06/17/05

Unresolved Mysteries About Parasites and our Oldest Ancestors
Mitchell L. Sogin, Marine Biological Laboratory
Introduction by Matthew Meselson, Harvard University

Lecture Abstract:

For nearly three billion years, the microbial world accounted for the only form of life on Earth. Today microbes of untold diversity are the principal agents for catalyzing bigeochemical transformations of our biosphere. Yet some are significant pathogens that affect human populations and economies on a global scale. How did these modest size life forms become so versatile and what can their genomes tell us about the origins of complex cellular architectures and multicellular organisms? The genes for ribosomal RNAs (core components of the machinery that assembles proteins) are present in all forms of life. Our studies of these genes have provided new insights on how medically important microbial protists have evolved. Giardia lamblia is of particular interest because this human parasite lacks many of the organelles (mitochondria, peroxisomes and stacked dictyosomes) normally present in eukaryotic cells. This suggests that cytoskeletal architecture and nuclear envelopes appeared before the origins of mitochondria. Today we must reconsider that interpretation in light of the discovery of a few mitochondrial-like genes in basal eukaryotic lineages. Were mitochondria present in the first eukaryotes or could they have arisen from many independent endosymbioses with alpha proteobacteria? To address this issue, we searched for mitochondrial targeted proteins in the G. lamblia genome. The absence of credible phylogenetic evidence of vestige genes from mitochondria or their endosymbiotic ancestors persuades us that Giardia is still our best candidate for exploring the origins of the eukaryotic cell.

Mitchell L. Sogin received his B.S. in Chemistry and Microbiology from the University of Illinois in 1967 and a Ph.D. in Microbiology and Molecular Biology in 1972. He was a Postdoctoral Fellow at the National Jewish Center in Denver, Colorado, where he subsequently joined the faculty as a Senior Staff Scientist. Dr. Sogin was also an Associate Professor in the Microbiology Department of the University of Colorado Health Sciences Center and a Miller Professor at the University of California at Berkeley. Dr. Sogin moved to the MBL in 1989 after establishing the summer Workshop in Molecular Evolution. He founded the Josephine Bay Paul Center for Comparative Molecular Biology and Evolution in 1996. The Center’s interlocking programs in Global Infectious Diseases, Molecular Evolution and Molecular Microbial Diversity seeks to understand processes that reshape genome architecture, identify and one day predict the origins and dispersal mechanisms of pathogenecity, and to develop systems level approaches to environmental microbiology. Over its short history, The Bay Paul Center has become a focal point for collaborative research between molecular biologists, biochemists, parasitologists, ecologists, and other colleagues from the MBL’s summer and resident communities, and from around the world.

Dr. Sogin’s research has changed our perceptions about the age and relationships amongst the major groups of eukaryotes. Using molecular techniques, he has shown that protist diversity dwarfs that of the plant, animal and fungal worlds. The early divergence of eukaryotes without mitochondria (diplomonads, trichomonads) is followed by a series of independent protist branchings. Late in evolution, five complex assemblages (plants, animals, fungi and two newly recognized "kingdoms") suddenly diverge, defining the "crown" of the eukaryotic subtree. This phylogenetic framework allows the positioning of many taxa of uncertain affiliations, some of which are important human pathogens. It argues for multiple endosymbiotic events leading to plastids. It argues that eukaryotes may be older than once thought possible. His current research explores the genomic structure of human parasites and the diversity of microbes in marine environments.
 
Dr. Sogin is a fellow of the American Association for the Advancement of Science, the American Academy of Arts and Sciences, and the American Academy of Microbiology. He has served on the National Research Council’s Space Studies Board and is a member of the American Society of Microbiology, the Society of Protozoologists, the Society for Molecular Biology and Evolution, and the American Society for Cell Biology. He serves on the editorial boards of Environmental Microbiology and Protist. The NIH, NSF and NASA support Dr. Sogin’s research on protist diversity and microbial evolution. Most recently he has initiated a global-wide effort to define microbial diversity in the world’s oceans funded through the Alfred P. Sloan Foundation. He is the author of over 180 publications.

Matthew Meselson is Thomas Dudley Cabot Professor of the Natural Sciences at Harvard University and an MBL Adjunct Scientist. Dr. Meselson received a Bachelor of Philosophy from the University of Chicago and a Ph.D. in chemistry, under Linus Pauling, from the California Institute of Technology (Caltech). Following a research fellowship at Caltech, he was an Assistant Professor of Chemistry there and then a Senior Research Fellow in Chemical Biology. In 1960 he was appointed an Associate Professor of Molecular Biology at Harvard University. Dr. Meselson is well-known for the Meselson-Stahl experiment which proved James Watson and Francis Crick’s model of semi-conservative DNA replication. Dr. Meselson first came to the MBL to work with Dr. Watson as a research assistant during the summer of 1954. In 2001 he established a satellite laboratory in the MBL’s Josephine Bay Paul Center for Comparative Molecular Biology and Evolution, where, together with his colleagues David and Jessica Mark Welch, his research focuses on the evolutionary problem of what causes the extinction of populations that abandon sexual reproduction. In addition, Dr. Meselson has dedicated much of his career to the elimination of chemical and biological weapons and has served as an advisor on these subjects to various U.S. government agencies. He currently co-directs the Harvard Sussex Program, a collaborative effort between Harvard University and the University of Sussex in the UK, which promotes communication and training in support of informed public policy regarding chemical and biological weapons.