Symbiotic Microorganisms Associated with Pentatomoidea 663
(Hosokawa et al. 2007a, 2008). Nymphs that did not ingest symbionts from the capsules were easily
identified by their wandering behavior and ultimate failure to flourish (Hosokawa et al. 2008).
14.3.2.2 Ishikawaella, Insect Development and Pest Status
Megacopta punctatissima is a legume pest of mainland Japan whereas M. cribraria, found across
the southwestern islands of Japan, is not (Hosokawa et al. 2007b). The difference between the pest
and non-pest status of these two plataspids appeared to be the gut symbiont Ishikawaella (Hosokawa
et al. 2007b).^1 Host eggs were, therefore, manipulated to determine if Ishikawaella, shown to co-
speciate with its plataspid host (Hosokawa et al. 2006) also was responsible for the pest status of
M. punctatissima. M. punctatissima eggs were combined with symbiont capsules from M. cribraria,
and M. cribraria eggs were combined with symbiont capsules from M. punctatissima. M. cribraria
that received capsules from M. punctatissima thrived on legumes, including soybean, whereas
M. punctatissima, that received symbiont capsules from M. cribraria had high mortality on soybean.
Hosokawa et al. (2007b) concluded, therefore, that it was the Ishikawaella gut symbiont in capsules
from M. punctatissima that were essential for normal development and reproduction as well as con-
ferred pest status on the host.
14.3.2.3 Genome Evolution
The Ishikawaella genome, like the genomes of bacteriocyte-associated endosymbionts, had similar pat-
terns of gene reduction (Hosokawa et al. 2006, Nikoh et al. 2011) but significant differences as well.
Nikoh et al. (2011) suggested that these differences might have functional and ecological consequences
for Ishikawaella. Unlike the bacteriocyte-associated endosymbionts, Buchnera in aphids (Shigenobu
et al. 2000) or Blochmannia in carpenter ants (Gil et al. 2003), Ishikawaella had more genes for the
synthesis of amino acids as well as some genes for the synthesis of vitamins and cofactors. Nikoh et al.
(2011) speculated that Ishikawaella either had a more recent evolutionary history or could supply more
nutritional needs to its host than either endosymbionts Buchnera or Blochmannia. Ishikawaella, how-
ever, had fewer genes for cofactor synthesis than bacteriocyte-associated endosymbionts Baumannia in
leafhoppers (Wu et al. 2006) or Wigglesworthia in tsetse flies (Akman et al. 2002), which reflected the
nutritional needs of their respective hosts (Nikoh et al. 2011). Ishikawaella compensated for the lack of
essential amino acids and vitamins in phloem (Nikoh et al. 2011) and Baumannia and Wigglesworthia
compensated for the lack of cofactors in the diet of their respective cicadellid and glossinid hosts (Akman
et al. 2002, Wu et al. 2006). Nikoh et al. (2011) also observed that, like endocellular symbiont genomes,
the Ishikawaella genome retained many genes for translation, replication, and energy production. Unlike
bacteriocyte-associated genomes, however, the Ishikawaella genome had fewer genes for cell wall syn-
thesis or lipid metabolism. The question thus arises, how does Ishikawaella survive the extracellular
condition?
Nikoh et al. (2011) speculated that the reductive evolution of the symbiont genome with its lack of
genes for cell wall synthesis or lipid metabolism may be possible because of the composition of the
vertically transmitted symbiont capsules. Ishikawaella cells are anchored within the capsules by a secre-
tion matrix (Hosokawa et al. 2005). When capsules are deposited upon oviposition, this matrix may be
simulating the intracellular composition of the cytoplasm and, therefore, supplying Ishikawaella with
metabolites that it cannot make with its reduced genome. These capsules also may serve to protect the
bacteria from dehydration and radiant energy (Nikoh et al. 2011).
Insect-bacterial mutualists are examples of “complementarity and syntrophy between host and
symbiont” (Shigenobu et al. 2000, International Aphid Genomics Consortium 2010, Wernegreen 2012).
Ishikawaella has a long history of mutually dependent co-evolution with its plataspid hosts including
Megacopta punctatissima and M. cribraria (Hosokawa et al. 2006). Genome evolution of the extracel-
lular Ishikawaella also is comparable to the evolutionary patterns of endocellular symbiont genomes,
(^1) For further discussion of the taxonomic status of M. cribraria and M. punctatissima, see Chapter 5.