needs, especially in the arena of vitamins and energy metabolism. They are also distinguished by the presence of a relatively restricted symbiont, Baumannia cicadellinicola, belonging to the Gammaproteobacteria and related to Buchnera (Moran et al., 2003; Takiya et al., 2006). Sharpshooters form a much younger group than the Auchenorrhyncha, with fossils not appearing until the Eocene. Phylogenetic analyses based on genes from both symbionts and insect hosts support the following evolutionary reconstruction: Sulcia was ancestrally present in a host lineage that acquired Baumannia at the same approximate time as the switch to xylem-feeding, consistent with the view that its nutrient-provisioning capabilities were a requirement for this lifestyle. After the acquisition of Baumannia, both Sulcia and Baumannia diversified in parallel with their sharpshooter hosts, through strict maternal transmission, based on the congruence of phylogenetic trees from the three clades (Takiya et al., 2006) (Fig. 9.1).
Other Auchenorrhynchan groups have recruited other bacterial and fungal symbionts, few of which have been studied beyond microscopy studies describing their morphology and transmission. Other cases of successive acquisition of symbionts are numerous, with cases documented in both aphids (Perez-Brocal et al., 2006) and weevils (Lefevre et al., 2004).
The nutritional role of symbionts associated with specialized organs (bacteriomes) (another of Buchner’s central theses) has been elucidated by cloning and sequencing of specific genes, complete genome sequencing, and studies of gene expression (Moran and Degnan, 2006). Thus, of four Buchnera genomes now fully sequenced, all are extremely small but nonetheless contain all or most pathways for synthesis of the amino acids that are required by animals (Shigenobu et al., 2000; van Ham et al., 2003; Perez-Brocal et al., 2006).
One genome of Sulcia has been partially sequenced, from the host species, Homalodisca coagulata (the “glassy-winged sharpshooter”) (Wu et al., 2006). As for Buchnera, Sulcia possesses a very small genome but retains pathways for synthesis of most essential amino acids, nutrients that are in short supply in both phloem and xylem sap, in both of which amino acid profiles are dominated by nonessential amino acids. A complete genome sequence for Baumannia of H. coagulata confirms that this symbiont plays a critical role in the dependence of sharpshooters on a xylem sap diet. Whereas Sulcia retains pathways for amino acid provisioning, Baumannia contains a large number of pathways for biosynthesis of vitamins (Wu et al., 2006). The complementarity between capabilities evident from the genomic sequences of the two symbionts is striking. For example,