Amphitremida (Poche, 1913) Is a New Major, Ubiquitous Labyrinthulomycete Clade

Micro-eukaryotic diversity is poorly documented at all taxonomic levels and the phylogenetic affiliation of many taxa – including many well-known and common organisms - remains unknown. Among these <i>incertae sedis</i> taxa are <i>Archerella flavum</i> (Loeblich and Tappan, 1961) and <i>Amphitrema wrightianum</i> (Archer, 1869) (Amphitremidae), two filose testate amoebae commonly found in <i>Sphagnum</i> peatlands. To clarify their phylogenetic position, we amplified and sequenced the SSU rRNA gene obtained from four independent DNA extractions of <i>A. flavum</i> and three independent DNA extractions of <i>A. wrightianum</i>. Our molecular data demonstrate that genera <i>Archerella</i> and <i>Amphitrema</i> form a fully supported deep-branching clade within the Labyrinthulomycetes (Stramenopiles), together with <i>Diplophrys</i> sp. (ATCC50360) and several environmental clones obtained from a wide range of environments. This newly described clade we named Amphitremida is diverse genetically, ecologically and physiologically. Our phylogenetic analysis suggests that osmotrophic species evolved most likely from phagotrophic ancestors and that the bothrosome, an organelle that produces cytoplasmic networks used for attachment to the substratum and to absorb nutrients from the environments, appeared lately in labyrithulomycete evolution.