Candidatus Carsonella ruddii – Wikipedia

Species of bacterium

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

This is the first, and as of February 2022 the only[3][4] species described from the genus Candidatus Carsonella, named after Rachel Carson.[5]


The species is an endosymbiont that is present in all species of phloem sap-feeding insects known as psyllids.[5][6] The endosymbionts occurs in a specialised structure known as the bacteriome.

C. ruddii is not completely parasitic in its relationship with its host insect; it supplies the host with some essential amino acids. It is therefore probably in the evolutionary process of becoming an organelle, similar to the mitochondria of eukaryotic cells that also evolved from an endosymbiont.[7]

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.[8]Nasuia deltocephalinicola is now considered to have the known smallest bacterial genome (112kb).[9]

C. ruddii and related species appear to be actively undergoing gene loss.[10]


  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. PMID 12572614.
  2. ^ a b Nakabachi A, Yamashita A, Toh H, Ishikawa H, Dunbar H, Moran N, Hattori M (2006). “The 160-kilobase genome of the bacterial endosymbiont Carsonella“. Science. 314 (5797): 267. doi:10.1126/science.1134196. PMID 17038615. S2CID 44570539.
  3. ^ “Taxonomy browser (“Candidatus Carsonella ruddii”)”.
  4. ^ Schoch, CL; Ciufo, S; Domrachev, M; Hotton, CL; Kannan, S; Khovanskaya, R; Leipe, D; Mcveigh, R; O’Neill, K; Robbertse, B; Sharma, S; Soussov, V; Sullivan, JP; Sun, L; Turner, S; Karsch-Mizrachi, I (1 January 2020). “NCBI Taxonomy: a comprehensive update on curation, resources and tools”. Database : The Journal of Biological Databases and Curation. 2020. doi:10.1093/database/baaa062. PMC 7408187. PMID 32761142.
  5. ^ a b Thao, MyLo L.; Moran, Nancy A.; Abbot, Patrick; Brennan, Eric B.; Burckhardt, Daniel H.; Baumann, Paul (July 2000). “Cospeciation of Psyllids and Their Primary Prokaryotic Endosymbionts”. Applied and Environmental Microbiology. 66 (7): 2898–2905. Bibcode:2000ApEnM..66.2898T. doi:10.1128/aem.66.7.2898-2905.2000. PMC 92089. PMID 10877784.
  6. ^ 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 (6): 419–21. doi:10.1007/s002840010240. PMID 11381334. S2CID 20069209.
  7. ^ Tortora(1), Funke(2), Case(3), Gerard J(1), Berdell R(2), Christine L(3) (2016). Microbiology: An Introduction (12th ed.). USA: Pearson. p. 315. ISBN 978-0321929150.{{cite book}}: CS1 maint: multiple names: authors list (link)
  8. ^ 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. PMID 24995872.
  9. ^ 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. PMC 3787670. PMID 23918810.
  10. ^ 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. PMC 3494270. PMID 22821013.

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