ABSTRACT
Introduction Many insect lineages, especially members of Sternorrhyncha (aphids, psyllids, whiteies, 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 con-ned 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 chloroplasts, 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).
