Symbiotic Microorganisms Associated with Pentatomoidea 665
pest-conferring Ishikawaella genome of M. punctatissima from Japan (Nikoh et al. 2011) showed little
difference. It was therefore hypothesized that the symbiosis introduced into the United States in 2009
was capable of being a legume pest and has maintained that status from 2009 to 2011.
14.3.3.3 Genomic Landscape
Except for being 46 bp longer than the Japanese reference genome (Nikoh et al. 2011), and having a single
difference in the riboflavin synthase (ribC) gene, the Ishikawaella genomes from all invasive Megacopta
cribraria examined were concordant in gene order and orientation (Brown et al. 2014) with the Japanese
Ishikawaella genome from the soybean pest M. punctatissima (Nikoh et al. 2011). No fixed differences
were observed to accumulate in the genomes over the 2 years of the study. Comparative analyses, how-
ever, across the United States-collected endosymbionts showed that negative allele frequency changes
including those for “lipid transport and metabolism, cell cycle control, cell division and chromosome
partitioning and amino acid metabolism” were not significantly different between endosymbionts taken
from plataspids feeding on kudzu and those feeding on soybean. Positive allele frequency changes for
functions such as replication, recombination and repair, biogenesis, and lipid and coenzyme transport
and metabolism were significantly different between Ishikawaella genomes from M. cribraria feeding
on kudzu and M. cribraria feeding on soybean.
Brown et al. (2014) observed increased allele frequency changes as the population rapidly expanded its
range from the end of 2010 into 2011, which is typical of an expanding population or negative (purifying)
selection. The correlation of increased allele frequency change with distance from the grand zero (the
point where the invasion started) also was correlated with the insect’s host plants, particularly soybean
(Brown et al. 2014, suggesting that the evolutionary processes that have occurred (or on-going) depend
on the host plants. This phenotypic plasticity could be supporting the adaptive potential of the insect by
allowing the symbiosis to respond differentially to novel environments (Hunt et al. 2011).
Except for a single difference in the riboflavin synthase (ribC) gene between the United States and
Japanese genomes, the United States Ishikawaella genome appeared to be functionally similar to the
Japanese Ishikawaella genome. Both had the same gene order and organizational landscape. Although
the significance of the point mutation, which showed consensus across the United States samples, in
ribC was unclear because there is a lack of understanding of how riboflavin availability differs across
legume species, it did suggest questions and lines of research. Ishikawaella, therefore, appeared to con-
fer pest status on its plataspid host upon arrival in northeast Georgia in 2009. Thus, the emergence of
Megacopta cribraria on soybean during the first year in the United States was not due to a change in
the endosymbiont. Jenkins and Eaton (2011) documented female lineage consensus in 83 individuals in
36 counties, which was suggestive of limited genetic diversity in the insect. The phenotypic variation
observed in host-plant distribution as well as phenotypic differences in insect color and size likely was
due, therefore, to the insect, not the endosymbiont (Brown et al. 2014).
14.3.4 Hypotheses from a Natural Laboratory Experiment
The Western Hemisphere has been an ongoing natural laboratory for testing two hypotheses as they
relate to a plataspid pest symbiosis indigenous to Asia and recently introduced into North America. The
hypotheses are as follows:
- A mutually obligate extracellular symbiosis indigenous to the Eastern Hemisphere will rapidly
adapt and expand its range into novel environments in the Western Hemisphere. This hypoth-
esis cannot be rejected based on the research reported. The symbiosis has rapidly expanded its
range across the southeastern United States and into Central America. - The symbiont genome will rapidly evolve as the symbiosis expands its range across the Western
Hemisphere. Population genome sequence studies of the United States samples across 23 sites
and in comparison to genome reference sequence from Asia (Nikoh et al. 2011) showed con-
sistency between gene order and orientation between the two genomes. The United States
genomes also showed patterns consistent with positive and negative selection, but more research