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Mitchell, Edward
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Mitchell, Edward
Affiliation principale
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Professeur ordinaire
Email
edward.mitchell@unine.ch
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Résultat de la recherche
Voici les éléments 1 - 5 sur 5
- PublicationAccès librerRNA Phylogeny of Arcellinida (Amoebozoa) Reveals that the Largest Arcellinid Genus, Difflugia Leclerc 1815, is not Monophyletic(2012)
; The systematics of lobose testate amoebae (Arcellinida), a diverse group of shelled free-living unicellular eukaryotes, is still mostly based on morphological criteria such as shell shape and composition. Few molecular phylogenetic studies have been performed on these organisms to date, and their phylogeny suffers from typical under-sampling artefacts, resulting in a still mostly unresolved tree. In order to clarify the phylogenetic relationships among arcellinid testate amoebae at the inter-generic and inter-specific level, and to evaluate the validity of the criteria used for taxonomy, we amplified and sequenced the SSU rRNA gene of nine taxa - Difflugia bacillariarum, D. hiraethogii, D. acuminata, D. lanceolata, D. achlora, Bullinularia gracilis, Netzelia oviformis, Physochila griseola and Cryptodifflugia oviformis. Our results, combined with existing data demonstrate the following: 1) Most arcellinids are divided into two major clades, 2) the genus Difflugia is not monophyletic, and the genera Netzelia and Arcella are closely related, and 3) Cryptodifflugia branches at the base of the Arcellinida clade. These results contradict the traditional taxonomy based on shell composition, and emphasize the importance of general shell shape in the taxonomy of arcellinid testate amoebae. - PublicationAccès libreMolecular phylogeny and taxonomy of Testate amoebae (protist) and host-symbiont evolutionary relationships within mixotrophic taxa(2012)
; Les recherches en phylogénie moléculaire ont considérablement avancé notre compréhension des relations entre eucaryotes. Les classifications récentes placent les protistes amoeboides dans plus de 30 lignées au sein des Amoebozoa, Rhizaria, Stramenopiles, Opisthokonta, et Excavata. Parmi celles-ci, certaines branches ont développé des thèques ou coquilles, souvent ornementées et caractéristiques qui ont été utilisées depuis plus de 150 ans comme caractère diagnostique pour décrire plus de 2000 espèces. Les thécamoebiens sont caractérisés par des pseudopodes lobés ou filamenteux et une thèque à une chambre pouvant être agglutinée, protéinique, calcaire ou siliceuse. L’acquisition de la thèque s’est faite plusieurs fois de manière indépendante au cours de l’évolution. De plus, et malgré la longue tradition de recherche en taxonomie sur les thécamoebiens, les relations entre les différents taxons demeurent largement non-résolue, l’affiliation phylogénétique de certains genres restant inconnue.
Le but de cette thèse était de construire une phylogénie fiable du plus grand ordre d’amibes, les Arcellinida, en utilisant des séquences du gène SSU rRNA et des analyses par microscopie électronique (chapitres 2 et 5). Les résultats révèlent des contradictions drastiques avec la taxonomie traditionnelle. Le genre Difflugia, le plus grand genre des Arcellinida, n’est pas monophylétique et est divisé en deux clades bien distincts regroupant respectivement les espèces allongées/pyriformes et les espèces globulaires. Le genre Netzelia est phylogénétiquement proche des Difflugia globulaires malgré les différences de structures de leur thèque.
Par ailleurs, les Arcellinida démontrent un conservatisme morphologique marqué; les types morphologiques similaires correspondant possiblement à des taxons génétiquement très distants. Nous démontrons la possibilité d’une évolution morphologique rapide an sein de ce groupe. Difflugia tuberspinifera, une espèce endémique d’Asie possède deux morpho-types (avec et sans cornes) possédant des séquences similaire du gène SSU rRNA gene (99.8%) et des introns et insertions identiques, mais pouvant toutefois être discriminés sur la base de leur séquences. Ceci suggère une évolution morphologique récente, possiblement liée à des facteurs écologiques à déterminer.
