Characterization of a novel Pantoea symbiont allows inference of a pattern of convergent genome reduction in bacteria associated with Pentatomidae.

Phytophagous stink bugs typically harbor nutritional symbiotic bacteria in their midgut, to integrate their unbalanced diet. In the Pentatomidae, most symbionts are affiliated to the genus Pantoea, and are polyphyletic. This suggests a scenario of an ancestral establishment of symbiosis, followed by multiple symbiont replacement events by akin environmental bacteria in different host lineages. In this study, a novel Pantoeaspecies ('CandidatusPantoea persica') was characterized from the gut of the pentatomid Acrosternum arabicum, and shown to be highly abundant in a specific portion of the gut and necessary for the host development. The genome of the symbiont (2.9 Mb), while presenting putative host-supportive metabolic pathways, including those for amino acids and vitamin synthesis, showed a high level of pseudogenization, indicating ongoing genome reduction. Comparative analyses with other free-living and symbiotic Pantoea highlighted a convergent pattern of genome reduction in symbionts of pentatomids, putatively following the typical phases modelized in obligate nutritional symbionts of insects. Additionally, this system has distinctive traits, as hosts are closely related, and symbionts originated multiple independent times from closely related free-living bacteria, displaying convergent and independent conspicuous genome reduction. Due to such peculiarities, this may become an ideal model to study genome evolutionary processes in insect symbionts.

Heritable Gammaproteobacterial Symbiont Improves the Fitness of Brachynema germari Kolenati (Hemiptera: Pentatomidae).

The pistachio green stink bug, Brachynema germari Kolenati, is an abundant and economic insect pest in most pistachio-growing regions. Some physiological and ecological features of this pest have been studied, but the microbiological nature of symbiotic bacteria and biological aspects of this host-symbiont interaction have been poorly understood. In the present study, we explored the host-associated environment, phylogeny, and acquisition features of the bacterial symbiont of the insect. Furthermore, the importance of the symbiont on the biological (i.e., lifespan, stage composition, and body weight) and behavioral characteristics (i.e., resting/wandering behaviors of the newborn nymphs) of the host were investigated. We found that a rod-shaped gammaproteobacterium was persistently colonized the fourth midgut region of the insect. Molecular phylogenetic and fluorescence in situ hybridization analyses strongly suggest that this symbiont should be placed in the genus Pantoea of the Enterobacteriales. Egg surface sterilization resulted in the aposymbiotic insects suggesting the vertical transmission of symbiont via egg surface smearing upon oviposition. Symbiotic and aposymbiotic B. germari showed no significant differences in the wandering behaviors of the first nymphal stages, whereas the symbiont-free insects exhibited retarded growth, lower longevity, and adult body weight. Taken together, these data provide a better understanding of the relationship between the bacterial symbiont and B. germari and demonstrate that the insect is heavily affected by the deprival of its gut symbionts.

Potential Management Tactics for Pistachio Stink Bugs, Brachynema germari, Acrosternum heegeri and Acrosternum arabicum (Hemiptera: Pentatomidae): High Temperature and Chemical Surface Sterilants Leading to Symbiont Suppression.

Insect symbionts offer an opportunity to deal with the anticipated elevated demand for novel pest management strategies. One approach is the disruption of essential symbionts required by the pests. In the present study, we examined the effects of symbiont elimination strategies, high temperature and sterilization agents, on the fitness of three stink bugs, Brachynema germari Kolenati, Acrosternum heegeri Fieber, and Acrosternum arabicum Wagner by using demographic approach. In the high-temperature experiments, almost all insects exhibited severe fitness defects, including elevated nymphal mortality and reduced population growth parameters (especially intrinsic rate of increase, r), as well as significant reductions in the gut symbiont titers. In the egg surface sterilization assays, we experimentally assessed the effects of sterilization agents on the bugs and their symbionts and observed similar fitness defects to those observed under the high-temperature condition. According to the results, we concluded that the host's defective phenotypes are attributable not to the heat stress itself but to the suppression of the symbiont titer, which highlights the possibility that global warming and elevated temperature may negatively affect this mutualism. Together, the results suggest the biological importance of the bacterial symbiont for the host that might help us for better management of these important pests in the future.