2001 Annual Science Report

Marine Biological Laboratory Reporting  |  JUL 2000 – JUN 2001

Eukaryotic Origins and the Evolution of Cellular Complexity -- Eukaryotic rRNA Evolution: Early Diverging Eukaryotes

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Eukaryotic rRNA Evolution: Early Diverging Eukaryotes (dm)

Pelobionts are flagellated protists that inhabit micro-oxic and anoxic environments. They represent one group of eukaryotes that has been identified as a potentially deep-branching eukaryotic lineage on the basis of ultrastructure and certain molecular phylogenies. Because they lack stacked dictyosomes, mitochondria, and outer dynein arms in their flagellar apparatus, pelobionts might represent early diverging eukaryotic lineages.

Molecular analyses of DNA-dependent RNA polymerase II suggest the pelobiont Mastigamoeba invertens diverged early. Analyses of SSU (small subunit) ribosomal RNAs, in contrast, position pelobionts near the eukaryotic “crown groups”, but not as a monophyletic group. We sequenced SSU rRNA genes from four additional pelobiont taxa (including two new isolates). This work confirmed the sequence for M. invertens by in situ hybridization, compared the ultrastructural features of these taxa, and subjected these new pelobiont sequences to a rigorous phylogenetic analysis in the context of a larger alignment of eukaryotes, including known sequences from other pelobionts.

Results show that despite the absence of typical eukaryotic organelles, e.g., mitochondria, dictyosomes, chloroplasts, coated vesicles, and peroxisomes, pelobionts are not basal to other eukaryotes in our molecular analyses nor are they specifically related to a previously described pelobiont-like organism, Mastigamoeba invertens. This result calls into question the taxonomic assignment of this organism and the phylogenetic methodologies that placed it as a deep-diverging eukaryotic lineage. Independent of the evolutionary models tested, sequence alignments employed, and taxon sampling, the pelobionts (exclusive of M. invertens) form a monophyletic group together with Endolimax nana in maximum likelihood, distance and parsimony inferences. Many analyses suggest the pelobionts share common ancestry with the Entamoebidae, with E. nana sharing an unique common evolutionary history with Mastigamoeba simplex. Pelobionts do not represent one of the earliest branching lineages as suggested by RNA polymerase II and ultrastructure.

  • PROJECT INVESTIGATORS:
  • PROJECT MEMBERS:
    Mitchell Sogin
    Project Investigator

    Virginia Edgcomb
    Co-Investigator

    Thomas Nerad
    Collaborator

    David Patterson
    Collaborator

    Linda Amaral Zettler
    Postdoc

    Jeffrey Silberman
    Postdoc

    Alastair Simpson
    Doctoral Student

  • RELATED OBJECTIVES:
    Objective 2.0
    Develop and test plausible pathways by which ancient counterparts of membrane systems, proteins and nucleic acids were synthesized from simpler precursors and assembled into protocells.

    Objective 4.0
    Expand and interpret the genomic database of a select group of key microorganisms in order to reveal the history and dynamics of evolution.

    Objective 7.0
    Identify the environmental limits for life by examining biological adaptations to extremes in environmental conditions.