2000 Annual Science Report
Marine Biological Laboratory Reporting | JUL 1999 – JUN 2000
Eukaryote Origins and the Evolution of Cellular Complexity - Eukaryotic rRNA Evolution
Early Diverging Eukaryotes (Edgcomb, Sogin, Silberman, Simpson, Patterson)
Pelobionts 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. Pelobionts are flagellated protists that inhabit micro-oxic and anoxic environments. Because they lack stacked dictyosomes, mitochondria and outer dynein arms in their flagellar apparatus, they might represent early diverging eukaryotic lineages. Molecular analyses of DNA-dependent RNA polymerase II (RPBI) suggest the pelobiont Mastigamoeba invertens diverged early whereas analyses of SSU ribosomal RNAs 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), 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 the pelobionts to be a monophyletic group, with the exception of M. invertens, calling into question the taxonomic assignment of this organism and the phylogenetic methodologies that placed it as a deep-diverging eukaryotic lineage. Pelobionts appear to have a sister-group relationship to the entamoebae, to be outside the crown, but they do not represent one of the earliest branching lineages as suggested by RPBI and ultrastructure.
Novel Eukaryotes at Hydrothermal Vents (Atkins, McArthur, Teske)
A cultivation survey of heterotrophic flagellates at Pacific hydrothermal vent sites yielded the flagellate Ancyromonas sigmoides at the 9°N East Pacific Rise vents. Molecular and morphological evidence point to Ancyromonas as a plausible candidate for the closest relative to the common ancestor of Metazoans, Fungi, and Choanoflagellates (the Opisthokonta), in other words, the origin of multicellularity among animals and fungi (Atkins et al., 2000). Using 18S rDNA sequences from most of the major eukaryotic lineages, maximum likelihood, minimum evolution and maximum parsimony analyses yielded congruent phylogenies supporting this hypothesis. Combined with ultrastructural similarities between Ancyromonas and opisthokonts, the evidence presented here suggests that Ancyromonas may form an independent lineage, the Ancyromonadida, closer in its relationship to the opisthokonts than is its nearest protist relatives, the Apusomonadida.
In addition to these studies, we have nearly completed the analysis of numerous other protist lineages from anoxic environments. Some of these, e.g., retortamonads, Carpediemonas, etc., are basal to other eukaryotes in molecular trees.
PROJECT MEMBERS:Virginia Edgcomb
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.
Expand and interpret the genomic database of a select group of key microorganisms in order to reveal the history and dynamics of evolution.
Define how ecophysiological processes structure microbial communities, influence their adaptation and evolution, and affect their detection on other planets.
Identify the environmental limits for life by examining biological adaptations to extremes in environmental conditions.