extending Buchner’s thesis that many of these associations have long evolutionary histories. Such studies have shown repeatedly that nutritional symbionts have evolved in parallel with their hosts, starting with studies of aphids and Buchnera and extending to whiteflies, scale insects, psyllids (Baumann, 2005), tsetse flies (Chen et al., 1999), stinkbugs (Hosokawa et al., 2006), carpenter ants (Schröder et al., 1996), and cockroaches (Lo et al., 2003).
The oldest such example of bacterial symbiosis underlying nutrition in an insect is that of Sulcia muelleri, a symbiotic clade in the bacterial phylum Bacteroidetes found in most groups of Auchenorrhyncha (Moran et al., 2005b). The Sulcia phylogeny matches that of hosts, based on current understanding, although this symbiont has been lost from numerous subclades of Auchenorrhyncha. The most plausible explanation for its occurrence is that an ancestral member of the Bacteroidetes, possibly a gut-dwelling associate, became an obligate symbiont of an insect host that was beginning to feed by sucking liquid from primitive vascular plants. This transition would have occurred by the time of the common ancestor of modern Auchenorrhyncha, implying an origin by the late Permian, at least 270 million years ago, as based on the insect fossil record (Moran et al., 2005b). Thus, a symbiotic event was critical to the emergence of one of the earliest major groups of herbivores on vascular plants and has been retained by many thousands of descendant species (Fig. 9.1).
Sulcia is typical of insect nutritional symbionts in that it inhabits the cytosol of specialized cells, grouped into a specialized host organ called the bacteriome (Fig. 9.2). In this case, this structure is a paired laterally positioned organ in the abdomen of adult insects; this structure appears to be homologous across the Auchenorrhyncha (Moran et al., 2005b). Symbionts are packaged into these specialized cell types during development, requiring specialized mechanisms of part of host and/or symbiont for limiting the location and growth of the bacteria. In aphids, cells destined to become bacteriocytes exhibit distinctive patterns of gene expression very early in development, before colonization by the symbiont population acquired from the maternal bacteriocytes (Braendle et al., 2003).
The elaborate symbiotic systems noted by Buchner for species of Auchenorrhyncha arise from the recruitment of additional symbionts in particular sublineages. In many cases, these later additions become obligate symbionts that coexist with Sulcia. The best-studied case to date is that of the sharpshooters, a subfamily of leafhoppers containing several thousand species. Sharpshooters are distinguished from related insects in that they have adopted a xylem sap diet, imposing distinct nutritional