Accueil > Séminaires de l’ISYEB


Les mardis de 12:30 à 13:30 dans l’amphithéâtre Rouelle, situé au rez de chaussée du bâtiment de la Baleine (derrière les wallabies) - Plan d’accès -

Le prochain séminaire aura lieu le 9 mai

- 9 mai : Jean-Lou Justine, ISYEB.
Présentation : Publications, Impact Factor, H-index, Altmetrics, Réseaux Sociaux - la vie du chercheur au début du XXI° siècle

- 16 mai : Flora Vincent, IBENS.

Diatom interactions in the open ocean : from the global patterns to the single cell

Diatoms are unicellular photosynthetic microeukaryotes that play a critical role in the functioning of marine ecosystems. They are responsible for 20% of global photosynthesis on Earth and lie at the base of marine food webs, ever more threatened by climate change. Diatoms establish microbial interactions with numerous organisms across the whole tree of life, through complex mechanisms including symbiosis, parasitism and competition. The goal of my thesis was to understand how those biotic interactions structure the planktonic community at large spatial scales, by using new approaches based on the unprecedented Tara Oceans dataset, a unique and worldwide circumnavigation that collected over 40.000 samples across 210 sites to explore the diversity and functions of marine microbes. Through the analysis of microbial association networks, I show that diatoms act as repulsive segregators in the ocean, in particular towards potentially harmful organisms such as predators as well as parasites, and that species co-occurrence is driven by environmental factors in a minority of cases. By leveraging the singularity of the Tara Oceans data, I provide a comprehensive characterization of a prevalent biotic interaction between a diatom and heterotrophic ciliates at large spatial scale, illustrating the success of data-driven research. Overall, my thesis contributes to our understanding and discovery of diatom biotic interactions, from the global patterns to the single cell.

- 23 mai : Pierre Brezellec, MC à l’université de Versailles Saint-Quentin, Membre de l’ABI - ISYEB.

Initiation de la réplication chez les bactéries : encore une fois, "rien n’a de sens (en biologie) si ce n’est a la lumiere de l’evolution"

Chez les bactéries, le début de l’initiation de la réplication est marquée par le recrutement et la charge de l’hélicase réplicative à l’origine de réplication. Nous verrons, dans un premier temps, que dnaC et dnaI - les chargeurs d’hélicase réplicative de coli et de subtilis - sont absents de la majorité des phyla bactériens. Nous montrerons ensuite que dnaC et dnaI sont très vraisemblablement des "éléments" d’origine phagique qui ont été domestiqués au dépend d’un gène - probablement présent chez l’ancêtre commun de toutes les bactéries - que nous avons identifié et appelé dciA (dnaC/I Antecedent). Durant cette première partie, nous soulignerons que i/ le concept d’organismes modèles (ici coli et subtilis comme modèle de la réplication) a longtemps masqué la découverte de dciA, i.e., dnaC/I ont longtemps été présentés comme "universel" alors qu’ils ne représentaient que des "exceptions" à une règle plus générale, ii/ un échantillonnage plus exhaustif de la "diversité" couplé à "une phylogénie" sont des leviers puissants de la découverte en biologie. Dans un second temps, nous montrerons que quelques ordres bactériens sont peuplés d’organismes ne possédant ni dnaC/I ni dciA. Nous verrons que chez les Cellvibrionales et une partie des Oceanospirillales, la perte de dciA est - encore une fois - probablement le résultat d’un transfert horizontal suivi de la domestication des deux gènes d’origine phagique. Nous finirons notre exposé en suggérant des pistes expliquant les raisons pour lesquelles dciA est "régulièrement "perdu" au profit "d’éléments de type phagique".

- 30 mai : Hervé Sauquet, Université Paris-Sud, Lab. Écologie, Systématique, Évolution (ESE)

The ancestral flower of angiosperms and its early diversification : results from the eFLOWER project

