Abstract: Symbionts universally exist in insects, not only providing host insects with nutrients such as amino acids and sterols that are necessary for their growth and development, but also improving the adaptability of insects to external adverse environmental factors such as high temperature, parasites and viruses. Insects provide a stable living environment and nutrients for symbionts. Symbionts and host insects are interdependent. In most cases, symbionts are vertically transmitted between generations of their host insects, from the mother to its offsprings. In this article, based on the relevant studies in recent years we reviewed the vertical transmission modes of different insect symbionts. Except for a small number of intestinal symbionts that can be transmitted vertically through contaminated egg shells and then eaten by host larvae, most of vertically-transmitted symbionts are transovarially transmitted. According to different infection periods, most of the transovarial transmission modes of symbionts can be divided into the following four types: infecting germline stem cells in host insect larvae, infecting germline stem cells in the young female adult of host insect, infecting mature oocytes in the female adult of host insect, and infecting blastocyst embryos of host insect. Among them, some symbionts enter the host ovary in the form of bacteriocyte. In addition, a few intestinal symbionts are also vertically transmitted through the ovary. Such symbionts firstly infect the lateral oviduct and accumulate in it, and then enter the oocyte when the oocyte is discharged into the lateral oviduct. We also explored the cellular and immune mechanisms of vertical transmission of insect symbionts, including the avoidance of host immune responses by symbionts, the entry of symbionts into ovaries by endocytosis, and the synergistic effects between different symbionts.

Key words: Insect, symbiont, transmission mode, vertical transmission, immune response, ovary