| Summary: | Dependence on multiple nutritional bacterial symbionts forming a metabolic unit has repeatedly evolved in many insect species that feed on nutritionally unbalanced diets such as plant sap. This is the case for aphids of the subfamilies Lachninae and Chaitophorinae, which have evolved di-symbiotic systems in which the ancient obligate nutritional symbiont <i>Buchnera aphidicola</i> is metabolically complemented by an additional nutritional symbiont acquired more recently. Deciphering how different symbionts integrate both metabolically and anatomically in such systems is crucial to understanding how complex nutritional symbiotic systems function and evolve. In this study, we sequenced and analyzed the genomes of the symbionts <i>B. aphidicola</i> and <i>Serratia symbiotica</i> associated with the Chaitophorinae aphids <i>Sipha maydis</i> and <i>Periphyllus lyropictus</i>. Our results show that, in these two species, <i>B. aphidicola</i> and <i>S. symbiotica</i> complement each other metabolically (and their hosts) for the biosynthesis of essential amino acids and vitamins, but with distinct metabolic reactions supported by each symbiont depending on the host species. Furthermore, the <i>S. symbiotica</i> symbiont associated with <i>S. maydis</i> appears to be strictly compartmentalized into the specialized host cells housing symbionts in aphids, the bacteriocytes, whereas the <i>S. symbiotica</i> symbiont associated with <i>P. lyropictus</i> exhibits a highly invasive phenotype, presumably because it is capable of expressing a larger set of virulence factors, including a complete flagellum for bacterial motility. Such contrasting levels of metabolic and anatomical integration for two <i>S. symbiotica</i> symbionts that were recently acquired as nutritional co-obligate partners reflect distinct coevolutionary processes specific to each association.
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