the single essential amino acid biosynthetic pathway that is retained by Baumannia, that for histidine, appears to be the only such pathway missing from the Sulcia genome (Wu et al., 2006). The two symbionts live in close proximity within the host bacteriome and sometimes with a single Sulcia cell surrounded by closely adjacent Baumannia cells (Fig. 9.3).

Obligate nutritional symbionts of insects provide prime examples of genome degradation in obligately host-associated bacterial lineages. These symbionts possess the smallest known genomes of cellular life forms with only 182–650 genes for fully sequenced cases (Wernegreen, 2002; Nakabachi et al., 2006; Perez-Brocal et al., 2006). In two symbiont clades, genomes of multiple representatives are available, with two for symbionts of carpenter ants [“Blochmannia” species (Degnan et al., 2005)] and four for Buchnera symbionts of aphids (Shigenobu et al., 2000; Tamas et al., 2002; van Ham et al., 2003; Perez-Brocal et al., 2006). Divergences in these two cases represent changes 30–200 million years of evolution, yet symbiont genomes show few changes. These cases are the extremes in genome

FIGURE 9.3 The two symbionts, Sulcia and Baumannia from the sharpshooter Graphocephala atropunctata. The cells are visualized by using fluorescent in situ hybridization with probes for taxon-specific 16S rRNA sequences (Wu et al., 2006). The large Sulcia cells are sometimes closely surrounded with Baumannia cells. (Photo by P. Tran and N. Moran, University of Arizona.)