Repeated replacement of an intrabacterial symbiont in the tripartite nested mealybug symbiosis

Abstract

Stable endosymbiosis of a bacterium into a host cell promotes cellular and genomic complexity. The mealybugPlanococcus citrihas two bacterial endosymbionts with an unusual nested arrangement: the γ-proteobacteriumMoranella endobialives in the cytoplasm of the β-proteobacteriumTremblaya princeps. These two bacteria, along with genes horizontally transferred from other bacteria to theP. citrigenome, encode gene sets that form an interdependent metabolic patchwork. Here, we test the stability of this three-way symbiosis by sequencing host and symbiont genomes for five diverse mealybug species and find marked fluidity over evolutionary time. AlthoughTremblayais the result of a single infection in the ancestor of mealybugs, the γ-proteobacterial symbionts result from multiple replacements of inferred different ages from related but distinct bacterial lineages. Our data show that symbiont replacement can happen even in the most intricate symbiotic arrangements and that preexisting horizontally transferred genes can remain stable on genomes in the face of extensive symbiont turnover.