show Abstracthide AbstractSymbiotic interaction between hosts and microbes promotes the establishment of new ecological niches and has profound impacts on the evolutionary trajectory of eukaryotic lineages. Many insects harbor maternally transmitted endosymbionts that provision hosts with key nutrients. One key evolutionary consequence of vertically transmitted symbiosis is rapid degradation of symbiont genomes. We use the system of aphids and their obligate endosymbionts, Buchnera aphidicola, to explore the evolutionary process of symbiont genome degradation and the functional consequences of this process.