Critical time of transovarial transmission of bacteriome-associated symbionts and related molecular mechanisms in cicada Hyalessa maculaticollis.

Obligate endosymbionts of sap-sucking auchenorrhynchan insects of Hemiptera colonize the bacteriomes and are transmitted vertically through the ovaries to the offspring of host insects, but the critical time of symbiont transmission and molecular mechanisms underlying the process remain unknown. We used histological and transmission electron microscopy, 16S rDNA amplification sequencing and transcriptome analyses to explore the vertical transmission of bacteriome-associated symbionts in the cicada Hyalessa maculaticollis. We find that the symbiont Candidatus Karelsulcia muelleri (hereafter Karelsulcia) proliferates and changes shape after the adult cicadas emerged for 3 h, which is then extruded to the hemolymph from the basal membrane of bacteriome units. The yeast-like fungal symbiont (YLS) harbored in bacteriome sheath cells is released freely along with Karelsulcia. As ovaries mature, Karelsulcia and YLS infect oocytes of cicadas that had emerged for 60 h, and begin to gather at the posterior pole of oocytes, where they form a symbiont ball in each oocyte. Expressions of genes associated with cytoskeletal organization, endocytosis, amino acid transporter and lipid synthesis increase in the newly emerged adults, mediating the transport of substances during the transmission of symbionts. The amino acid-sensitive mechanistic target of the rapamycin pathway is one of the crucial pathways coordinating the vesicle-mediated symbiotic transmission. The insulin signaling pathway potentially together with insect hormones synergically regulate insect fertility and affect yolk deposition, which is closely related to the symbiont infection of ovaries. This study highlights the importance of signaling pathways in regulating the vertical transmission of symbionts in sap-feeding auchenorrhynchan insects.