Nous avons déterminé la position phylogénétique des deux genresincertae sedis bien connus de la famille des Amphitrematidae, Amphitrema et Archerella (chapitre 3), qui de manière surprenante sont apparentés Labyrinthulomycetes (Stramenopiles), formant ainsi un nouveau clade de thécamoebiens indépendants des autres (c.à.d. Amoebozoa & Rhizaria). Cette étude illustre également que la taxonomie et la phylogénie des protistes en général est d’une importance cruciale pour comprendre l’évolution de la diversité des eucaryotes.
Les thécamoebiens forment souvent des associations avec les organismes photosynthétiques dont l’identité demeure toutefois inconnue. Nous avons identifié les symbiontes de quatre thécamoebiens différents sur la base du gène chloroplastique rbcL (ribulose-1, 5-diphosphate carboxylase/oxygénase grande sub-unité) utilisé comme gène de barcoding. La majorité des symbiontes de thécamoebiens ont pu être raisonnablement associés à une seule espèce, malgré le fait que leurs hôtes étaient taxonomiquement très distants. Fait intéressant, les Chlorelles symbiontes des thécamoebiens étaient très proches de Chlorella variabilis ainsi que des symbiontes de Paramecium bursaria. A la lumière de ces résultats, nous proposons un scénario d’évolution de l’association entre hôtes hétérotrophes et leur symbiontes photosynthétiques.
De manière générale, ma thèse illustre qu’une phylogénie fiable des thécamobiens basée sur les approches morphologiques et moléculaires est non-seulement un prérequis essentiel pour comprendre leur évolution, mais contribuera aussi à résoudre des débats concernant leur diversité et leur biogéographie, et en augmentera en général leur utilisé comme groupe modèle d’organismes pour les recherches en écologie appliquée., Molecular phylogenetic studies have considerably advanced our understanding of the relationships among eukaryotes. In recent classification schemes, amoeboid protists appeared scattered in more than 30 lineages within Amoebozoa, Rhizaria, Stramenopiles, Opisthokonta, and Excavata. Amongst these, some branches tended to develop a test or shell, often ornamented and conspicuous, which has been used for more than 150 years as a diagnostic character to describe more than 2000 species. Testate amoebae are characterized by lobose or filose pseudopodia and one chamber shell that can be agglutinated, proteinaceous, calcareous or siliceous. The acquisition of the shell happened several times independently in the course of evolution. Furthermore, and in spite of the long taxonomic tradition in testate amoebae research, the relationships between the different taxa remained largely unresolved, some genera remaining still without known phylogenetic affiliation.
In this thesis, we aimed at constructing a reliable phylogeny of the largest testate amoebae order, the Arcellinida, using SSU rRNA gene sequences and scanning electron microscopy analyses (chapters 2 and 5). Our results revealed drastic contradictions with traditional taxonomy. Genus Difflugia, the largest Arcellinid genus appeared not monophyletic, and divided in two major and distantly related clades that grouped respectively the elongated/pyriform and the globular species. Genus Netzelia was phylogenetically very closely related to the globular Difflugia despite the inconsistencies in their shell structure.
In addition, Arcellinida tended to show an important morphological conservatism, and closely related morphologies can possibly hide important genetic distances. We also demonstrated that fast morphological evolution could also be possible in this group. Difflugia tuberspinifera, an Asian endemic species had two morphotypes (spiny and spineless) which shared highly similar SSU rRNA gene sequences (99.8%) and identical introns and insertions, but could be nevertheless discriminated on the base of their sequences. This result suggested a recent morphological evolution, presumably due to some differing ecological factors that still need to be clarified.
We determined also the phylogenetic position of two well known incertae sedis genera of family Amphitremida, Amphitrema and Archerella (chapter 3), which appeared surprisingly to be related to Labyrinthulomycetes (Stramenopiles), thus forming a new clade of testate amoebae independent from others (i.e Amoebozoa, Rhizaria). This study also illustrated that accurate taxonomy and phylogeny of protists in general is of crucial important for understanding the evolution and diversity of eukaryotes.
