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Yeast-like symbionts (YLSs) are transovarially transmitted between brown planthopper generations, but the transmission mechanism is poorly understood. In the present study, we found that YLSs do not enter the brown planthopper oocyte until it develops to late vitellogenesis. During oocyte vitellogenesis, the diversification and morphological changes of the follicular cells surrounding the oocyte are associated with the accumulation and polarization of vitellogenin (Vg) and lipids in the oocyte, indirectly affecting the entry of YLSs into the oocyte. Additionally, the posterior follicular cells, one of the diversified follicular cell populations, form the epithelial plug that is the only structure for YLSs to enter the oocyte. In addition, actin is rearranged within the follicular cells to build an actin bridge for YLSs to move from the epithelial plug to the oocyte, indicating that actin mediates the entry of YLSs into the brown planthopper oocyte during oocyte vitellogenesis. Taken together, our results demonstrate that the entry of YLSs into the brown planthopper oocyte is triggered by oocyte vitellogenesis. [ABSTRACT FROM AUTHOR]