Genetics of the silkworm: revisiting an ancient model system

Introduction The earliest known silk textile is nearly five thousand years old (Kuhn, 1988). The condition and weave of the fiber indicate that the insect that produced it, Bombyx mori , was already domesticated, implying that it had long been subjected to inbreeding and selection. In modern times, Bombyx has been used as a model for genetic studies since the birth of genetics as a formal science in the early 1900s. As early as 1905, Toyama, one of the founders of silkworm genetics, was breeding genetic hybrids between Thai and Japanese silkworms for improved vigor and silk production (Yokoyama, 1968). He first reported discovery of a chorion mutation that affects the shape and transparency of the eggshell in 1910 (Tazima, 1964), the same year as the publication of Morgan's famous white-eyed mutant of Drosophila melanogaster . Japanese geneticists maintained active research into the fundamental principles of genetics, keeping pace with the field and often leading with early reports of such phenomena as dominance (Toyama, 1906, cited in Tazima, 1964), sex linkage (Tanaka, 1914, cited in Sturtevant, 1915), maternal inheritance (Toyama, 1912), homeotic mutants (Suzuki, 1929, Sasaki, 1930, and Suzuki and Ohta, 1930 – all cited in Tazima, 1964), enzyme polymorphisms (Matsumura, 1934, cited in Kikkawa, 1953), and unstable mutations (Hatamura, 1939). Although studies with many Lepidoptera have made important contributions to genetics, today, with more than two hundred mutations mapped, the silkworm stands as the only member of this taxonomic group whose genetic system is well-enough established to consider adopting it as a molecular genetic model for solving a broad range of fundamental biological problems.