The incredible shrinking bacterium
Inside sap-sucking insects called psyllids lives a bacterium named Candidatus Carsonella ruddii. This microbe is the result of a long-standing partnership, an endosymbiosis where one organism lives inside another. Over millions of years, Carsonella has become entirely dependent on its insect host. This dependency shows in its genome, which is one of the smallest ever found in a cellular organism. The first strain to be fully sequenced, found in the psyllid Pachypsylla venusta, has a circular chromosome of just 159,662 base pairs. This tiny genome contains only 182 predicted protein-coding genes. For comparison, the common gut bacterium Escherichia coli has over 4,000 genes in a genome of more than 4 million base pairs.
Carsonella resides within specialized host cells called bacteriocytes, which group together to form an organ called a bacteriome. The psyllid's diet of plant phloem is high in sugar but poor in essential amino acids. Carsonella's primary job is to synthesize these missing amino acids for the psyllid. This relationship is so established that the bacteria are passed down directly from a mother psyllid to her offspring through her ovaries, a process called transovarial transmission. Many psyllid species are agricultural pests in California and worldwide, including the Asian citrus psyllid, Diaphorina citri, which also hosts Carsonella.