Patrick Keeling, Ph.D.
University of British Columbia, Department of Botany
Endosymbiosis and Plastid Evolution
Thursday - March 19, 2009
Pacific Forum – 3:00 p.m.
Plastids and mitochondria both are now well known to have originated though the endosymbiotic uptake of a cyanobacterium and an alpha-proteobacterium, respectively. In the case of plastids, however, this was only the beginning, since plastids have since been moved between eukaryotic lineages by further rounds of eukaryote-eukaryote endosymbiosis. The number of times this has happened and the effects of these events on the partners and their genomes are both controversial issues. Current evidence suggests that three major rounds of this secondary endosymbiosis took place, two involving green algae and one involving a red alga, the latter giving rise to a major fraction of marine primary producers, the chromalveolates (including diatoms, kelp, dinoflagellates, haptophytes, cryptomonads, apicomplexa, etc.). Within chromalveolates, the plastids of dinoflagellates and apicomplexa (e.g. the malaria parasite) have evolved in particularly odd ways. Genome reductions, new genetic codes, the loss of photosynthesis, unusual modes of gene expression, and fragmented genes are all commonplace in these lineages. We recently sequenced the plastid genome of a newly discovered coral symbiont, Chromera velia, and this photosynthetic relative of apicomplexan parasites has shed a great deal of light on the evolution of plastids in both apicomplexa and dinoflagellates. Lastly, dinoflagellates are unique in having undergone an additional level of fusion, called tertiary endosymbiosis. This has given rise to some of the most complex cells known, in one case still harboring at least five and perhaps six distinct genomes, raising questions of how functional and genomic redundancy is dealt with in the course of time.