Two bacteria that generate amino acids for their insect host, the mealybug, live one inside the other in an unprecedented nested arrangement.

Credit: J. BURGESS/SPL

John McCutcheon at the University of Montana in Missoula and Carol von Dohlen at Utah State University in Logan sequenced and analysed the genomes of the two bacterial species. One is Candidatus 'Tremblaya princeps'; the other is an unnamed species that lives in Tremblaya's cytoplasm, which the authors propose to name Candidatus 'Moranella endobia'.

The duo found that Tremblaya has the smallest reported bacterial genome, with just 121 protein-coding genes. Although Tremblaya has retained several genes involved in the biosynthesis of essential amino acids, it lacks any complete pathways. Moranella complements several missing genes, patching holes in the pathways and enabling the production of essential amino acids for the mealybug (Planococcus citri; pictured).

Curr. Biol. 10.1016/j.cub.2011.06.051 (2011)