644 Invasive Stink Bugs and Related Species (Pentatomoidea)
14.1 Microbial Symbiosis in Pentatomomorphan Insects: An Overview
14.1.1 Introduction
Heinrich Anton De Barry (1879), a German plant pathologist, defined “symbiosis” as a close association
between different organisms. Among diverse symbiotic associations in nature, the most cohesive form is
“endosymbiosis,” an association in which one partner is a “symbiont” (usually microorganisms including
fungi, Archaea, and bacteria) that lives inside the body of and intimately interacts with the other partner
called “host” (usually animals, plants, and some protists). Sometimes, the microbial associates are harm-
ful or even lethal to the host organisms, and, in those cases, the microbe is regarded as a “pathogen”
or “parasite.” Some microorganisms that have neutral effects on the host organisms are called “com-
mensalists.” In contrast, several microbial associates benefit the host organisms owing to their versatile
metabolic abilities that result in significant ecological and evolutionary advantages to the host organ-
isms; such microbial associates are called “mutualists.” For example, leguminous plants can grow under
nitrogen-deficient soil conditions; this ability is conferred to the plant by Rhizobium symbionts that
fix atmospheric dinitrogen and provide the hosts with fixed nitrogen compounds such as nitrate and
amino acids (Gualtieri and Bisseling 2000). Bobtail squids harbor luminescent Vibrio in a specific light
organ and use the bacteria to hide from predators (Nyholm and McFall-Ngai 2004). Corals are tightly
associated with photosynthetic dinoflagellates (Muscatine 1973). The human gut is colonized by a large
number of diverse microorganisms that play pivotal roles in the processes of metabolism, immunity, and
development of gut organs (Ley et al. 2006). In addition, the evolutionary origin of mitochondria and
chloroplasts is believed to be endosymbiotic microorganisms (Margulis and Fester 1991). Hence, nature
is full of endosymbiosis that have accelerated organismal evolution.
Recent development of molecular experimental techniques such as polymerase chain reaction (PCR),
DNA sequencing, molecular phylogenetic analysis, in situ hybridization, and next-generation sequencing
has revolutionized the entire world of microbial ecology, which has in turn revealed the ecological and
evolutionary aspects of endosymbiosis. In this chapter, we present an overview of the current knowl-
edge concerning the diversity of endosymbioses in pentatomomorphan insects, with a special focus
on the well-studied symbiotic systems of the southern green stink bug, Nezara viridula (L.) (family
Pentatomidae) and the kudzu bug, Megacopta cribraria ( F.) (family Plataspidae).
14.1.2 Microbial Symbiosis in Insects
Symbiotic associations with bacteria occur in many animals, plants, fungi, and protists (Margulis and
Fester 1991, Ruby et al. 2004), among which insects are regarded as the largest group that takes signifi-
cant advantages of bacterial symbionts (Buchner 1965, Bourtzis and Miller 2003, Kikuchi 2009). The
insects that feed exclusively on restricted diets, such as plant sap, vertebrate blood, or woody materials,
usually host symbiotic microorganisms in their bodies, where the symbionts are involved in the provi-
sion of essential nutrients and/or digestion of food materials for the host (Douglas 1989). Carnivorous
and herbivorous insects that feed on animal or plant tissues usually do not possess specific symbionts.
Symbiotic bacteria generally show strict host tissue tropism and are localized in specialized body
parts called “symbiotic organs.” Locations of symbionts are diverse in insects, ranging from extracel-
lular within the lumen of intestinal symbiotic organs called “crypts” to intracellular within specialized
cells called “bacteriocytes” (Kikuchi 2009). Bacteriocytes are enlarged cells specialized for harboring
symbiotic bacteria in the cytoplasm, which are sometimes clustered and form a large symbiotic organ
called a “bacteriome” in many insect species that are associated with intracellular symbionts.
Endosymbiosis between aphids and their intracellular bacterium Buchnera aphidicola are among the
most well-known examples of such nutritional mutualistic associations. Almost all aphids possess bacte-
riocytes for housing the intracellular symbionts (Douglas 1989, Baumann 2005). Buchnera provides the
host aphid with essential amino acids that are almost lacking in the plant phloem sap. The symbiont is
passed from the mother to offspring via transovarial transmission, where Buchnera directly infects the
embryos from the mother’s bacteriocytes (Miura et al. 2003, Koga et al. 2012). The symbiont phylogeny