Kyushu University and National institute of Agrobiological Science of Japan (NIAS) function as center for silkworm genetic resources in Japan. The genetical resources of NIAS include many kinds of mutants and pure lines derived from various regions and nations. On the other hand, Kyushu University completely focuses on the conservation for genetic mutants, and the number of those about genetical mutants is larger than those of NIAS. It seems that we can see more than 90 percentages of mutants now being in the world in about 460 strains maintain at Kyushu University. Besides of these two institutes, Hokkaido University, Shinshu University, Tokyo University of Agriculture and Technology, University of Tokyo, Kyoto Institute of Technology, Nagoya University, Institute of Sericulture and so on make an effort to maintain peculiar mutants, respectively.
Closed line culture system is employed in stock maintenance, because difficulties due to inbreeding depression are encountered. Usually, for maintenance of one line culture, three hundred larvae derived from 15-30 batched eggs are rearing in spring, when best season for rearing. In northern area of Kyushu island where Kyushu University locates, it is possible to rear silkworm from middle of April to middle of November. Thus long available period for rearing is enable to culture five generations in one year. This is advantage for genetical experiments. In main island of Japan, Honshu island, rearing season is shorter than Kyushu island. The genetical traits (characteristics) of each strain are checking at all developmental stages by expert staffs, especially it is doing strictly at last instar larval stage and pupal stage to prevent genetical contamination.
So far, method of long-term preservation more than one year has not been successful in this insect. For maintenance of silkworm strain, it is therefore necessary to rear each strain year after year. When users want to get some mutant for research or culture, it is necessary to order delivery of eggs before the start of spring rearing. Because, rearing for maintenance is usually done only one time in spring.
Recently two original methods for silkworm transformation have developed (Yamao et al., 1999; Tamura et al., 2000), and then transgenic lines by using those methods are accumulated rapidly. It is serious problems to maintain new transgenic line, because rearing is hard work. As the means of solving this matter, it is noteworthy the artificial insemination using a cryopreserved sperm established by Takemura et al. (1999, 2000).