Genomic sequencing of Pseudococcus longispinus and its endosymbionts

Long-term stable endosymbiosis of bacterium into a host cell can enable cellular and genetic complexity. The mealybug Planococcus citri has two bacterial endosymbionts; remarkably, however, the gammaproteobacterium Moranella endobia lives in the cytoplasm of the betaproteobacterium Tremblaya princeps. These two bacteria, along with genes horizontally transferred from other bacteria to the insect genome, have been shown to encode complementary gene sets that form a complex metabolic patchwork. Here we test the stability of this three-way symbiosis by sequencing host-symbiont genome pairs for five diverse mealybug species. We find remarkably fluidity over evolutionary time: while Tremblaya is the result of a single infection in the ancestor of mealybugs, the innermost gammaproteobacterial symbionts result from multiple replacements of inferred different ages from related but distinct bacterial lineages. Our data highlight the possibility of ongoing turnover even in the most intricate symbiotic arrangements, and show that ancient horizontally transferred genes can remain stable on genomes in the face of extensive symbiont turnover.