Abstract: 【Aim】 The aim of this study is to clarify the effect of specific strains in the symbiotic bacterium Wolbachia on the host fitness and its molecular mechanism. 【Methods】 The 3rd generation of parthenogenetic adult Trichogramma pretiosum was continuously fed with 20 mg/mL tetracycline hydrochloride honey solution. The parasitism rate, emergence rate and proportion of females were calculated. The lg(absolute copy number) of wsp of Wolbachia in parthenogenetic adult T. pretiosum was detected by absolute quantitation qPCR (AQ-PCR) to analyze the Wolbachia titer. The Illumina platform was used to sequence the transcriptome of Wolbachia in the parthenogenetic adult T. pretiosum with the Wolbachia genome as the reference genome, and the differentially expressed genes in the transcriptome of Wolbachia in parthenogenetic adult T. pretiosum before and after tetracycline hydrochloride treatment were screened and analyzed by databases GO/KEGG/COG. RT-qPCR was used to detect the expression levels of five genes (w1260, w0535, w3585, w4570 and w2910) of Wolbachia in the parthenogenetic adult T. pretiosum. 【Results】 The parasitism rate, emergence rate, proportion of females and the lg(absolute copy number) of wsp in the parthenogenetic adult T. pretiosum in the tetracycline hydrochloride treatment group were significantly lower than those in the control group and decreased significantly with the increase of generation. A total of 28 differentially expressed genes (6 down-regulated) were identified from the transcriptome of Wolbachia in parthenogenetic adult T. pretiosum before and after tetracycline hydrochloride treatment and significantly enriched in multiple pathways related to growth and development, such as metabolic process, oxidative phosphorylation, energy production and conversion. Six down-regulated genes were aligned to three Wolbachia strains, wCIFem, wfur and wSca  by NCBI Blast. RT-qPCR verification showed that the expression trends of five differentially expressed genes (w1260, w0535, w3585, w4570 and w2910) in the parthenogenetic adults after tetracycline hydrochloride treatment were consistent with the transcriptome sequencing results.【Conclusion】 The decreased titer of Wolbachia weakens the energy supply and development efficiency of the host by inhibiting the key metabolic pathways (such as oxidative phosphorylation) and scavenging functional strains (wCIFem, wfur and wSca), resulting in a decrease in reproductive fitness. In this study, the Wolbachia genome was used as a reference genome to analyze the molecular mechanism of symbiotic bacteria-host interaction for the first time. The results showed that Wolbachia maintained symbiotic homeostasis through strain-specific gene network, revealing the molecular mechanism of different strains of Wolbachia regulating host reproductive fitness through multiple pathways, and providing theoretical support for the establishment of targeted regulation technology system based on symbiotic bacterial genome.

Key words: Trichogramma pretiosum, symbiotic bacteria; Wolbachia, transcriptome, biological characteristics, reproductive fitness