Symbiotic Microorganisms Associated with Pentatomoidea 659
Benitsuchiphilus tojoi in Parastrachia japonensis (Parastrachiidae) (Hosokawa et al. 2010a). Although it
remains to be discovered what evolutionary mechanisms have led to these peculiar genetic traits in insect
symbionts, theoretical studies have suggested that the endosymbiotic lifestyle may relax the symbiont’s
genome against environmental selection pressures, whereas severe bottlenecks during vertical transmis-
sion and absence of recombination and/or horizontal gene transfer may accelerate molecular evolution
and genome reduction (Moran 1996, Wernegreen 2002).
Gene compositions of Candidatus Ishikawaella and Candidatus Tachikawaea symbionts are basically
similar to those of Buchnera, probably because the host stink bugs feed exclusively on plant phloem
sap (Nikoh et al. 2011, Kaiwa et al. 2014). These symbionts, having host-dependent lifestyles, have lost
several genes that are involved in the primary metabolic processes such as replication, transcription, and
energy production (e.g., TCA cycle) and cell structures such as cell walls, flagella, and pilli. In contrast,
these symbiotic bacteria retain several biosynthesis pathways for nutrients that are limited in plant sap
and, thus, are demanded by the host insects such as essential amino acids and vitamins (for Ishikawaella
symbiont, see Section 14. 3 for more information). The gene composition of Candidatus Pantoea carbekii
symbiont in Halyomorpha halys is similar to those of Ishikawaella and Tachikawaea, whereas the sym-
biont retains intact TCA cycle and cell-wall synthesis pathways (Kenyon et al. 2015).
Probably because the Burkholderia symbionts possess a free-living life stage in the environmental
soil, in contrast to the vertically transmitted symbionts, genome sizes of the Burkholderia symbionts are
remarkably larger with a genome size and lower AT content of 6.96 Mb and 36.8% in strain RPE64 and
8.69 Mb and 36.6% in strain RPE67 (Shibata et al. 2013, Takeshita et al. 2014). Their genomes consist of
almost a full-set of genes of primary metabolic processes and cell structures and also include a number
of genes with no homologous genes.
14.2 Symbiosis in the Southern Green Stink Bug, Nezara viridula
14.2.1 Over view
Nezara viridula (Pentatomidae) is distributed worldwide (Todd 1989) and is an economically impor-
tant pest of many crops such as soybean, cotton, and macadamia nuts (Schaefer and Panizzi 2000).
N. viridula can cause direct damage, as with other heteropterans, by sucking the developing seeds and
reducing yield and quality of various commercial crops (Schaefer and Panizzi 2000, Jones et al. 2001).
In addition, the insects cause physical damage to plants by their probing behavior, which may facilitate
the penetration of opportunistic bacterial and fungal pathogens into the plant tissues (Russin et al. 1988,
Lee et al. 1993). Furthermore, the bugs act as vectors of plant pathogens, causing diseases such as yeast-
spot disease in soybean (Clarke and Wilde 1970a, b). Because N. viridula is economically important,
extensive research has been conducted on various aspects of its biology, and rearing and experimental
systems have been established. For these reasons, N. viridula was chosen as a model species for studying
bacterial symbionts associated with midgut crypts.
14.2.2 Fitness Effect of Symbiotic Bacteria
Hirose et al. (2006) and Prado et al. (2006) consistently found a bacterium associated with the midgut
M4 region of stink bugs (Figure 14.3). Hirose et al. (2006) investigated symbionts inside the midgut of
insects from Brazil; and Prado et al. (2006) investigated symbionts of insects from Hawaii, California,
and South Carolina. Although the M4 region has a low concentration of cultivable bacteria, molecular
analyses have revealed that midgut crypts consist of a large number of Erwinia-like bacterial symbionts
(Hirose et al. 2006; Prado et al. 2006, 2009; Tada et al. 2011, Prado and Zucchi 2012).
Early studies on the effects of surface sterilization of pentatomid eggs on insect development had
shown conflicting results; however, increased knowledge in this field and the development of molecular
tools have confirmed that egg-surface sterilization eliminates the crypt-associated bacterium, generat-
ing aposymbiotic insects. The influence of this aposymbiotic status on insect fitness, however, varies
between individuals and/or populations (Prado et al. 2006, 2009; Tada et al. 2011; Prado and Zucchi