Expression of ecdysone receptor isoforms and trehalase in the anterior silk gland ofBombyx mori during an extra larval molt and precocious pupation induced by 20-hydroxyecdysone administration

Abstract

When newly molted 5th (last) instar larvae of Bombyx mori were fed an artificial diet containing 20-hydroxyecdysone (20E), an extra larval molt was induced and the cuticle layer of the anterior silk gland was renewed. In contrast, feeding 20E to 5th instar larvae allatectomized in the mid-4th instar induced pupational responses and anterior silk gland degeneration. Topical application of a juvenile hormone analogue, fenoxycarb, to allatectomized larvae before 20E treatment nullified the effects of allatectomy and induced an extra molt. In the two cases where an extra larval molt was induced, mRNA expression of the ecdysone receptor A isoform (EcR-A) preceded that of the B1 isoform (EcR-B1) in the anterior silk gland, while EcR-A and EcR-B1 were expressed synchronously during precocious pupation. Precedence of EcR-A over EcR-B1 and the synchronization of both are expression patterns observed during the normal 4th and 5th instars. These results suggest that temporal expression profiles of EcR isoforms are important for regulating stage-specific responses of this tissue to ecdysteroids. Trehalase was also identified as a metamorphosis-specific gene in the anterior silk gland by mRNA differential display between the extra larval molt and precocious pupation. During normal 4th and 5th instar, trehalase mRNA was expressed at the end of the 5th instar only. Thus, extra larval molt and precocious pupation induced by dietary application of 20E mimic normal larval molt and pupation well and can be a useful tool in isolating stage-specific ecdysteroid-regulated genes. Arch. Insect Biochem. Physiol. 41:79–88, 1999. © 1999 Wiley-Liss, Inc.