No abstract

Dynamins are a eukaryote-specific family of GTPases. Some family members are involved in diverse and varied cellular activities. Here, we report that the primitive red alga Cyanidioschyzon merolae retains only one dynamin homolog, CmDnm1, belonging to the mitochondrial division subfamily. Previously, the bacterial cell division protein, FtsZ, was shown to localize at the mitochondrial division site in the alga. We showed that FtsZ and dynamin coexist as mitochondrial division-associated proteins that act during different phases of division. CmDnm1 was recruited from 10 -20 cytoplasmic patches (dynamin patches) to the midpoint of the constricted mitochondrion-dividing ring (MD ring), which was observed as an electron-dense structure on the cytoplasmic side. CmDnm1 is probably not required for early constriction; it forms a ring or spiral when the outer mitochondrial membrane is finally severed, whereas the FtsZ and MD rings are formed before constriction. It is thought that the FtsZ, MD, and dynamin rings are involved in scaffolding, constriction, and final separation, respectively. In eukaryotes, mitochondrial severance is probably the most conserved role for the dynamin family.

Red algae are one of the main photosynthetic eukaryotic lineages and are characterized by primitive features, such as a lack of flagella and the presence of phycobiliproteins in the chloroplast. Recent molecular phylogenetic studies using nuclear gene sequences suggest two conflicting hypotheses (monophyly versus non-monophyly) regarding the relationships between red algae and green plants. Although kingdom-level phylogenetic analyses using multiple nuclear genes from a wide-range of eukaryotic lineages were very recently carried out, they used highly divergent gene sequences of the cryptomonad nucleomorph (as the red algal taxon) or incomplete red algal gene sequences. In addition, previous eukaryotic phylogenies based on nuclear genes generally included very distant archaebacterial sequences (designated as the outgroup) and/or amitochondrial organisms, which may carry unusual gene substitutions due to parasitism or the absence of mitochondria. Here, we carried out phylogenetic analyses of various lineages of mitochondria-containing eukaryotic organisms using nuclear multigene sequences, including the complete sequences from the primitive red alga Cyanidioschyzon merolae. Amino acid sequence data for two concatenated paralogous genes (alpha- and beta-tubulin) from mitochondria-containing organisms robustly resolved the basal position of the cellular slime molds, which were designated as the outgroup in our phylogenetic analyses. Phylogenetic analyses of 53 operational taxonomic units (OTUs) based on a 1525-amino-acid sequence of four concatenated nuclear genes (actin, elongation factor-1alpha, alpha-tubulin, and beta-tubulin) reliably resolved the phylogeny only in the maximum parsimonious (MP) analysis, which indicated the presence of two large robust monophyletic groups (Groups A and B) and the basal eukaryotic lineages (red algae, true slime molds, and amoebae). Group A corresponded to the Opisthokonta (Metazoa and Fungi), whereas Group B included various primary and secondary plastid-containing lineages (green plants, glaucophytes, euglenoids, heterokonts, and apicomplexans), Ciliophora, Kinetoplastida, and Heterolobosea. The red algae represented the sister lineage to Group B. Using 34 OTUs for which essentially the entire amino acid sequences of the four genes are known, MP, distance, quartet puzzling, and two types of maximum likelihood (ML) calculations all robustly resolved the monophyly of Group B, as well as the basal position of red algae within eukaryotic organisms. In addition, phylogenetic analyses of a concatenated 4639-amino-acid sequence for 12 nuclear genes (excluding the EF-2 gene) of 12 mitochondria-containing OTUs (including C. merolae) resolved a robust non-sister relationship between green plants and red algae within a robust monophyletic group composed of red algae and the eukaryotic organisms belonging to Group B. A new scenario for the origin and evolution of plastids is suggested, based on the basal phylogenetic position of the red algae within the large clade (Group B plus red algae)...