Many insects are naturally infected with symbiotic bacteria, which are typically transmitted vertically from mother to offspring but are not transmitted horizontally. Understanding the effects of these symbionts is important in terms of insect pest management as they can significantly affect the biology and reproduction of insects.

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The predatory mirid bug, Nesidiocoris tenuis, which preys on agricultural pests such as whiteflies and thrips, is an important biological control agent. Although the symbiotic bacterium Rickettsia is often found in N. tenuis, its effects on the host have not been clarified.

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A research team led by NARO and the University of Miyazaki has revealed that Rickettsia induces strong cytoplasmic incompatibility (CI) in N. tenuis. CI is a phenomenon where eggs laid by uninfected females fail to hatch when mated with infected males. In this study, mating experiments using Rickettsia-infected insects and antibiotic-treated uninfected insects demonstrated that eggs did not hatch, specifically in the combination of CI. This study newly adds Rickettsia to the list of CI-inducing symbiotic bacteria.

Horizontal gene transfer

The genome analysis revealed that the Rickettsia strain in N. tenuis is closely related to the Bellii group, a group of symbiotic Rickettsia commonly found in insects. Notably, on the plasmids of this Rickettsia strain, there existed homologs of the CI factor gene (cif gene), which is known as the causal gene of CI in Wolbachia, another symbiotic bacterium known to induce CI. This suggests that the ability to induce CI may have been acquired through horizontal gene transfer between Wolbachia and Rickettsia, providing insights into the evolution of symbiont-induced reproductive manipulation in insects.

This discovery has significant implications for biological control which involves the reproduction of predatory insects that are used for pest management. If predatory insects do not reproduce as expected, CI could be the cause. Therefore, managing the infection status of symbiotic bacteria in the predatory insects could contribute to effective pest management in agriculture.

Predatory insects

The research team highlighted the importance of assessing the frequency of CI in wild populations of N. tenuis. They also noted, “The wide distribution of N. tenuis and related species across Europe, Asia, and other regions offers potential for better use of predatory insects in agriculture and to explore the evolutionary origins of CI.”

Future research will focus on the mechanism of Rickettsia-induced CI, which is important for the effective management of N. tenuis as a biological control agent, as well as for a better understanding of host manipulation by symbiotic bacteria.

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