Candidatus Carsonella ruddii

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Candidatus Carsonella ruddii
Scientific classification
Kingdom: Bacteria
Phylum: Proteobacteria
Class: Gamma Proteobacteria
Order: unclassified
Genus: Candidatus Carsonella
Species: Ca. C. ruddii
Binomial name
Candidatus Carsonella ruddii
Thao et al. 2000

Candidatus Carsonella ruddii is an obligate endosymbiotic Gamma Proteobacterium[1] with one of the smallest genomes of any characterised bacteria.[2]


The species is an endosymbiont that is present in all species of phloem sap-feeding insects known as psyllids.[3][4] The endosymbionts occurs in a specialised structure known as the bacteriome.


In 2006 the genome of Ca. 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 genome consists of a circular chromosome of 159,662 base pairs and that it has a high coding density (97%) with many overlapping genes and reduced gene length. The number of predicted genes was 182, also the lowest on record (NCBI-Genome). In comparison, Mycoplasma genitalium, which has the smallest genome of any free-living organism, has a genome of 521 genes. Numerous genes considered essential for life seem to be missing, suggesting that the species may have achieved organelle-like status.[2]

At the time of its sequencing, C. ruddii was thought to have the smallest genome of any characterized bacterial species.[5] Nasuia deltocephalinicola is now considered to have the known smallest bacterial genome (112kb).[6]

C. ruddii and related species appear to be actively undergoing gene loss.[7]


  1. ^ Spaulding, A. W.; von Dohlen, C. D. (1998). "Phylogenetic Characterization and Molecular Evolution of Bacterial Endosymbionts in Psyllids (Hemiptera: Sternorrhyncha)". Molecular Biology and Evolution 15 (11): 1506–1513. doi:10.1093/oxfordjournals.molbev.a025878. 
  3. ^ Thao, M.L. (2000). "Cospeciation of Psyllids and Their Primary Prokaryotic Endosymbionts". Applied and Environmental Microbiology 66: 2898–2905. doi:10.1128/aem.66.7.2898-2905.2000. 
  4. ^ Thao, M.L. (2001). "Phylogenetic analysis of vertically transmitted psyllid endosymbionts (Candidatus Carsonella ruddii) based on atpAGD and rpoC: comparisons with 16S-23S rDNA-derived phylogeny". Current Microbiology 42: 419–21. doi:10.1007/s002840010240. PMID 11381334. 
  5. ^ Moran, Nancy A.; Bennett, Gordon M. (8 September 2014). "The Tiniest Tiny Genomes". Annual Review of Microbiology 68 (1): 195–215. doi:10.1146/annurev-micro-091213-112901. 
  6. ^ Bennett, G. M.; Moran, N. A. (5 August 2013). "Small, Smaller, Smallest: The Origins and Evolution of Ancient Dual Symbioses in a Phloem-Feeding Insect". Genome Biology and Evolution 5 (9): 1675–1688. doi:10.1093/gbe/evt118. PMID 23918810. 
  7. ^ Sloan, D. B.; Moran, N. A. (19 July 2012). "Genome Reduction and Co-evolution between the Primary and Secondary Bacterial Symbionts of Psyllids". Molecular Biology and Evolution 29 (12): 3781–3792. doi:10.1093/molbev/mss180. 

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