Evolution

Growing Closer Together

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Science  30 Jul 2010:
Vol. 329, Issue 5991, pp. 493
DOI: 10.1126/science.329.5991.493-a

The fern (left) and the bacterium (right).

CREDIT: RAN ET AL., PLOS ONE 5, E11486 (2010)

The conversion of a free-living organism into a domesticated organelle is a remarkable event, and mitochondria and chloroplasts have become virtually indispensable components of eukaryotic life. Ran et al. have sequenced the genome of a nitrogen-fixing symbiont of the water fern Azolla filiculoides and suggest that this cyanobacterium may have been partly domesticated by its host. They note that there appears to have been co-evolution between the two organisms, as revealed in the intricate means by which Azolla maintains cyanobacterial colonies through successive generations. Furthermore, hallmarks of genome reduction in the symbiont, such as excessive pseudogenization, were identified, and the pattern of gene losses resembled that of another plant symbiont, rather than closely related, free-living cyanobacteria. Several basic metabolic processes such as glycolysis, replication, and nutrient import had suffered losses, yet nitrogen-fixing pathways had remained intact. This apparent streamlining of the genome indicates that progress toward a full mutualism between the plant and the cyanobacterium may be well under way.

PLoS ONE 5, e11486 (2010).

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