**5. Aphid endosymbionts**

The plant phloem sap is a highly unbalanced diet composed principally of sugars and amino acids with high C:N content. To cope with excess of sugars in their diet, aphids have evolved modification in their intestinal tract and filter out excess of sugars and water in the form of honeydew [33]. The most of amino acids are present at very low concentrations. Despite their nutritionally poor diet, aphids exhibit high growth and reproduction rates. Since aphids directly feed on the sugars and amino acids, they need not spend extra energy to digest complex nutrients such as proteins which remarkably increases their assimilation efficiency. In addition to this, the essential amino acids required by their growth and development are synthesized by symbiotic bacteria present in their body. Generally two types of symbiotic bacteria are known to be present in aphids: the primary (obligate) symbionts and secondary (facultative) symbionts. *Buchnera aphidicola* (γ3-proteobacteria: *Escherichia coli* is also a member of this group) is the most common vertically transmitted primary symbiont present in most aphid species [34]. Some species of aphids also bear other bacteria, i.e., "secondary symbionts." These include several species of γ-proteobacteria such as *Serratia symbiotica*, *Regiella insecticola*, and *Hamiltonella defensa* [35–43]. *B. aphidicola* is a coccoid hosted in the cytoplasm of specialized cells called mycetocytes/bacteriocytes in the hemocoel of insect. These endosymbionts upgrade the aphid diet by converting nonessential amino acids to essential amino acids. The evolution of symbiotic relationship with endosymbionts has enabled aphids to exploit new ecological niches, i.e., to feed on the plant phloem sap which is otherwise the nutritionally poor diet.
