title: Complete genome of a nonphotosynthetic cyanobacterium in a diatom reveals recent adaptations to an intracellular lifestyle.
abstract: The evolution of mitochondria and plastids from bacterial endosymbionts were key events in the origin and diversification of eukaryotic cells. Although the ancient nature of these organelles makes it difficult to understand the earliest events that led to their establishment, the study of eukaryotic cells with recently evolved obligate endosymbiotic bacteria has the potential to provide important insight into the transformation of endosymbionts into organelles. Diatoms belonging to the family Rhopalodiaceae and their endosymbionts of cyanobacterial origin (i.e., "spheroid bodies") are emerging as a useful model system in this regard. The spheroid bodies, which appear to enable rhopalodiacean diatoms to use gaseous nitrogen, became established after the divergence of extant diatom families. Here we report what is, to our knowledge, the first complete genome sequence of a spheroid body, that of the rhopalodiacean diatom Epithemia turgida. The E. turgida spheroid body (EtSB) genome was found to possess a gene set for nitrogen fixation, as anticipated, but is reduced in size and gene repertoire compared with the genomes of their closest known free-living relatives. The presence of numerous pseudogenes in the EtSB genome suggests that genome reduction is ongoing. Most strikingly, our genomic data convincingly show that the EtSB has lost photosynthetic ability and is metabolically dependent on its host cell, unprecedented characteristics among cyanobacteria, and cyanobacterial symbionts. The diatom-spheroid body endosymbiosis is thus a unique system for investigating the processes underlying the integration of a bacterial endosymbiont into eukaryotic cells.
authors: Center for Computational Sciences, University of Tsukuba, Tsukuba 305-8577, Japan;Department of Biochemistry and Molecular Biology, Canadian Institute for Advanced Research, Program in Integrated Microbial Biodiversity, Dalhousie University, Halifax, NS, Canada B3H 4R2;National Institute for Environmental Studies, Tsukuba 305-8506, Japan; ntakuro@ccs.tsukuba.ac.jp.
authors: Takuro
authors: T
authors: Nakayama
authors: Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba 305-8572, Japan;Graduate School of Global Environmental Studies, Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan;
authors: Ryoma
authors: R
authors: Kamikawa
authors: Department of Biochemistry and Molecular Biology, Canadian Institute for Advanced Research, Program in Integrated Microbial Biodiversity, Dalhousie University, Halifax, NS, Canada B3H 4R2;Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba 305-8572, Japan;
authors: Goro
authors: G
authors: Tanifuji
authors: Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan;Department of Environmental and Biological Chemistry, Fukui University of Technology, Fukui 910-8505, Japan; andPrecursory Research for Embryonic Science and Technology, Japan Science and Technology Agency, Chiyoda 153-8902, Japan.
authors: Yuichiro
authors: Y
authors: Kashiyama
authors: Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan;
authors: Naohiko
authors: N
authors: Ohkouchi
authors: Department of Biochemistry and Molecular Biology, Canadian Institute for Advanced Research, Program in Integrated Microbial Biodiversity, Dalhousie University, Halifax, NS, Canada B3H 4R2;
authors: John M
authors: JM
authors: Archibald
authors: Center for Computational Sciences, University of Tsukuba, Tsukuba 305-8577, Japan;Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba 305-8572, Japan;
authors: Yuji
authors: Y
authors: Inagaki
mesh_terms: D000222:Adaptation, Physiological; D002876:Chromosomes, Bacterial / Q000235:genetics; D000458:Cyanobacteria / Q000235:genetics*; D017377:Diatoms / Q000382:microbiology*; D004579:Electron Transport; D059646:Genome Size; D016680:Genome, Bacterial / Q000235:genetics*; D042541:Intracellular Space / Q000382:microbiology*; D008969:Molecular Sequence Data; D009586:Nitrogen Fixation / Q000235:genetics; D015388:Organelles / Q000378:metabolism; D010788:Photosynthesis / Q000235:genetics*; D013559:Symbiosis / Q000235:genetics; D020524:Thylakoids / Q000378:metabolism
keywords: organelle evolution; photosynthesis; pseudogenization