Chemosynthetic Symbiont with a Drastically Reduced Genome Serves As Primary Energy Storage in the Marine Flatworm Paracatenula
Chemosynthetic symbiont with a drastically reduced genome serves as primary energy storage in the marine flatworm Paracatenula Oliver Jäcklea, Brandon K. B. Seaha, Målin Tietjena, Nikolaus Leischa, Manuel Liebekea, Manuel Kleinerb,c, Jasmine S. Berga,d, and Harald R. Gruber-Vodickaa,1 aMax Planck Institute for Marine Microbiology, 28359 Bremen, Germany; bDepartment of Geoscience, University of Calgary, AB T2N 1N4, Canada; cDepartment of Plant & Microbial Biology, North Carolina State University, Raleigh, NC 27695; and dInstitut de Minéralogie, Physique des Matériaux et Cosmochimie, Université Pierre et Marie Curie, 75252 Paris Cedex 05, France Edited by Margaret J. McFall-Ngai, University of Hawaii at Manoa, Honolulu, HI, and approved March 1, 2019 (received for review November 7, 2018) Hosts of chemoautotrophic bacteria typically have much higher thrive in both free-living environmental and symbiotic states, it is biomass than their symbionts and consume symbiont cells for difficult to attribute their genomic features to either functions nutrition. In contrast to this, chemoautotrophic Candidatus Riegeria they provide to their host, or traits that are necessary for envi- symbionts in mouthless Paracatenula flatworms comprise up to ronmental survival or to both. half of the biomass of the consortium. Each species of Paracate- The smallest genomes of chemoautotrophic symbionts have nula harbors a specific Ca. Riegeria, and the endosymbionts have been observed for the gammaproteobacterial symbionts of ves- been vertically transmitted for at least 500 million years. Such icomyid clams that are directly transmitted between host genera- prolonged strict vertical transmission leads to streamlining of sym- tions (13, 14). Such strict vertical transmission leads to substantial biont genomes, and the retained physiological capacities reveal and ongoing genome reduction.
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