Abstract: 【Aim】 This study aims to further understand the symbiotic relationship between auchenorrhynchan insects of the order Hemiptera and endosymbionts by investigating the diversity and functions of obligate and facultative symbiotic bacteria in bacteriomes, fat bodies and other related tissues of adult Tettigetta isshikii at the ultrastructural and genomic levels. 【Methods】 Field-collected female and male adults of T. isshikii were investigated to clarify the distribution of symbiotic bacteria Karelsulcia, Hodgkinia and Wolbachia in the bacteriomes, fat bodies, ovaries, spermathecae, salivary glands, conical segment, filter chamber and gut of female adults, and testes of male adults through transmission electron microscopy and fluorescence in situ hybridization. Metagenomic sequencing, assembly and functions by genome annotation of symbiotic bacteria in the bacteriomes and fat bodies of female adults, and testes of male adults of T. isshikii were conducted. The phylogenetic relationships of Wolbachia in T. isshikii and other insects were determined using maximum-likelihood and Bayesian inference methods to determine the phylogenetic position of Wolbachia. 【Results】 The obligate symbiotic bacteria Karelsulcia and Hodgkinia are harbored in the bacteriomes of female adults of T. isshikii, and the facultative symbiotic bacterium Wolbachia, belonging to the supergroup F, was harbored not only in the cytoplasm and nuclei of the epithelial cells of testicular follicles but also in the nuclei of sperms of male adults and fat bodies of female adults. Genome annotation analysis revealed that genes of Karelsulcia and Hodgkinia were involved in the synthesis of essential amino acids and vitamins for the host cicada, while genes of Wolbachia were involved in riboflavin metabolism, heme pathway, and biosynthesis of purine and pyrimidine. The duplication of Wolbachia occured within the nuclei of sperms, which ultimately may lead to the rupture of nuclei. Phylogenetic relationship revealed that this Wolbachia belongs to the F supergroup and was closely related to the Wolbachia harbored in other arthropods and nematodes. 【Conclusion】 This study clarified the potential nutritional functions of symbionts Karelsulcia and Hodgkinia in the bacteriomes of T. isshikii adults, and elucidated a unique phenomenon that Wolbachia may have both beneficial and detrimental effects for the host insects. The results of this study contribute to a further understanding of the symbiotic relationship and complex co-evolution between Cicadidae and symbiotic bacteria.

Key words:  Tettigetta isshikii, sap-feeding insects, symbiotic bacteria, symbiotic relationship, genome reduction