Differential expression of two RXR/ultraspiracle isoforms during the life cycle of the hemimetabolous insect Blattella germanica (Dictyoptera, Blattellidae).

In insects, the molecular basis of ecdysteroid action has been analysed in great detail in flies and moths, but rarely in primitive orders. Using the primitive hemimetabolous insect Blattella germanica, the German cockroach, as a model, we isolated two cDNAs of RXR/USP, a component of the heterodimeric ecdysone receptor. These two cDNAs correspond to two isoforms, named BgRXR-S (short form) and BgRXR-L (long form). Both are identical except for a 23-amino acid deletion/insertion located in the loop between helices H1 and H3 of the ligand-binding domain. Pattern expression studies show that the two isoforms are differentially expressed throughout the life cycle of B. germanica. During embryogenesis, BgRXR-L occurs in early embryos, whereas BgRXR-S is highly expressed in middle and late embryogenesis. In the penultimate and last larval instars, BgRXR-S mRNA is the predominant form in the fat body and in the prothoracic gland. In the adult female, BgRXR-S mRNA predominates in the fat body, whereas BgRXR-L mRNA predominates in the ovary. Experiments performed with fat body and embryo cells incubated in vitro showed that the expression of BgRXR-S and BgRXR-L is not affected by 20-hydroxyecdysone or by juvenile hormone III.