Symbiotic Microorganisms Associated with Pentatomoidea 657
(see Section 14. 3). Furthermore, a novel type of symbiont transmission was reported recently in the
Urostylididae. During egg oviposition, females cover their egg clusters with a symbiont-containing gel-
like substance, called “symbiont jelly,” and the hatchlings feed on the jelly to acquire the symbiont
(Kaiwa et al. 2014).
Coreoid and lygaeoid species associated with Burkholderia symbionts do not vertically transmit
the symbiont but, rather, acquire the microbes from the surrounding environment in every generation
(Kikuchi et al. 2007, 2011; Olivier-Espejel et al. 2011; Boucias et al. 2012; Itoh et al. 2014). Such a trans-
mission manner, called “environmental acquisition” (or environmental transmission), is common among
symbiotic associations in marine invertebrates such as squid-Vibrio symbiosis (Nyholm and McFall-
Ngai 2004), and in terrestrial plants such as legume-Rhizobium symbiosis (Gualtieri and Bisseling 2000).
However, this type of symbiont transmission/acquisition rarely has been reported in insects, except for
coreoid and lygaeoid species. Although most of the vertically transmitted symbionts are unculturable
outside their hosts, probably because of their host-dependent lifestyles and extremely reduced genomes,
the Burkholderia symbionts are easily culturable (Kikuchi et al. 2007, 2011). Experimental inspections
of the bean bug, Riptortus pedestris Thunberg [= R. clavatus (F.)], have revealed that the host acquires
the Burkholderia symbionts mainly during the second stadium, and that a 50% infective dose (ID 50 : the
amount of symbionts required for the colonization of 50% of the tested insects) was remarkably low;
only 80 symbiont cells were required, suggesting high efficiency of the symbiont acquisition (Kikuchi
and Yumoto 2013), which probably stabilizes the symbiotic association without vertical transmission.
14.1.3.5 Biological Functions of Symbiotic Bacteria
Numerous studies have demonstrated repeatedly that symbiotic bacteria play a pivotal role in the life
of a pentatomomorphan host; experimental elimination of symbionts by egg-surface sterilization or by
removing capsule/jelly causes retarded growth, nymphal mortality, small body size, and/or abnormal
body coloration (Table 14.2).
Buchner (1965) noted a general pattern that insects living on nutritionally unbalanced diets, such as
plant sap or vertebrate blood, tend to harbor endosymbiotic microorganisms, whereas carnivorous and
herbivorous insects feeding on animal or plant tissues do not, which implies that symbiotic bacteria
provide essential nutrients that are limited in species that feed on insects. For instance, on the basis on
physiological and genomic analyses, Buchnera spp. provide their aphid hosts with essential amino acids
that are mostly lacking in plant phloem sap (Sasaki and Ishikawa 1998, Shigenobu et al. 2000). The same
situation also may apply to the sap-feeding groups of the Pentatomomorpha, such as the Plataspidae and
Urostylididae; this is strongly supported by recent genome analyses of their symbionts (Nikoh et al. 2011,
Kaiwa et al. 2014) (also see Sections 14.1.3.6 and 14.3.2.3). However, in phytophagous pentatomomor-
phans, not only sap feeders but also many seed feeders possess well-developed endosymbiotic systems.
Because plant seeds seem to be more nutritionally rich and balanced than plant phloem sap, biological
functions of the symbiotic bacteria in seed feeders remain unclear. The use of an artificial diet for the
rearing of African cotton stainer, Dysdercus fasciatus Signoret (Pyrrhocoridae), elucidated this long-
standing question and clearly demonstrated that symbiotic bacteria provide B-vitamins to the host insect
(Salem et al. 2014). Such a nutritional supplement is probably a major biological function of the symbiotic
bacteria in seed-feeding pentatomomorphans.
14.1.3.6 Symbiont Genome
The genome sequence of symbiotic bacteria has been reported so far from Candidatus Ishikawaella
capsulata in Megacopta punctatissima (Montandon) (Plataspidae) (Nikoh et al. 2011), Candidatus
Tachikawaea gelatinosa in Urostylis westwoodii Scott (Urostylididae) (Kaiwa et al. 2014), Candidatus
Pantoea carbekii in Halyomorpha halys (Stål) (Pentatomidae) (Kenyon et al. 2015), and two strains of
the Burkholderia symbiont in Riptortus pedestris (Alydidae) (Shibata et al. 2013, Takeshita et al. 2014).
Intracellular obligate symbionts of diverse insects, including Buchnera in aphids and Wigglesworthia
in tsetse flies, share unique genetic traits such as an AT (Adenine, Thymine)-biased nucleotide com-
position, accelerated molecular evolution, and remarkably reduced genome size. In comparison with a