RNAi-Mediated Functional Analysis Reveals the Regulation of Oocyte Vitellogenesis by Ecdysone Signaling in Two Coleoptera Species

SIMPLE SUMMARY: In Coleoptera, successful female reproduction partially relies on effective vitellogenesis, characterized by vitellogenin (Vg) synthesis in the fat body, secretion into hemolymphs, transport through intercellular channels in follicular epithelium, endocytosis mediated by Vg receptor (VgR), and absorption and storage by mature oocytes. In two representative Coleoptera species, Leptinotarsa decemlineata (Chrysomelidae) and Henosepilachna vigintioctopunctata (Coccinellidae), we performed RNA interference targeting ecdysone receptor (EcR) or ultraspiracle (usp) genes. Depletion of the expression level of EcR or usp inhibited oocyte development and dramatically repressed the transcription of Vg in fat bodies. Our findings indicate that 20E signaling plays an indispensable role in the stimulation of Vg synthesis and uptake in the two beetle species. ABSTRACT: Coleoptera is the largest taxa of animals by far. The robust reproductive capacity is one of the main reasons for such domination. Successful female reproduction partially relies on effective vitellogenesis. However, the hormone regulation of vitellogenesis remains to be explored. In the present paper, in vitro culture of Leptinotarsa decemlineata 1-day-old adult fat bodies in the 20E-contained median did not activate juvenile hormone production and insulin-like peptide pathways, but significantly stimulated the expression of two LdVg genes, in a cycloheximide-dependent pattern. In vivo RNA interference (RNAi) of either ecdysone receptor (LdEcR) or ultraspiracle (Ldusp) by injection of corresponding dsRNA into 1-day-old female adults inhibited oocyte development, dramatically repressed the transcription of LdVg genes in fat bodies and of LdVgR in ovaries; application of JH into the LdEcR or Ldusp RNAi L. decemlineata females did not restore the oocyte development, partially rescued the decreased LdVg mRNA levels but over-compensated LdVgR expression levels. The same RNAi experiments were performed in another Coleoptera species, Henosepilachna vigintioctopunctata. Little yolk substances were seen in the misshapen oocytes in the HvEcR or Hvusp RNAi ovaries, in contrast to larger amounts of yolk granules in the normal oocytes. Correspondingly, the transcript levels of HvVg in the fat bodies and ovaries decreased significantly in the HvEcR and Hvusp RNAi samples. Our results here show that 20E signaling is indispensable in the activation of vitellogenesis in the developing oocytes of the two beetle species.