Vertebrate estrogen regulates the development of female characteristics in silkworm, Bombyx mori.

The vertebrate estrogens include 17-ß-estradiol (E2), which has an analog in silkworm ovaries. In this study, the Bombyx mori vitellogenin gene (BmVg) was used as a biomarker to analyze the function of the E2 in silkworm. In most oviparous animals, Vg has female-specific expression. However, BmVg expression was also detected in B. mori males. Stage specific fluctuation of BmVg expression was similar in males and females, but expression levels in males were lower than in females. E2 treatment by injection or feeding of male larvae in the final instar stage induced and stimulated male BmVg transcription and protein synthesis. When silkworm ovary primordia were transplanted into males, BmVg was induced in male fat bodies. Transplanted ovaries primordia were also able to develop into ovaries and produce mature eggs. When females were treated with E2 promoted BmVg/BmVn protein accumulation in hemolymph, ovaries and eggs. However, BmVg transcription was decreased in female fat bodies. An E2 analog was identified in the hemolymph of day 3 wandering silkworms using high-performance liquid chromatography. Estradiol titers from fifth late-instar larvae to pupal stage were determined by enzyme-linked immunosorbent assay. The results suggested that silkworms synthesized a vertebrate E2 analog. This study found that E2 promoted the synthesis of BmVg, a female typical protein in silkworms.

Female qualities in males: vitellogenin synthesis induced by ovary transplants into the male silkworm, Bombyx mori.

Female qualities in males are common in vertebrates but have not been extensively reported in insects. Vitellogenin (Vg) is highly expressed in the female fat body and is generally required for the formation of yolk proteins in the insect egg. Vg upregulation is generally regarded as a female quality in female oviparous animals. In this study, we found that Bombyx mori Vg (BmVg) is especially highly expressed in the female pupa. Downregulation of the BmVg gene in the female pupa by RNA interference (RNAi) interfered with egg formation and embryonic development, showing the importance of BmVg in these processes. So, we used BmVg as a biomarker for female qualities in the silkworm. Hematoxylin-eosin staining and immunofluorescence histochemistry showed that ovary transplants induced BmVg synthesis in the male pupa fat body. Ovaries transplanted into male silkworms produced only a few eggs with deformed yolk granules. These results suggested that the amount of BmVg in the male silkworm was insufficient for eggs to undergo complete embryonic development. After 17-beta-estradiol was used to treat male pupae and male pupal fat bodies, BmVg was upregulated in vivo and in vitro. These findings indicated that the male silkworm has innate female qualities that were induced by a transplanted ovary and 17ß-estradiol. However, in silkworms, female qualities in males are not as complete as in females.

The Broad Complex isoform 2 (BrC-Z2) transcriptional factor plays a critical role in vitellogenin transcription in the silkworm Bombyx mori.

BACKGROUND: Vitellogenin (Vg) is synthesized in the fat body of the female silkworm Bombyx mori and transported to the oocyte as a source of nutrition for embryo development. It is well known that ecdysone regulates physiological, developmental and behavioral events in silkworm. However, it is still not clear how the ecdysone regulates B. mori Vg (BmVg) transcription. METHODS: Electrophoretic mobility shift assay (EMSA) and cell transfection assay were used to reveal whether BmBrC-Z2 is involved in regulating BmVg transcription. RNAi was employed to illustrate the function of BmBrC-Z2 in the silkworm egg formation and development. RESULTS: (1) The transcription of BmVg can be induced by ecdysone in the female fat body. (2) Three putative BrC-Z2 cis-response elements were mapped to regions flanking the BmVg gene. (3) BmBrC-Z2 required direct binding to the cis-response elements on the BmVg promoter. (4) Over-expression of three BmBrC isoforms in the cell line showed that only BmBrC-Z2 could induce the BmVg promoter activity. (5) RNA interference (RNAi) of BmBrC-Z2 in female remarkably reduced BmVg synthesis and led to destructive affection on egg formation. The dsRNA of BmBrC-Z2 treated moths laid fewer and whiter eggs compared to the control. CONCLUSIONS: BmBrC-Z2 transported the ecdysone signal then regulated BmVg transcription directly to control vitellogenesis and egg formation in the silkworm. GENERAL SIGNIFICANCE: The results of this study revealed that BmBrC-Z2 as a key factor to mediate ecdysone regulates reproduction in the silkworm.