Recent progress in silkworm genetics and genomics

The domesticated silkworm, Bombyx mori, has long served as a model for lepidopteran biology. Its relatively short, predictable life cycle, large size, and adaptation to mass rearing and laboratory culture, coupled with the high degree of fundamental knowledge needed for successful practice of sericulture, explains in part the silkworm’s use as a representative for Lepidoptera in basic research. It became an object of genetic studies early in the twentieth century (for accounts of the history of silkworm genetics and sericulture, see Eickbush 1995; Yasukochi, Fujii, and Goldsmith 2008), leading to the collection of many spontaneous mutations found in the course of mass rearing or introgressed from its nearest wild relative, Bombyx mandarina, which is present in mulberry fields in China, Japan, and Korea, and can still form partially fertile hybrids with its domesticated cousin. Major germ plasm collections in China, Japan, Korea, and India maintain hundreds of “geographic” or local strains, and genetically improved varieties that, like any agricultural breeding stock, are classified according to their economic characters, such as rearing properties and robustness, cocoon and silk qualities, fertility, and fecundity (Sohn 2003; Table 2.1). These centers also maintain various morphological, biochemical, and behavioral mutations affecting all metamorphic stages (about 430 mutations in Japan (Fujii 1998; Table 2.1) and more than 600 mutant strains in China (Lu, Dai, and Xiang 2003). The silkworm was used for irradiation studies shortly after the first report of using X-rays to mutate Drosophila by Müller in 1927 (reviewed in Tazima 1964). This early work led to the induction and selection of many morphological and behavioral mutations and chromosome aberration stocks. Among the latter are sex-linked translocations involving visible mutations affecting egg, larva, or cocoon, which were transferred from autosomes to the female W chromosome, and have been used to breed autosexing stocks for sericulture (Tazima 1964; Nagaraju 1996; Fujii et al. 2006). These aberrations have also been useful for fine structure mapping of the affected autosomes (Fujiwara and Maekawa 1994) and the W chromosome itself, which carries the putative female sex-determining gene, Fem (see Chapter 4 for a discussion of W-translocations and their application in studies of sex determination)