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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
    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
    Métadonnées seulement
    The testate lobose amoebae (order Arcellinida Kent, 1880) finally find their home within Amoebozoa
    (2005)
    Nikolaev, Sergey I
    ;
    Mitchell, Edward 
    ;
    Petrov, Nikolay B
    ;
    Berney, Cédric
    ;
    Fahrni, José
    ;
    Pawlowski, Jan
    Testate lobose amoebae (order Arcellinida Kent, 1880) are common in all aquatic and terrestrial habitats, yet they are one of the last higher taxa of unicellular eukaryotes that has not found its place in the tree of life. The morphological approach did not allow to ascertain the evolutionary origin of the group or to prove its monophyly. To solve these challenging problems, we analyzed partial small-subunit ribosomal RNA (SSU rRNA) genes of seven testate lobose amoebae from two out of the three suborders and seven out of the 13 families belonging to the Arcellinida. Our data support the monophyly of the order and clearly establish its position among Amoebozoa, as a sister-group to the clade comprising families Amoebidae and Hartmannellidae. Complete SSU rRNA gene sequences from two species and a partial actin sequence from one species confirm this position. Our phylogenetic analyses including representatives of all sequenced lineages of lobose amoebae suggest that a rigid test appeared only once during the evolution of the Amoebozoa, and allow reinterpretation of some morphological characters used in the systematics of Arcellinida. (c) 2005 Published by Elsevier GmbH.
  • Publication
    Accès libre
    Changes in testate amoebae (Protists) communities in a small raised bog. A 40-year study
    (2005)
    Kishaba, Keiko
    ;
    Mitchell, Edward 
    We analyzed the testate amoebae communities from two sets of moss samples taken forty years apart ( 1961 and 2001) in the same locations of a peat bog of the Swiss Jura Mountains. Peat Cutting and lateral drainage of Le Cachot bog have caused a clear increase in tree cover, especially near the edges. Changes affecting herbaceous plants, mosses. or soil organisms can be subtle, and may easily be overlooked. We hypothesized that we would see changes in the dominant Sphagnum species and the structure of testate amoebae communities living in the mosses. More specifically, we hypothesized that the frequency of bryophyte and testate amoebae species indicative for dry conditions Would increase and that the frequency of species indicative for wet conditions would decrease. The mean testate amoebae species richness per sample decreased from 11.9 to 9.6, but the overall species richness was identical (33 species) in both years. Three species increased significantly in mean relative abundance: Nebela tincta s.l. (+97 %), Bullinularia indica (+ 810 %), and Cyclopyxis eurystoma (+ 100 %; absent in 1961), while two species decreased significantly: Assulina muscorum (- 63 %) and Euglypha compressa (-93 %). The testate amoebae communities clearly differed among microhabitat types (hummocks, lawns, hollows), but no overall significant change in the community was detectable between the two sampling dates (Mantel test). These results could signify that changes at the microscopic level had already taken place by 1961 and no further overall significant changes in micro-enviromental conditions took place during the 40-year period of this Study. This Would agree with the faster response time usually attributed to microorganisms and would also imply that the bushes and trees may be poor predictors of the response of microorganisms if they are themselves in a non-equilibrium stage. Other possible causes for the lack of overall differences are discussed.
  • Publication
    Métadonnées seulement
    Vertical micro-distribution and response to nitrogen deposition of testate amoebae in Sphagnum
    (2004)
    Mitchell, Edward 
    ;
    Gilbert, Daniel
    Previous studies have shown the existence of a vertical micro-distribution of testate amoebae in the first centimeters of Sphagnum and their response to nutrient enrichment. In order to test the response of testate amoebae to depth and N addition in dry moss carpets recolonizing cutover peatlands, we sampled Sphagnum that had received 0, 1, 3, or log N M-2 yr(-1) for three years. The mosses were cut into three segments: 0-1 cm, 1-3 cm and 3-5 cm and analyzed for testate amoebae. The overall diversity (22 taxa) was high considering the dryness of the site, but the species richness of individual samples was low (mean 6.6). The presence of several species characteristic of wetter conditions suggests that they have a broader tolerance than usually believed and/or have a high colonization potential. Species richness increased with depth. Assulina muscorum was most abundant in the top segment, while Phryganella acropodia, Heleopera rosea and Nebela militaris were most abundant in the deepest segment. Neither the metabolism type nor the shell characteristics significantly explained the vertical distribution of species. There was no overall response of testate amoebae to N, although one species, Bullinularia indica, was significantly more abundant in the fertilized than in the control plots.
  • Publication
    Accès libre
    Horizontal distribution patterns of testate amoebae (Protozoa) in a Sphagnum magellanicum carpet
    (2000-5-2)
    Mitchell, Edward 
    ;
    Borcard, D.
    ;
    Buttler, Alexandre
    ;
    Grosvernier, Philippe
    ;
    Gilbert, Daniel
    ;
    Gobat, Jean-Michel 
    The distribution of soil microorganisms is generally believed to be patchy and to reflect habitat heterogeneity. Despite this general rule, the amount of existing data oil species distribution patterns is scarce. Testate amoebae (Protozoa; Rhizopoda) are an important component of soil microbial communities and are increasingly used in ecological and paleoecological studies of Sphagnum-dominated peatlands, but data on the spatial structure of communities are completely lacking. This is an important aspect since quantitative models used for paleoecological reconstruction and monitoring are based on species assemblages. We explored the distribution patterns of testate amoebae distribution in a macroscopically homogeneous Sphagnum carpet, down to a scale of several centimeters. Distributions maps of the species and spatially constrained sample groups were produced. Multivariate and individual spatial autocorrelations were calculated. The importance of spatial structure was quantified by canonical correspondence analysis. Our ultimate goal is to find the finest resolution of environmental monitoring using testate amoebae. The distribution patterns differed among species, resulting in a complex spatial structure of the species assemblage in a whole. Spatial structure accounted for 36% of the total variation of species abundance in a canonical correspondence analysis constrained by spatial variables. This structure was partly correlated to altitude (microtopography) at a very fine scale. These results confirmed the existence of significant broad-and fine-scale spatial structures within restate amoebae communities that could in parr be interpreted as effects of ecological gradients. This shows that, on a surface area of 0.25 m(2), ecological conditions which look uniform from a macroscopic point of view are not perceived as such by Sphagnum-inhabiting organisms. Therefore, restate amoebae could prove very useful to monitor fine-scale ecological processes or disturbances. Studies of the species' spatial distribution patterns in combination with autoecological studies are needed and should be included in the toolbox of biomonitoring itself.