<i>Polynucleobacter necessarius</i> , a model for genome reduction in both free-living and symbiotic bacteria

Significance We have investigated multiple aspects of the Euplotes - Polynucleobacter system, which provides a unique opportunity for the study of an obligate symbiont with a closely related free-living organism that itself possesses a peculiarly reduced genome and metabolism. We confirmed the robustness and generality of patterns in the evolution of bacterial symbionts’ genome, adding at the same time new elements and hypotheses concerning genome reduction in both symbiotic and free-living bacteria. We argue that this system will provide an exceptionally useful model for investigations on symbiosis, because of its peculiarities and the commonness and ease of handling of the ciliate hosts. Genome sequences for independently derived Polynucleobacter symbionts will be particularly telling.