Retention of an endosymbiont for the production of a single molecule.

Sap-feeding insects often maintain two or more 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 together these bacteria have genomes which encode interdependent 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 unique genes that produce the host nutrition-related molecule chorismate. All three bacterial endosymbionts have tiny genomes, suggesting that they have been co-evolving inside their insect host for millions of years.