Testate amoebae have been also often found in association with some photosynthetic organisms whose identity remained unknown. We identified the symbionts of four different testate amoeba species using the chloroplastic gene rbcL (ribulose-1, 5-diphosphate carboxylase/oxygenase large subunit) as a barcoding gene. The majority of testate amoeba symbionts formed a consistent group with very few sequence diversity that could be reasonably associated to a single species, in spite of the fact that host species were taxonomically distantly related. Interestingly, testate amoebae Chlorella symbionts were very closely related to Chlorella variabilis and to Paramecium bursaria Chlorella symbionts. In the light of these results, we proposed a general evolutionary scenario for association between heterotrophic hosts and their photosynthetic symbionts.
Overall, my thesis illustrated that the reliable phylogeny of testate amoebae based on molecular and morphological approaches is not only essential prerequisite for understanding their evolution, but it also will contribute in resolving debates concerning their diversity and biogeography, and in general will increase their utility as a model group of organisms for applied ecological research. - PublicationAccès libreMorphological and Molecular Diversification of Asian Endemic Difflugia tuberspinifera (Amoebozoa, Arcellinida): A Case of Fast Morphological Evolution in Protists?
; - PublicationAccès libreOne Alga to Rule them All: Unrelated Mixotrophic Testate Amoebae (Amoebozoa, Rhizaria and Stramenopiles) Share the Same Symbiont (Trebouxiophyceae)
; ; Endosymbiosis is a central and much studied process in the evolution of eukaryotes. While plastid evolution in eukaryotic algae has been extensively studied, much less is known about the evolution of mixotrophy in amoeboid protists, which has been found in three of the five super groups of Eukaryotes. We identified the green endosymbionts in four obligate mixotrophic testate amoeba species belonging to three major eukaryotic clades, Hyalosphenia papilio and Heleopera sphagni (Amoebozoa: Arcellinida), Placocista spinosa (Rhizaria: Euglyphida), and Archerella flavum (Stramenopiles: Labyrinthulomycetes) based on rbcL (ribulose-1,5-diphosphate carboxylase/oxygenase large subunit) gene sequences. We further investigated whether there were different phylotypes of algal endosymbionts within single H. papilio cells and the degree of host-symbiont specificity by amplifying two genes: COI (mitochondrial cytochrome oxydase subunit 1) from the testate amoeba host, and rbcL from the endosymbiont. Results show that all studied endosymbionts belong to genus Chlorella sensu stricto, closely related to Paramecium bursaria Chlorella symbionts, some lichen symbionts and also several free-living algae. Most rbcL gene sequences derived from symbionts from all testate amoeba species were almost identical (at most 3 silent nucleotides difference out of 780 bp) and were assigned to a new Trebouxiophyceae taxon we named TACS (Testate Amoeba Chlorella Symbionts). This “one alga fits all mixotrophic testate amoeba” pattern suggests that photosynthetic symbionts have pre-adaptations to endosymbiosis and colonise diverse hosts from a free-living stage. - PublicationAccès libreUsing DNA-barcoding for sorting out protest species complexes:: A case study of the Nebela tincta–collaris–bohemica group (Amoebozoa; Arcellinida, Hyalospheniidae)
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We examined the relationship between morphological and genetic diversity within this species complex by combined analyses of light microscopy imaging and Cytochrome Oxidase Subunit 1(COI) sequences obtained from the same individual amoeba cells. Our goals were (1) to clarify the taxonomy and the phylogenetic relationships within this group, and (2) to evaluate if individual genotypes corresponded to specific morphotypes and the extent of phenotypic plasticity.
We show here that small variations in test morphology that have been often overlooked by traditional taxonomy correspond to distinct haplotypes. We therefore revise the taxonomy of the group. We redefine Nebela tincta (Leidy) Kosakyan et Lara and N. collaris (Ehrenberg 1848) Kosakyan et Gomaa, change N. tincta var. rotunda Penard to N. rotunda (Penard 1890), describe three new species: N. guttata n. sp. Kosakyan et Lara, N. pechorensis n. sp. Kosakyan et Mitchell, and N. aliciae n. sp. Mitchell et Lara.