Date Published: June 13, 2019
Publisher: Public Library of Science
Author(s): Minhyung Jung, Doo-Hyung Lee, Maohua Chen.
Riptortus pedestris is a major agricultural pest on leguminous plants in South Korea and Japan. Recent studies have revealed that R. pedestris can form beneficial symbiosis with bacteria belonging to genus Burkholderia acquired from soil newly for every generation. Although their physiological interactions are relatively well-understood, infection rate and abundance of the Burkholderia in overwintering natural populations of R. pedestris remain unknown. Therefore, the objective of this study was to characterize Burkholderia infection ratio and clade composition of overwintering R. pedestris populations as well as prevalence and diversity of the genus Burkholderia in soil by conducting a two-year field survey. From the field survey, we found 29 overwintering R. pedestris adults in forested areas nearby soybean fields. Diagnostic PCR analysis revealed that overall infection rate of the symbiotic Burkholderia was 93.1% from overwintering adults. Among the Burkholderia-infected R. pedestris, 70.4% of individuals harbored unclassified Burkholderia clades whereas 22.2% and 7.4% of R. pedestris harbor stinkbug-associated beneficial and environmental (SBE) group and Burkholderia cepacia and complex (BCC), respectively. All R. pedestris were infected with a single clade of Burkholderia. In soil, 56.2% of soil samples were Burkholderia positive, and unlike R. pedestris, multiple Burkholderia clades were detected from 62.2% of those samples. Clade composition of the genus Burkholderia in the samples with the bacteria was 91.1%, 60.0%, 31.1% and 8.8% for plant-associated beneficial and environment (PBE), BCC, SBE and unclassified clade, respectively.
During winter season, insects face thermally stressful environments. They have developed various strategies to successfully withstand and survive inhospitable environments through evolutionary interactions with organisms and abiotic conditions [1,2]. Their physiological strategies for cold hardness can be classified into two groups: freeze tolerant (synthesis of ice nucleating agents) and freeze avoidance (removal of all potential nucleates) [2–5]. In addition, insects can behaviorally avoid inhospitable environments through long or short distance movement to less stressful conditions [2,6,7].
Infection rate and clade composition of symbiotic Burkholderia in R. pedestris field populations provides critical information to better design and implement pest management programs against this economically-important pest. Although it is currently unknown how symbionts affect the biology of overwintering R. pedestris, it is expected that higher infection rates in overwintering populations would impose higher risk to crops by survived pests in the spring. This is because symbiotic Burkholderia are known to provide a series of enhancement for R. pedestris fitness, including larger body size, increased fecundity, and enhanced insecticide resistance [24,25,28]. For this reason, this study investigated infection rate of Burkholderia in overwintering R. pedestris populations in nature for the first time. In this study, DNA sample was extracted from the whole abdomen of overwintering R. pedestris, which might have resulted in detection of Burkholderia from gut sections other than the crypt-bearing M4 region, where the symbiont is known to colonize in the insect. Nevertheless, overwintering individuals generally clear their gut lumen to prevent freeze damage because the particles in gut lumen can act as ice nucleators, reducing the chance of detecting the symbionts from other gut sections .