Introduction

The body of an insect is constructed in segmental fashion, with segmented subdivisions of the head, a set of mouthparts, the thorax, the abdomen, and the terminal region (Snodgrass, 1935; Anderson, 1973). In early embryogenesis, after determination of the anteroposterior and dorsoventral axes of the embryo, the segments of the mouthparts, the thorax, and the abdomen are specified and formed, acquiring their individual identities (Sander, 1976).

The specific identities of body segments are known to be determined by homeotic genes in many kinds of insects (Ouweneel, 1976). In the silkworm, Bombyx mori, for instance, a homeotic gene complex (the E complex) that specifies the identities of body segments has long been known (Hashimoto, 1941). In other insects, notably Drosophila melanogaster and the flour beetle Tribolium castaneum, homeotic genes are also clustered. In Drosophila the Antennapedia complex (ANT-C) (Wakimoto and Kaufman, 1981) and the bithorax complex (BX-C) (Lewis, 1978), which are located on the right arm of the third chromosome, specify identities of the body segments. The ANT-C determines identities from the head to the mesothoracic segment, and the BX-C determines identities of the metathoracic segment and all the abdominal segments. In Tribolium, similar homeotic genes are known to determine identities of the body segments, where Beeman demonstrated six clustered homeotic genes in the second linkage group (Beeman, 1987). These loci include elements with apparent homology to the homeotic genes in the ANT-C and BX-C in Drosophila.

We are studying the molecular structure and function of the homeotic genes in Bombyx. These genes specify identities of the segments during early embryogenesis. The silkworm is unique among the lepidoptera because its developmental processes can be studied by genetic analysis.