Flowers are central to the biology and evolution of angiosperms (flowering plants). Here, we focus on the history of floral diversification through deep evolutionary time. A comprehensive understanding of global patterns of floral evolution has been prevented so far by the lack of an adequate morphological data set spanning all major angiosperm lineages. Using a new flexible and innovative approach, centered on a multi-user database (PROTEUS), we have built such a data set. Unlike most earlier studies of character evolution at the scale of angiosperms, we have recorded exemplar species instead of higher taxa such as genera or families. This approach allows for explicit reconstructions without assumptions about ancestral states or monophyly of supraspecific taxa, although we acknowledge that it also entails the risk of undersampling morphological variation. Importantly, the exemplar approach allows a direct match with the species sampled in molecular phylogenetic trees, providing the possibility to take branch lengths into account and to explore character evolution with probabilistic models. We report our first results from a data set of floral traits for 792 species representing all orders and 367 families (87%) of angiosperms, sampled from a recently published timetree calibrated with 136 fossil age constraints. Using this data set and tree, we reconstruct ancestral states for 27 key floral traits using parsimony, maximum likelihood, and Bayesian approaches and infer ancestral flowers for 15 key nodes (incl. Angiospermae, Mesangiospermae, Magnoliidae, Monocotyledoneae, Eudicotyledoneae, Pentapetalae, Rosidae, and Asteridae) as well as all extant orders of flowering plants. Further, we explore the sensitivity of our results to phylogenetic, dating, and model uncertainty. Importantly, all of our analyses converge on the same results in most cases, except near the root of the angiosperm tree, where parsimony continues to provide equivocal answers for some important floral traits, consistent with previous work. Lastly, the use of an explicit temporal framework for reconstructing ancestral states allows, for the first time, a direct comparison of inferred ancestral flowers with the known fossil record of each geological time interval. These results shed new light on long-standing questions on floral evolution across angiosperms, with important implications for a wide range of disciplines including Evo-Devo, pollination biology, and paleobotany.

- 6 juin : Julien Clavel, Institut de Biologie de l’École Normale Supérieure (IBENS).

Modelling phenotypic evolution on phylogenetic trees

- 13 juin : Vyacheslav Yurchenko, Associate Professor and Lab Head,
Laboratory of Molecular Protozoology, University of Ostrava, Czech Republic - SYNTHESIS UMR 7245

Trypanosomatids in NGS era

In this talk I will focus on two recently described trypanosomatid species which allow us to dissect molecular mechanisms governing some fundamental biological processes.

Part 1 will be devoted to Blastocrithidia spp., a clade of trypanosomatids with a unique nuclear genetic code. A limited number of non-canonical genetic codes have been described in eukaryotic nuclear genomes. Most involve reassignment of one or two termination codons as sense ones, but no code variant is known that would have reassigned all three termination codons. Here, we describe such a variant that we discovered in a clade of trypanosomatids comprising nominal Blastocrithidia spp. In these protists, UGA has been reassigned to encode tryptophan, while UAG and UAA (UAR) have become glutamate encoding. Strikingly, UAA and, less frequently, UAG also serve as bona fide termination codons. The release factor eRF1 in Blastocrithidia contains a substitution of a conserved serine residue predicted to decrease its affinity to UGA, which explains why this triplet can be read as a sense codon. However, the molecular basis for the dual interpretation of UAR codons remains elusive. Our findings expand the limits of comprehension of one of the fundamental processes in molecular biology.

Part 2 will deal with a recently established trypanosomatid-endosymbiont association. Recently, we described a novel symbiotic association between a kinetoplastid protist, Novymonas esmeraldas gen. nov., sp. nov., and an intracytoplasmic bacterium, “Candidatus Pandoraea novymonadis” sp. nov., discovered as a result of a broad-scale survey of insect trypanosomatid biodiversity in Ecuador. We characterize this association by describing the morphology of both organisms, as well as their interactions, and by establishing their phylogenetic affinities. Importantly, neither partner is closelyrelated to other known organisms previously implicated in eukaryote-bacterial symbiosis. This symbiotic association seems to be relatively recent, as the host does not exert a stringent control over the number of bacteria harbored in its cytoplasm. We argue that this unique relationship may represent a suitable model for studying the initial stages of establishment of endosymbiosis between a single-cellular eukaryote and a prokaryote. Based on phylogenetic analyses, Novymonas could be considered a proxy for the insect-only ancestor of the dixenous genus Leishmania and shed light on the origin of the two-host life cycle within the subfamily Leishmaniinae.

- 20 juin : Ray Tangney, Principal Curator Botany (Cryptogams), National Museum Wales, Cardiff

- 27 juin : André Nel, ISYEB

Les insectes fossiles, témoins privilégiés de l’histoire de la biodiversité terrestre

- 12 septembre : Roseli Pellens, ISYEB

- 10 octobre : Guillaume Sapriel, ABI, ISYEB

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