Involvement of the Bombyx Scr gene in development of the embryonic silk gland.

Homeotic selector genes determine the identity of each segment and induce the differentiation of segment-specific organs. To analyze how the silk glands of the lepidopteran, Bombyx mori, develop, we cloned and identified two genes that encode the homeodomain and its flanking regions identical to the corresponding regions of Deformed and Sex combs reduced. Using in situ hybridization and immunohistochemistry, we analyzed the expression patterns of these genes during Bombyx embryogenesis. Bombyx Deformed is expressed in the mandibular and maxillary segments, whereas expression of Bombyx Sex combs reduced is first limited to the labial segment and at later stages extended to the anterior part of the prothoracic segment. The expression of Bombyx Sex combs reduced then disappears from the invaginating placodes of silk glands where expression of Bombyx fork head/SGF-1 follows. In the Nc/Nc mutant embryos, which lack the 3' end region of Bombyx Antennapedia, in addition to the expression in the labial segment, the Bombyx Sex combs reduced is expressed ectopically in the thoracic and abdominal regions, and Bombyx fork head/SGF-1 is also ectopically expressed in the T1, T2, and T3 segments, resulting the ectopic induction of the silk gland invaginations. These results suggest that Bombyx homeobox genes such as the Bombyx Deformed and Sex combs reduced are associated with determination of the segment identities and Bombyx Sex combs reduced is involved in the induction of silk gland development.