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The Japanese common broad-headed bugs Riptortus clavatus and Leptocorisa chinensis possess a number of crypts in the posterior region of the midgut, whose lumen contains a copious amount of bacterial cells. We characterized the gut symbiotic bacteria by using molecular phylogenetic analysis, light and electron microscopy, in situ hybridization, and PCR-based detection techniques. Restriction fragment length polymorphism analysis of 16S rRNA gene clones suggested that a single bacterium dominated the microbiota in the crypts of the both bug species. The predominant 16S rRNA gene sequences obtained from different individuals and species of the bugs were not identical but were very similar to each other. Homology searches in the DNA databases revealed that the sequences showed the highest levels of similarity (96% to 99%) to the sequences of Burkholderia spp. belonging to the beta subdivision of the class Proteobacteria. In situ hybridization with specific oligonucleotide probes confirmed the localization of the Burkholderia symbiont in the lumen of the midgut crypts. Electron microscopy showed that the lumen of the crypts was filled with rod-shaped bacteria of a single morphotype. Molecular phylogenetic analysis demonstrated that the Burkholderia symbionts of the bugs formed a well-defined monophyletic group, although the group also contained several environmental Burkholderia strains. The phylogenetic relationship of the Burkholderia symbionts did not reflect the relationship of the host bug species at all. The sequences from R. clavatus and the sequences from L. chinensis did not form clades but were intermingled in the phylogeny, suggesting that horizontal transmission of the symbiont might have occasionally occurred between populations and species of the bugs.
Kikuchi et al. (Fri,) studied this question.