Abstract
Cyanobacteria, the major photosynthetic prokaryotic lineage, are also known as a major nitrogen fixer in nature. N<sub>2</sub>-fixing cyanobacteria are frequently found in symbioses with various types of eukaryotes and supply fixed nitrogen compounds to their eukaryotic hosts, which congenitally lack N<sub>2</sub>-fixing abilities. Diatom species belonging to the family Rhopalodiaceae also possess cyanobacterial symbionts called spheroid bodies. Unlike other cyanobacterial N<sub>2</sub>-fixing symbionts, the spheroid bodies reside in the cytoplasm of the diatoms and are inseparable from their hosts. Recently, the first spheroid body genome from a rhopalodiacean diatom has been completely sequenced. Overall features of the genome sequence showed significant reductive genome evolution resulting in a diminution of metabolic capacity. Notably, despite its cyanobacterial origin, the spheroid body was shown to be truly incapable of photosynthesis implying that the symbiont energetically depends on the host diatom. The comparative genome analysis between the spheroid body and another N<sub>2</sub>-fixing symbiotic cyanobacterial group corresponding to the UCYN-A phylotypes – both were derived from cyanobacteria closely related to genus <em>Cyanothece</em> – revealed that the two symbionts are on similar, but explicitly distinct tracks of reductive evolution. Intimate symbiotic relationships linked by nitrogen fixation as seen in rhopalodiacean diatoms may help us better understand the evolution and mechanisms of bacterium-eukaryote endosymbioses.
Highlights
Nitrogen is one of the most important and fundamental elements for all living cells
N2-fixing cyanobacteria are frequently found in symbioses with various types of eukaryotes and supply fixed nitrogen compounds to their eukaryotic hosts, which congenitally lack N2-fixing abilities
The comparative genome analysis between the spheroid body and another N2-fixing symbiotic cyanobacterial group corresponding to the UCYN-A phylotypes – both were derived from cyanobacteria closely related to genus Cyanothece – revealed that the two symbionts are on similar, but explicitly distinct tracks of reductive evolution
Summary
Nitrogen is one of the most important and fundamental elements for all living cells. only prokaryotic species are able to fix and utilize the dinitrogen that abundantly exists in the atmosphere [1]. The first spheroid body genome from a rhopalodiacean diatom has been completely sequenced. The comparative genome analysis between the spheroid body and another N2-fixing symbiotic cyanobacterial group corresponding to the UCYN-A phylotypes – both were derived from cyanobacteria closely related to genus Cyanothece – revealed that the two symbionts are on similar, but explicitly distinct tracks of reductive evolution.
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