MITOCHONDRIAL DNA IN THE OOGAMOCHLAMYS CLADE (CHLOROPHYCEAE): HIGH GC CONTENT AND UNIQUE GENOME ARCHITECTURE FOR GREEN ALGAE(1).

Most mitochondrial genomes in the green algal phylum Chlorophyta are AT-rich, circular-mapping DNA molecules. However, mitochondrial genomes from the Reinhardtii clade of the Chlorophyceae lineage are linear and sometimes fragmented into subgenomic forms. Moreover, Polytomella capuana, from the Reinhardtii clade, has an elevated GC content (57.2%). In the present study, we examined mitochondrial genome conformation and GC bias in the Oogamochlamys clade of the Chlorophyceae, which phylogenetic data suggest is closely related to the Reinhardtii clade. Total DNA from selected Oogamochlamys taxa, including four Lobochlamys culleus (H. Ettl) Pröschold, B. Marin, U. G. Schlöss. et Melkonian strains, Lobochlamys segnis (H. Ettl) Pröschold, B. Marin, U. G. Schlöss. et Melkonian, and Oogamochlamys gigantea (O. Dill) Pröschold, B. Marin, U. G. Schlöss. et Melkonian, was subjected to Southern blot analyses with cob and cox1 probes, and the results suggest that the mitochondrial genome of these taxa is represented by multiple-sized linear DNA fragments with overlapping homologies. On the basis of these data, we propose that linear mitochondrial DNA with a propensity to become fragmented arose in an ancestor common to the Reinhardtii and Oogamochlamys clades or even earlier in the evolutionary history of the Chlorophyceae. Analyses of partial cob and cox1 sequences from these Oogamochlamys taxa revealed an unusually high GC content (49.9%-65.1%) and provided evidence for the accumulation of cob and cox1 pseudogenes and truncated sequences in the mitochondrial genome of all L. culleus strains examined.