The Long and Winding Road for Symbiont and Yolk Protein to Host Oocyte

Many insects are intimately associated with microbial symbionts, which are passed to developing oocytes in the maternal body for ensuring vertical transmission to the next generation. Previous studies uncovered that some symbionts utilize preexisting host’s molecular and cellular machineries for targeting oocytes. For example, the major yolk protein vitellogenin (Vg) is massively produced in fat body cells, processed and transported to ovaries, and incorporated into developing oocytes via Vg receptor (VgR)-mediated endocytosis, and some symbiotic bacteria were reported to interact with Vg and migrate to oocytes by hitchhiking the VgR-mediated endocytotic mechanism. In a recent study, Mao et al. (mBio 12:e01142-20, 2020, https://doi.org/10.1128/mBio.01142-20) reported that, in some leafhoppers, a considerable proportion of Vg is incorporated into symbiotic bacteria and translocated into oocytes by hitchhiking the symbiont’s vertical transmission mechanism, uncovering the host’s cooption of the symbiont’s oocyte-targeting machineries and highlighting complicated trajectories toward host-symbiont coevolution and integration.