The evolution of interdependence in a four-way mealybug symbiosis

Mealybugs maintain one to three bacterial endosymbionts inside specialized cells, called bacteriocytes. Previous work has demosntrated the metabolic interdependence of endosymbionts and host in Planococcus citri, a species of mealybugs that maintains two ancient endosymbionts in a nested organization (one endosymbiont resides within another). Here, we analyze another species of mealybug, Pseudococcus longispinus, which harbors three species of endosymbionts, two of which have recently transitioned into an endosymbiotic lifestyle. The fact that these two bacterial species only recently transitioned from a free-living to a host-restricted intracellular nice is evidenced by the extreme level of erosion that we observe in each genome. We combined previously published Illumina sequence data of Pseudococcus longispinus with newly generated PacBio sequence reads, allowing us to generate high-quality assemblies of both species of the nascent endosymbionts. Both of these endosymbionts harbor thousands of pseudogenes, which have yet to be deleted, and hundreds of transposases (many of which are near-identical), indicating recent proliferation. Through careful analysis of key metabolic pathways (amino acid, vitamin, and peptidoglycan biosynthesis), we demonstrate evolution of genomic complementarity among these young endosymbionts and host. Our work suggests rapid development of metabolic co-dependence relatively early in this complex endosymbiotic system.