimental results, show that cooperativity is reduced to a great extend in the interlamellar amorphous regions. The evolution of the modes on crystallization temperature reveals that large scale movements are progressively replaced by more localized ones, with higher frequency, as crystallization takes place at higher temperatures. As a consequence, the glass transition temperature of the amorphous interlamellar phase tends to lower values for higher annealing temperatures. Evolution of calorimetric scans of the glass transition are simulated from the obtained results and show the same behaviour. The interpretation of these results in terms of current views about secondary crystallization is discussed.Comment: 30 pages, 5 tables, 12 figures; figure 5 modifie

Dielectric study of the glass transition: correlation with calorimetric data

The glass transition in amorphous poly(ethylene terephthalate) is studied by thermally stimulated depolarization currents (TSDC) and differential scanning calorimetry (DSC). The ability of TSDC to decompose a distributed relaxation, as the glass transition, into its elementary components is demonstrated. Two polarization techniques, windows polarization (WP) and non-isothermal windows polarization (NIW), are employed to assess the influence of thermal history in the results. The Tool-Narayanaswami-Moynihan (TNM) model has been used to fit the TSDC spectra. The most important contributions to the relaxation comes from modes with non-linearity (x) around 0.7. Activation energies yield by this model are located around 1eV for polarization temperature (Tp) below 50C and they raise up to values higher than 8eV as Tp increases (up to 80C). There are few differences between results obtained with WP and NIW but, nonetheless, these are discussed. The obtained kinetic parameters are tested against DSC results in several conditions. Calculated DSC curves at several cooling and heating rates can reproduce qualitatively experimental DSC results. These results also demonstrate that modelization of the non-equilibrium kinetics involved in TSDC spectroscopy is a useful experimental tool for glass transition studies in polar polymers.Comment: 13 pages, 2 tables, 10 figures; minor change

Photoluminescence, recombination induced luminescence and electroluminescence in epoxy resin

Dielectric breakdown of epoxies is preceded by light emission, or so-called electroluminescence, from the solid-state material. Very little is known about the luminescence properties of epoxies. The aim of this paper is to derive information that can be used as a basis to understand the nature of the excited states and their involvement in electrical degradation processes. Three different kinds of stimulation were used to excite the material luminescence. Photoluminescence was performed on the base resin, the hardener and the cured resin. Luminescence excited by a silent discharge has been analysed to identify which of the luminescent centres are optically active upon the recombination of electrical charges and could therefore act as charge traps. Finally, the electroluminescence spectrum has been acquired and compared with the previous ones. Although the identification of the origin of these emissions is far from being complete, it has been found that the photoluminescence from the cured resin is due to in-chain chromophores, which acts as trapping centres. The excited states involved in photoluminescence also seems to be involved in electroluminescence, but other components are detected as well, which could be due to the degradation of the resin molecule under the effect of the electric stress

Anatomical bases of sex- and size-related acoustic variation in herring gull alarm calls

The hypothesis that anatomical or physiological factors can constrain the production of vocalizations is supported by an increasing number of examples from a range of taxa, where acoustic variation is related to sex, body-size or condition. In this study, we combine anatomical and acoustic investigations in herring gulls Larus argentatus to 1) identify co-variation between sex, body size and the dimensions of the vocal apparatus and 2) test the possible effect of this co-variation on interindividual variation in the acoustics of alarm calls. We found that the vocal apparatus was sexually dimorphic, with males having longer trachea and bigger vibratile membranes than females. We also identified a relationship between the head–bill length – a secondary sexual trait – and the length of the trachea in males only. However, we failed to identify corresponding sex- and body-size related variation in the acoustic components of alarm calls. We suggest that this absence of a relationship between anatomical and acoustic dimensions may reflect the lack of biomechanical constraints exerted during the production of alarm calls, and that such relationships are more likely to be expressed in this species’ sexual calls, whose production is characterised by more pronounced, ritualised postures that are more likely to highlight inter-individual size variation