THE SILKMOTH HOMOLOG-ASTERISK OF THE DROSOPHILA ECDYSONE RECEPTOR (B1ISOFORM) - CLONING AND ANALYSIS OF EXPRESSION DURING FOLLICULAR CELL-DIFFERENTIATION

To understand the role that 20-hydroxy-ecdysone (20E) plays during ova rian development in Bombyx mori, we have undertaken the cloning of the silkworm ecdysone receptor (EcR) and a study of its expression during follicular cell differentiation. We have cloned a cDNA that contains a complete open reading frame for a 68.1 kDa polypeptide that shares e xtensive similarities with the B1 isoform of the Drosophila EcR, The p resumed silkmoth EcR (BmEcR) is encoded by a single copy gene whose le ngth is in excess of 23 kb. A portion of this gene encompassing seven exons that constitute the cloned BmEcR cDNA was also characterized. Em ployment of monoclonal antibodies, directed against the DNA binding do main of the Drosophila EcR, in Western blot analyses revealed the pres ence of a major 70 kDa polypeptide in extracts of follicular cells and other silkmoth tissues. The mRNA and protein encoded by BmEcR are pre sent in constant amounts in follicular cells throughout vitellogenesis but disappear transiently at the onset of choriogenesis and reappear during the later stages of choriogenesis. The down-regulation of BmEcR in follicular cells during oogenesis suggests a complex relationship between 20E, the induction of the program of chorion gene expression i n follicular cells during mid-vitellogenesis and the execution of this program at the end of vitellogenesis.