Genome Canada 1996, Grantee Meeting of the Canadian Genome Analysis
and Technology Program.
Ottawa June 14-16, 1996. Invited presentation.
MITOCHONDRIAL GENOMES AS A UNIQUE WINDOW TO RESOLVE THE PHYLOGENY
OF THE EUKARYOTES.
M.W. Gray*, G. Burger, R. Cedergren, B. Golding, C. Lemieux, T.G.
Littlejohn, D. Sankoff, M. Turmel and B.F. Lang*.
(Presented by *)
Organelle Genome Megasequencing Program, Canadian Institute for
Advanced Research, Departement de Biochimie, Universite de
Montreal, Montreal, Quebec H3C 3J7, Canada.
Mitochondrial genomes are rather conservative in the genetic
functions they encode, but they differ radically in genome size,
structure, patterns of genome organization and expression, and
mode and tempo of evolution. Because of their relatively small
sizes, mitochondrial genomes provide excellent material for a
comparative genomics approach to genome evolution. Comprehensive
sequencing of mitochondrial DNAs (mtDNAs) from diverse organisms
is expected to provide a wealth of molecular biological data with
which to make inferences about the pathways and mechanisms of
mitochondrial genome change and diversification.
Toward this end, the Organelle Genome Megasequencing Program
(OGMP) is systematically investigating the mitochondrial genomes
of unicellular eukaryotes (protists). From a phylogenetic
perspective, and compared to fungi, animals and plants, protists
are by far the most diverse eukaryotic group, and the least well
characterized at the level of the mitochondrial genome. To date,
we have completely sequenced the mitochondrial genomes of
Prototheca wickerhamii (green alga), Porphyra purpurea (red
alga), Rhodomonas salina (cryptomonad alga), Ochromonas danica
(chrysophyte alga), Cafeteria roenbergensis (bicosoecid
zooflagellate), Acanthamoeba castellanii (rhizopod amoeba) and
Tetrahymena pyriformis (ciliate protozoon). The sequencing of
other mitochondrial genomes is in progress, including those of
Pedinomonas minor (green alga); Reclinomonas americana,
Malawimonas jakobiformis and Jakoba libera (jakobid flagellates);
Chrysodidymus synuroideus (golden alga); and Thraustochytrium
aureum (stramenopile). The data set from this broad selection of
taxa has allowed us to undertake a detailed comparison of the
gene contents and patterns of mitochondrial genome organization,
expression and evolution in these protists. We will present
several examples of unusual genes, gene arrangements, and modes
of gene expression that exemplify the kinds of molecular data
that have emerged from the systematic approach taken by OGMP, and
the utility of these data for making evolutionary inferences. In
addition, phylogenetic reconstructions based on multiple
mitochondrial protein sequences will be emphasized as a powerful
approach to resolving the branching order of eukaryotic
assemblages whose evolutionary relationships have so far remained
unresolved by other approaches. For example, our results
support, with high bootstrap probability, the idea that red and
green algae shared a common ancestor that emerged later in
eukaryote evolution than cryptophyte algae. Finally, the
molecular data gathered by OGMP, by highlighting ancestral
characteristics of present-day mitochondrial genomes, provide
increased support for a monophyletic origin of mitochondria, and
give us a much better understanding than was previously possible
of what the proto-mitochondrial genome was probably like, and
what genes it contained. Supported by MRC Canada (SP-34) and
CGAT (GO-12323).
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