by Merry Youle
Glassy-winged sharpshooters, long notorious in wine country as an insect vector for Pierce's disease in grapes, have recently achieved notice for employing not one but two bacterial endosymbionts to provide essential nutrients. One provides amino acids while the other supplies cofactors, especially water-soluble B-family vitamins.
Sharpshooters are sucking insects that feed on xylem fluid, the nutrient-poor sap that conveys water and salts from the roots to the rest of the plant. In contrast, aphids and many other sap-feeding insects tap into the phloem tissues. Phloem fluid contains more nutrients than the xylem sap but still lacks some essentials. Most aphids rely on a single endosymbiont, typically a Buchnera, to supply the needed amino acids in exchange for simple sugars. Interestingly, two or more endosymbionts are typically found associated with xylem feeders. In the case of this sharpshooter, the authors reported that the two endosymbionts complement one another metabolically with little overlap in biosynthetic pathways, thus potentially nourishing each other as well as their host. The authors also comment that since both symbionts have undergone major reduction in their genome size while maintaining their three-way complementary capabilities, this raises "the question of how the steps in genome reduction have been coordinated."
Wondering why these insects are called sharpshooters? Two versions of the story focus on their feeding habits, but from different points of view. The first attributes the name to the tiny "bullet-holes" left by the piercing-sucking mouthparts when feeding. The other version notes that each day sharpshooters consume 100 to 300 times their dry body weight in xylem fluid in order to acquire sufficient nutrients. The excess filtered liquid is excreted, falling like a light rain beneath heavily infested trees.