Retention of an endosymbiont for the production of a single molecule

Sap-feeding insects often maintain multiple nutritional endosymbionts, which act in concert to produce compounds essential for insect survival. Many mealybugs have endosymbionts in a nested configuration: one or two bacterial species reside within the cytoplasm of another bacterium, and to-gether these bacteria have genomes which encode interdependent but complete sets of genes needed to produce key nutritional molecules. Here we show that the mealybug Pseudococcus viburni has three endosymbionts, one of which contributes only two genes that produce a single host nutrition-related molecule. All three bacterial endosymbionts have tiny genomes, suggesting that they have been co-evolving inside their insect host for millions of years. SignificanceNutritional endosymbionts synthesize (or contribute to the synthesis of) key metabolites such as essential amino acids and vitamins for their host organism. These nutrients are required by hosts because of their restricted diets, which in the case of mealybugs consists solely of plant phloem sap. Genome sequencing of insect endosymbionts has shown that their genomes can be very small, encoding few genes outside of core bacterial processes and nutrient provisioning. Here we highlight an example that has taken this reductive process to the extreme: a mealybug endosymbiont contributes only a single essential compound, chorismate, to the symbiosis.