CLONING OF 2 PUTATIVE ECDYSTEROID RECEPTOR ISOFORMS FROM TENEBRIO-MOLITOR AND THEIR DEVELOPMENTAL EXPRESSION IN THE EPIDERMIS DURING METAMORPHOSIS

Using the Drosophila EcR-B1 cDNA as a probe, we have cloned the putati ve ecdysteroid receptor from the mealworm Tenebrio molitor. We have is olated two cDNAs with different 5' termini that contain a complete ope n reading frame. These two cDNAs encode two proteins with distinct N-t erminal regions corresponding to two isoforms. The coleopteran recepto r is obviously related to the ecdysteroid receptor of other insects, b ut shares only 89% and 61% amino acid identities with the DNA-binding and ligand-binding domains of the Drosophila receptor, respectively. I ts expression pattern has been examined in the epidermis during the la st larval instar and pupal stage of T. molitor, in correlation with th e hemolymph ecdysteroid titer. Hybridizations revealed two transcripts of 7 kb and 6.5 kb detected in most stages during metamorphosis and c orresponding to the A and B1 isoforms. These two mRNAs are highly evid ent just before the rise of each ecdysteroid peak both in prepupae and in pupae. They show almost the same expression pattern in epidermis e xcept for the second part of the pupal stager during which only the A isoform is detected.