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.
Life on the edge
The genome of Carsonella ruddii is so reduced that it challenges the definition of a living organism. It has lost the genes required for many processes considered essential for life. Analysis of its gene content reveals it lacks sufficient machinery for DNA replication, transcription, and translation, the fundamental processes of cellular function. It cannot create its own cell envelope and must rely on its host for many metabolic functions. Once the psyllid host took over these responsibilities, the corresponding bacterial genes were lost.
This extreme genetic reduction has led scientists to propose that Carsonella is in an evolutionary transition, moving from a distinct organism to becoming a permanent fixture of its host cell, much like an organelle. Mitochondria, the powerhouses of animal and plant cells, are themselves the descendants of ancient endosymbiotic bacteria that became fully integrated into their host cells. Carsonella has lost so many genes that it can no longer produce all the essential amino acids its host needs, and some psyllid species host a secondary symbiont to pick up the slack. The bacterium has outsourced its survival to such an extent that it exists in a boundary between an independent life form and a specialized cellular component.