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Mitchell, Edward

Nom
Mitchell, Edward
Affiliation principale
Site web
Fonction
Professeur ordinaire
Email
edward.mitchell@unine.ch
Identifiants
https://libra.unine.ch/handle/123456789/357
_
0000-0003-0358-506X

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  • Publication
    Métadonnées seulement
    The curse of taxonomic uncertainty in biogeographical studies of free-living terrestrial protists: a case study of testate amoebae from Amsterdam Island
    (2009)
    Heger, Thierry J
    ;
    Mitchell, Edward 
    ;
    Ledeganck, Pieter
    ;
    Vincke, Sofie
    ;
    Van de Vijver, Bart
    ;
    Beyens, Louis
    Aim A current debate in microbial biogeography contrasts two views concerning the distribution of free-living microorganisms. The first view assumes a ubiquitous distribution, while the second assumes that at least some species have limited geographical distributions. We tested for limited geographical distributions by identifying testate amoebae morphospecies from an extremely remote oceanic island where the potential for endemism is high. Location Amsterdam Island, Indian Ocean. Methods Sixty moss and water samples collected from the top of the volcano to the lowland were investigated for their testate amoeba content. Due to taxonomic uncertainties among the Argynnia (Nebela) dentistoma species complex (including A. antarctica), we also performed light and scanning electron microscopy investigations on the shell ultrastructure and biometric analyses on several specimens of this taxon. Results We identified a total of 43 testate amoeba taxa belonging to 15 genera. Only four testate amoeba taxa had previously been recorded on this island. Testate amoeba communities of Amsterdam Island are dominated by cosmopolitan ubiquitous euglyphid taxa such as Trinema lineare, Assulina muscorum and Corythion dubium. The length and width ranges for Argynnia dentistoma on Amsterdam Island overlap with other records of this species and of A. antarctica, suggesting that A. antarctica is not a distinct taxon. Main conclusions Although Amsterdam Island is among the most remote islands in the world, an extensive inventory of testate amoeba morphospecies provided no clear evidence for endemism. On the one hand, our detailed morphometric analysis of the A. dentistoma complex revealed that A. antarctica, a morphospecies previously suggested to display endemism, cannot be confidently distinguished from the cosmopolitan morphospecies A. dentistoma. On the other hand, five morphotaxa could not be identified with certainty and might represent new species, potentially with limited distribution. These examples illustrate how taxonomic uncertainties undermine biogeographical studies of testate amoebae. In order to allow better interpretation of morphology-based testate amoeba distribution data, an assessment of genetic diversity among and within morphotaxa in relation to geographical distance for some common testate amoebae should be given high priority.
  • Publication
    Métadonnées seulement
    Ribosomal RNA genes challenge the monophyly of the hyalospheniidae (Amoebozoa : Arcellinida)
    (2008)
    Lara, Enrique 
    ;
    Heger, Thierry J
    ;
    Ekelund, Flemming
    ;
    Lamentowicz, Mariusz
    ;
    Mitchell, Edward 
    To date only five partial and two complete SSU rRNA gene sequences are available for the lobose testate amoebae (Arcellinida). Consequently, the phylogenetic relationships among taxa and the definition of species are still largely dependant on morphological characters of uncertain value, which causes confusion in the phylogeny, taxonomy and the debate on cosmopolitanism of free-living protists. Here we present a SSU rRNA-based phylogeny of the Hyalospheniidae including the most common species. Similar to the filose testate amoebae of the order Euglyphida the most basal clades have a terminal aperture; the ventral position of the pseudostome appears to be a derived character. Family Hyalospheniidae appears paraphyletic and is separated into three clades: (1) Heleopera sphagni, (2) Heleopera rosea and Argynnia dentistoma and (3) the rest of the species from genera Apodera, Hyalosphenia, Porosia and Nebela. Our data support the validity of morphological characters used to define species among the Hyalospheniidae and even suggest that taxa described as varieties may deserve the rank of species (e.g. N. penardiana var. minor). Finally our results suggest that the genera Hyalosphenia and Nebela are paraphyletic, and that Porosia bigibbosa branches inside the main Nebela clade. (c) 2007 Elsevier GmbH. All rights reserved.
