Molecular cloning and expression pattern of an armadillo homologue from the mulberry silkworm Bombyx mori

Armadillo/beta-catenin, encoded by the segment polarity gene armadillo functions as a downstream effector of canonical Wnt signals. The expression patterns of Bm wnt-1, -Ci and -engrailed, suggested the presence of Wnt-1 network in the middle silkglands of Bombyx mori. To test this, a homologue of armadillo from B. mori (Bm arm) was cloned by PCR using degenerate primers, designed based on the conserved regions of the protein, and characterized. The cloned region harboured one complete and two incomplete Armadillo (Arm) repeat motifs and was highly conserved across species. Genomic Southern analysis suggested Bm arm to be a multi-copy gene. The expression of Bm arm RNA was first detected at stage 6 of embryogenesis, which reached maximum levels at stage 21C and was maintained until larval hatching. The RNA was expressed uniformly in the embryos, whereas the Arm protein was localized in a segmentally reiterated striped pattern, in conformity with its predicted segment polarity nature. Bm arm was also expressed in the entire silkgland and the transition from third to fourth and fifth larval intermoults was accompanied by an increase in the transcript levels. However, the Arm protein was predominantly localized to the middle silkglands, especially the middle and posterior sub-compartments. The silkglands represented a novel expression domain for arm in Bombyx, and the results were consistent with the existence of a canonical Wnt network in the middle silkglands.