Many insect lineages, especially members of Sternorrhyncha (aphids, psyllids, whiteflies, and scale insects), Auchenorrhyncha (cicadas, leafhoppers, treehoppers, spittlebugs, and planthoppers), Blattaria (cockroaches), and Coleoptera (beetles), have bacteriocytes (also called mycetocytes), cells that are differentiated to harbor obligate mutualistic intracellular bacteria (Buchner, 1965) . The bacteria, usually called primary symbionts, are confined to the cytoplasm of bacteriocytes except during transmissions to eggs or progeny and have been vertically transmitted through host generations for hundreds of millions of years (Moran et al ., 1993; Chen et al ., 1999; Thao et al ., 2000; Lo et al ., 2003; Thao and Baumann, 2004; Baumann and Baumann, 2005; Moran et al ., 2005b; Takiya et al ., 2006; Gruwell et al ., 2007) . The host insects and the primary symbionts are indispensable to each other for their growth and reproduction: the symbionts cannot proliferate out of bacteriocytes, whereas the host insects grow poorly and are sterile when they are deprived of symbionts (Douglas, 1989) . Such indivisibility is reminiscent of the association between extant eukaryotic cells and organelles such as mitochondria and chloro-plasts, which are now widely acknowledged to be descendants of free-living bacteria that invaded into ancient ancestors of eukaryotes far more than one billion years ago (Margulis, 1970; Dyall et al ., 2004; Poole and Penny, 2007).

Buchner (1965) advocated that bacteriocyte symbioses have evolved multiple times between various insect groups and a diverse array of bacteria, and this outlook was verified by molecular phylogenetic analyses . The primary symbionts of aphids (Buchnera aphidicola), psyllids (Carsonella ruddii), whiteflies (Portiera aleyrodidarum), some leafhopper species (Bau-mannia cicadellinicola), tsetse flies (Wigglesworthia glossinidia), and ants (Blochmannia) belong to y-Proteobacteria (Munson et al ., 1991; Chen et al ., 1999; Thao et al., 2000; Moran et al ., 2003; Thao and Baumann, 2004), whereas those of many auchenorrhynchan insects (Sulcia muelleri) and armored scale insects belong to Bacteriodetes (Moran et al., 2005b; Gruwell et al ., 2007). A fascinating case was exemplified in mealybugs, where P-proteobacterial primary symbionts (Tremblaya princeps) contain y-proteobacterial "secondary" symbionts (von Dohlen et al , 2001) These endosymbionts of different host groups have evolved as independent lineages from free-living bacteria . In general, primary symbionts appear to have a nutritional role, as the host insects in many cases feed on very specialized diets such as plant sap (Auchenorrhyncha and Sternorrhyncha) or blood (tsetse flies), which are poor in essential amino acids (amino acids that metazoa cannot synthesize: tryptophan, lysine, methionine, phenylalanine, threonine, valine, leucine, isoleucine, arginine, and his-tidine) and B-complex vitamins, respectively (Douglas, 1989; Sandstrom and Moran, 1999) . Physiological studies have corroborated that Buchnera and Wigglesworthia provide hosts with these nutrients (Nogge, 1981; Sasaki and Ishikawa, 1995; Douglas, 1998) .

Although the inseparability of two partners has been the core interest, it also limited investigations of the host-symbiont interactions . A breakthrough was brought about by the advent of genomics and transcriptomics, which enable us to obtain comprehensive genetic information on the bacteriocyte symbioses In this chapter, I review the genomics of bacteriocyte-restricted primary symbionts and the transcriptomics of the host bacterio-cytes, featuring the smallest cellular genome of Carsonella, and the transcriptome analysis of the aphid bacteriocyte that harbors Buchnera

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