  • Publication
    Accès libre
    SSU rRNA reveals a sequential increase in shell complexity among the euglyphid testate amoebae (Rhizaria : Euglyphida)
    (2007)
    Lara, Enrique 
    ;
    Heger, Thierry J
    ;
    Mitchell, Edward 
    ;
    Meisterfeld, Ralf
    ;
    Ekelund, Flemming
    The existing data on the molecular phylogeny of filose testate amoebae from order Euglyphida has revealed contradictions between traditional morphological classification and SSU rRNA phylogeny and, moreover, the position of several important genera remained unknown. We therefore carried out a study aiming to fill several important gaps and better understand the relationships among the main euglyphid testate amoebae and the evolutionary steps that led to the present diversity at a higher level. We obtained new SSU rRNA sequences from five genera and seven species. This new phylogeny obtained shows that (1) the clade formed by species of genera Assulina and Placocista branches unambiguously at the base of the subclade of Euglyphida comprising all members of the family Trinematidae and genus Euglypha, (2) family Trinematidae (Trachelocorythion, Trinema, and Corythion) branches as a sister group to genus Euglypha, (3) three newly sequenced Euglypha species (E. cf. ciliata, E. penardi, and E. compressa) form a new clade within the genus. Since our results show that Assulina and Placocista do not belong to the Euglyphidae (unless the Trinematidae are also included in this family), we propose the creation of a new family named Assulinidae. Consequently, we give a family status to the genera Euglypha and (tentatively) Scutiglypha, which become the new family Euglyphidae. The evolutionary pattern suggested by SSU rRNA phylogeny shows a clear tendency towards increasing morphological complexity of the shell characterised by changes in the symmetry (migration of the aperture to a ventral position and/or compression of the shell) and the appearance of specialised scales at the aperture (in families Trinematidae and Euglyphidae). (C) 2007 Elsevier GmbH. All rights reserved.
  • Publication
    Accès libre
    Testate Amoebae Like It Hot: Species Richness Decreases Along a Subalpine-Alpine Altitudinal Gradient in Both Natural Calluna vulgaris Litter and Transplanted Minuartia sedoides Cushions
    Heger, Thierry J
    ;
    Derungs, Nicolas 
    ;
    Theurillat, Jean-Paul
    ;
    Mitchell, Edward 
    Most groups of higher organisms show a decrease in species richness toward high altitude, but the existence of such a pattern is debated for micro-eukaryotes. Existing data are scarce and mostly confounded with the diversity of habitats that also decreases with elevation. In order to disentangle these two factors, one approach is to consider only similar types of habitats occurring across an elevational gradient. We assessed the diversity and community structure of testate amoebae in two specific habitats: (1) natural Calluna vulgaris litter and (2) Minuartia sedoides cushions 7 years after their transplantation along a vertical transect from 1770 to 2430 m in the subalpine and alpine zones of the Swiss Alps. Analyses of co-variance and variance showed that testate amoeba species richness, equitability, and diversity declined with elevation and were significantly correlated to habitat type. In a redundancy analysis, the variation in the relative abundance of the testate amoeba taxa in Calluna vulgaris litter was equally explained by elevation and litter pH. This is the first study documenting a monotonic decrease of protist diversity in similar habitats across an elevational gradient.
  • Publication
    Accès libre
    One Alga to Rule them All: Unrelated Mixotrophic Testate Amoebae (Amoebozoa, Rhizaria and Stramenopiles) Share the Same Symbiont (Trebouxiophyceae)
    Gomaa, Fatma
    ;
    Kosakyan, Anush 
    ;
    Heger, Thierry J
    ;
    Corsaro, Daniele
    ;
    Mitchell, Edward 
    ;
    Lara, Enrique 
    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.