654 Invasive Stink Bugs and Related Species (Pentatomoidea)
14.1.3.2 Symbiotic Organ: Midgut Crypts, M3 Bulb, and Bacteriocytes
As discussed above, phytophagous species of the superfamilies Pentatomoidea, Coreoidea, and
Lygaeoidea commonly develop midgut crypts in the posterior region of the midgut, although the mor-
phology and arrangement of the symbiotic organ are diverse. In the Pentatomoidea and Coreoidea,
several small sacs are arranged in rows of two or four along the midgut (Figure 14.1B,D,F). In the
Lygaeoidea, most species, except members of the family Pachygronthidae, develop tubular-type crypts
in the M4 region. Pachygronthid species commonly possess small sac-type crypts, as reported in the
pentatomoid and coreoid insects. Although the luminal cavity of the midgut crypts generally open into
the lumen of the gut (Goodchild 1963), members of the family Acanthosomatidae develop an exclu-
sively unique type of crypt organization, in which the crypts superficially connect to the midgut, but the
entrance into the crypts remains completely closed (Rosenkranz 1939). In the family Plataspidae, the
alimentary tract is completely disconnected between the M3 and M4 regions, and the two gut portions
are connected only by a tiny threadlike connecting tissue in adult insects (Buchner 1965, Fukatsu and
Hosokawa 2002, Hosokawa et al. 2006). However, the lumen of the two gut portions are connected in
hatchlings so that symbiotic bacteria can pass through and colonize in the M4 region.
Although several species of the superfamily Lygaeoidea harbor extracellular symbionts in the mid-
gut crypts, the members of the family Lygaeidae generally lack the symbiotic organ. On the other
hand, several lygaeid species possess large, red-colored bacteriome(s) housing intracellular symbionts
in their body cavities (Figure 14.1I–J) (Buchner 1965; Küchler et al. 2010, 2011, 2012; Matsuura et
al. 2012, 2015). Such intracellular symbiosis also has been reported in the genus Ischnodemus of the
family Blissidae, although other members of this family harbor extracellular gut symbionts in the
tubular-type midgut crypts (Buchner 1965, Küechler et al. 2012). Those species that possess intracel-
lular symbionts commonly have neither gut symbionts nor midgut crypts (Figure 14.1I). Although
most lygaeoid species possess extracellular gut symbionts of the class Betaproteobacteria, intracel-
lular symbionts of these same species belong to the Gammaproteobacteria (see Section 14.1.3.3).
This strongly suggests that the gut symbionts have been replaced by the intracellular symbionts in
the evolution of the Lygaeoidea (Matsuura et al. 2012). It is unclear why such a symbiotic transition
has occurred in the lygaeoid lineage. One possibility is that, although their localization pattern is quit
different, there is no functional difference between the extracellular and intracellular symbiotic bac-
teria, and the transition has occurred by chance. Alternatively, the intracellular symbionts may confer
ecological and evolutionary advantages on the host insects that the gut symbionts do not, resulting in
the shift of the symbionts.
The members of the family Pyrrhocoridae (superfamily Pyrrhocoroidea) lack midgut crypts. Instead,
M3 of these species is remarkably enlarged and swollen, the lumen of which harbors a large number of
bacterial cells (Kaltenpoth et al. 2009, Sudakaran et al. 2015).
14.1.3.3 Phylogenetic Diversity of Symbiotic Bacteria
Many pentatomomorphan symbionts are unculturable outside their hosts (Y. Kikuchi, unpublished data).
Therefore, phylogenetic placements of the symbionts has been estimated based on DNA sequences,
usually of the 16S rRNA gene. Figure 14.2 shows the molecular phylogeny of the symbiotic bacteria
in pentatomomorphan species. Microbial symbiosis in the Pentatomomorpha is usually of the mono-
association type, wherein each host species houses a single species/strain of bacteria. Symbiotic bacteria
of the superfamily Pentatomoidea (including the families Pentatomidae, Scutelleridae, Acanthosomatidae,
Cydnidae, Parastrachiidae, Plataspidae, and Urostylididae) investigated so far belong to the bacterial fam-
ily Enterobacteriaceae of the class Gammaproteobacteria (Hosokawa et al. 2006, 2010a, 2012b, 2016;
Kikuchi et al. 2009; Prado and Almeida 2009a; Kaiwa et al. 2010, 2011; Bansal et al. 2014; Bistolas et
al. 2014; Kaiwa et al. 2014; Matsuura et al. 2014). In the families Plataspidae, Acanthosomatidae, and
Urostylididae, symbiotic bacteria form a monophyletic group that depends on the host families, which
suggests that each family had a single evolutionary origin of symbiosis (Hosokawa et al. 2006, Kikuchi
et al. 2009, Kaiwa et al. 2014). Symbionts of other families such as the Pentatomidae, Scutelleridae, and
Cydnidae are paraphyletic and phylogenetically more diverse (Prado and Almeida 2009a; Kaiwa et al. 2010,