In 2006, Carsonella ruddii was reported as the smallest of the small: this microbe contains the smallest genome identified at the time, clocking in at 159,662 base pairs encoding 182 genes. The Escherichia coli genome on the other hand contains over 4 million base pairs encoding about 4,000 genes.
Unlike E. coli, Carsonella ruddii is not a free-living microorganism. It dwells within bacteriocytes in the abdomen of sap-feeding insects known as psyllids. Since these insects are restricted to plant sap, their diet is high in sugar but lacking in most essential amino acids and vitamins. Thus, C. ruddii was considered an endosymbiont because it also contains genes for the biosynthesis of essential amino acids.
In comparison to other minimal genomes known at the time, the C. ruddii genome is much smaller. It’s half the size of that of B. aphidicola BCc, another insect endosymbiont which encodes 362 proteins from a 422 kb genome. Since the sequencing of the C. ruddii genome, a handful of other obligate insect symbionts have been identified with genomes of similar size.
Javier Tamames from the Universitat de Valѐcia found it surprising that the C. ruddii genome is so much smaller than the previously identified minimal genomes. The lab wondered if C. ruddii can actually be considered an endosymbiont, or even a living organism? To meet this requirement, the genes involved in essential living functions and those needed to maintain host fitness must be preserved in the genome.
After examining the genome sequence, Tamames and colleagues found that the C. ruddii genome is not sufficient for DNA replication, transcription, and protein synthesis (B. aphidicola BCc, which has a genome twice the size, does retain complete machinery for DNA replication, transcription, and translation). Moreover, the C. ruddii genome fails to provide its host with all essential amino acids – the pathways for histidine, phenylalanine, and tryptophan are absent. These researchers conclude that C. ruddii cannot be considered an endosymbiont or even a living organism.
It is not uncommon for endosymbionts to lose essential endosymbiotic functions in insect-bacterial relationships. However in these instances, another endosymbiont appears to complement the insect’s diet. For C. ruddii and its psyllid host, no other symbiont could be found.
The ultimate fate of C. ruddii remains a mystery. Tamames and colleagues propose that possibly C. ruddii is heading towards an existence in the zone between a living cell and organelle, taking advantage of mitochondrial functions encoded by the nucleus to carry out the basic necessities for life. In the case that a second endosymbiont has escaped detection, this could mean that C. ruddii could soon be driven to extinction and replaced by the second symbiont.
Further reading
Smallest Genome Clocks in at 182 Genes, Nature, 2006.
Tiny Genome May Reflect Organelle in the Making, Scientific American, 2006.
