Coevolution and Functional Effects of Endosymbiotic <i>Rickettsia</i> in <i>Leptocybe invasa</i> Fisher &amp; <scp>LaSalle</scp> (Hymenoptera: Eulophidae) Across China

<i>Rickettsia</i> is an endosymbiotic bacterium that infects various arthropods, affecting the host's biology, ecology, and evolution. <i>Leptocybe invasa</i> is an invasive pest that severely damages eucalyptus plants. A comprehensive investigation of <i>Rickettsia</i> in 313 female <i>L. invasa</i> individuals from 17 Chinese populations revealed a 100% infection prevalence. Sequencing of three host molecular markers-mitochondrial <i>COI</i>, nuclear <i>ITS</i>, and <i>28S</i>-led to the identification of a novel <i>L. invasa</i> haplotype, designated Haplotype 1 × 2, which exhibits mito-nuclear discordance. Concurrently, sequencing of four <i>Rickettsia</i> genes (<i>16S</i> rRNA, <i>gltA</i>, <i>atpA</i>, <i>rpmE</i>) revealed two distinct strains, termed STRiA and STRiB. These strains demonstrated a specific association with the host lineages, where STRiA was exclusively associated with lineage A (comprising Haplotype 1 and Haplotype 1 × 2), and STRiB was linked to lineage B. Phylogenetic analysis of the multigene datasets from both the host and <i>Rickettsia</i> revealed a high degree of topological congruence between their inferred trees. Correlation analysis further demonstrated a moderate positive association (<i>r</i> = 0.307). The significance of this relationship was supported by a Mantel test (<i>p</i> < 0.005), suggesting coevolution. Low-dose tetracycline treatment effectively eliminated <i>Rickettsia</i> from <i>L. invasa</i>. <i>L. invasa</i> treated with tetracycline exhibited a significantly higher proportion of male offspring, reduced <i>Rickettsia</i> expression, and decreased body length and lifespan in female offspring. Transcriptome analysis comparing <i>Rickettsia</i>-free and <i>Rickettsia</i>-infected <i>L. invasa</i> following antibiotic treatment identified 178 differentially expressed genes (122 up-regulated, 56 down-regulated). These genes were enriched in GO terms related to metabolic processes, cellular processes, cellular components, binding functions, and catalytic activities. KEGG pathway analysis revealed enrichment of differentially expressed genes primarily in metabolic pathways, insect hormone biosynthesis, and thermogenesis. Additionally, enrichment was observed in key signaling pathways, including Ras, MAPK, NF-κB, TGF-β, TNF, and Apelin. These findings elucidate the coevolutionary relationship and functional roles of <i>Rickettsia</i> in <i>L. invasa</i>, providing a foundation for symbiont-mediated biological control.