Embryonic silk gland development in Bombyx: molecular cloning and expression of the Bombyx trachealess gene

We describe embryonic development of the Bombyx silk gland. To extend the a nalysis further we isolated a Bombyx counterpart gene of the Drosophila tra chealess (trh) gene. Bombyx trh encodes a protein of 849 amino acids. When compared with the amino acid sequence of Drosophila trh, the identity of Bo mbyx bHLH, PAS-A and PAS-B domains is 100%, 97%, and 80%, respectively. Nor thern blot analysis revealed the presence of a single Bombyx trh transcript of 5.4 kb. We analyzed the expression pattern of the Bombyx trh transcript during embryogenesis by in situ hybridization. Bombyx trh mRNA was first d etected in the tracheal primordial cells at around embryonic stage 18. Ther eafter levels of Bombyx trh mRNA increased, and the high expression level w as maintained until hatching. At embryonic stage 19 the transcript was also detected in the posterior basal region of the labial segment from where th e silk gland invaginates. By the blastokinesis stage (around stage 23), the silk gland was lengthened, and, interestingly, the Bombyx trh transcript w as restricted to the anterior silk gland. These results suggest that Bombyx trh plays a role in the formation of the trachea and the anterior silk gla nds.