Symbiotic microorganisms are universally found in the gut, body cavity, or cells of a wide variety of insects Some obligate symbionts are of a mutualistic nature and contribute to the fitness of their hosts, whereas other facultative symbionts are rather parasitic and tend to cause negative effects on their hosts (Bourtzis and Miller, 2003, 2006)

In particular, the most intimate mutualistic associations are found in obligate endocel-lular symbionts like Buchnera in aphids and Wigglesworthia in tsetse flies . In these insects, the symbiotic bacteria are housed in the cytoplasm of large specialized cells called bac-teriocytes (or mycetocytes) In the body of the insects, these cells aggregate into a large symbiotic organ called bacteriome (or mycetome), wherein the inhabiting symbionts play their physiological roles such as provisioning of essential nutrients for the host insects (Douglas, 1998; Baumann et al , 2000; Shigenobu et al , 2000; Akman et al , 2002)

Meanwhile, facultative endosymbionts such as Wolbachia in diverse insects and Spiro-plasma in fruit flies are generally not restricted to specialized cells in the host body, are not essential for survival and reproduction of their hosts, and are almost neutral or rather negative to the host fitness Some of them cause reproductive aberrations of their hosts such as cytoplasmic incompatibility, male-killing, parthenogenesis, and feminization, whereby the maternally inherited endosymbionts increase their infection frequencies in the host populations often at the expense of the host fitness (O'Neill et al., 1997; Werren, 1997; Bourtzis and Miller, 2003)

Regardless of their obligate or facultative nature, these endosymbiotic bacteria are generally passed to the next host generation vertically in the maternal body at early stages of oogenesis or embryogenesis, wherein the symbiont transmission is integrated into the intricate developmental process of the host insects (Braendle et al , 2003; Veneti et al , 2004; Frydman et al , 2006) In obligate associations, neither the host nor the symbiont can survive without their partner, constituting an almost inseparable biological entity

Among diverse insect-microbe symbioses, several model systems, such as Buchnera in aphids, Wigglesworthia in tsetse flies, Wolbachia in fruit flies, etc , have been investigated preferentially These conventional models for insect symbiosis studies have their own merits and demerits The Buchnera-aphid relationship has attracted much attention because of the obligate nature of the association, the endosymbiotic system with highly developed bacteriocytes, the easy rearing and handling of the insect in the laboratory, and the importance of the host insect as notorious agricultural pest The Wigglesworthia-tsetse relationship has been well studied, despite the tediousness of the insect maintenance, because of the medical importance of the host insect as the vector of African sleeping disease trypano-somes The Wolbachia-Drosophila relationship has been widely investigated because of the sophisticated genetic and molecular tools available with the model insect, the easy rearing and handling of the insect in the laboratory, and the cytoplasmic incompatibility pheno-type induced by the symbiont that is of great interest from the viewpoint of both basic and applied biology In these conventional model systems, however, because the host and the symbiont are endocellularly integrated in an inseparable manner, experimental manipulation of the host-symbiont associations, which is essential for understanding of the functional and biological aspects of the relationships, are generally not easy to perform

Recently, we have established and developed a novel model system, the capsule-transmitted gut symbiotic bacteria of plataspid stinkbugs, which enable unprecedented experimental, functional, and genomic approaches to the insect-bacterium mutualism In this chapter, we review the recent advances in the studies on the plataspid symbiosis . For details, please refer to the following original literatures (Fukatsu and Hosokawa, 2002; Hosokawa et al ., 2005, 2006, 2007a, 2007b, 2008) .

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