2000 Annual Science Report
Marine Biological Laboratory Reporting | JUL 1999 – JUN 2000
Eukaryote Origins and the Evolution of Cellular Complexity - Evolution of Tubulins
Jakobids are free-living, heterotrophic flagellates that might represent early diverging, mitochondriate protists. They share ultrastructure similarities with eukaryotes that occupy basal positions in molecular phylogenies, and their mitochondrial genome architecture is eubacteria-like, suggesting a close affinity with the ancestral alpha-proteobacterial symbiont that gave rise to mitochondria and hydrogenosomes. To elucidate relationships amongst jakobids and other early-diverging eukaryotic lineages, we have characterized alpha- and beta-tubulin genes from four jakobids: Jakoba libera, Jakoba incarcerata, Reclinomonas americana (the “core jakobids”), and Malawimonas jakobiformis. These are the first reports of nuclear genes from these organisms. Phylogenies based on alpha-, beta-, and combined alpha- plus beta-tubulin protein datasets do not support the monophyly of the jakobids. While beta-tubulin and combined alpha- plus beta-tubulin phylogenies show a sister-group relationship between J. libera and R. americana, the two other jakobids, M. jakobiformis and J. incarcerata have unclear affinities. In all three analyses J. libera, R. americana and M. jakobiformis emerge from within a well-supported large “plant-protist” clade that includes plants, green algae, cryptophytes, stramenopiles, alveolates, Euglenozoa, Heterolobosea, several other protist groups, but not animals, fungi, microsporidia, parabasalids or diplomonads. A preferred branching order within the plant-protist clade is not identified, but there is a tendency for the J. libera-R. americana lineage to group with a clade made up of the heteroloboseid amoeboflagellates and euglenozoan protists. Jakoba incarcerata branches within the plant-protist clade in beta- and alpha- plus beta-tubulin phylogenies. In alpha-tubulin trees J. incarcerata occupies an unresolved position, weakly grouping with the animal/fungal/microsporidian group or with amitochondriate parabasalid and diplomonad lineages, depending on the phylogenetic method employed. Tubulin gene phylogenies are in general agreement with mitochondrial gene phylogenies and ultrastructural data in indicating that the “jakobids” may be polyphyletic. Relationships to the putatively deep-branching amitochondriate diplomonads remain uncertain.
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.