Candidatus Carsonella ruddii

Overview
Candidatus Carsonella ruddii is an endosymbiotic Gamma Proteobacteria; it has the smallest genome of any characterised bacteria.

The species is an endosymbiont that is present in all species of phloem sap-feeding insects known as psyllids. The endosymbionts occurs in a specialised structure known as the bacteriome.

In 2006 the genome of C. ruddii strain Pv (Carsonella-Pv) of the hackberry petiole gall psyllid, Pachypsylla venusta was sequenced at RIKEN in Japan and the University of Arizona. It was shown that the species genome consists of a circular chromosome of 159,662 base pairs and that the genome has a high coding density (97%) with many overlapping genes and reduced gene length. The number of predicted genes was 182, also lowest in record. In comparison, Mycoplasma genitalium, which has the smallest genome of any free-living organism, has a genome of 580,000 base pairs. Numerous genes considered essential for life are missing, suggesting that the species may have achieved organelle-like status.