Genetic Studies on Tropical Races of Silkworm (Bombyx mori) with Special Reference to Cross Breeding Strategy between Tropical and Temperate Races

In Japan a dozen of multivoltine strains had been maintained even for a short period from the second half of the 19th century to the first stage of the next centuryi0>. It seems to be likely to suppose that these multivoltine strains would be derived from the tropical races. Some of these multivoltine strains had practically been cultured for the production of silk threads and/or the exportation of eggs. Since then, very little is known about the practical utilization of the multivoltine strains in our country. Nevertheless, several tropical multivoltine strains, Cambodge, Mysore, Pure Mysore, Annam and Ringetsu, have been maintained in the National Institute of Sericultural and Entomological Science, M.A.F.F. Recently, the tropical multivoltine races have been highlighted in our country by reason of physical nature (softness, fineness, etc.) of their products or silk threads. It should be noted here that the multivoltine silkworm strains raised in Japan cease embryonic development (or hibernation) during the unfavorable season from October to the coming April. This event and others are strongly suggestive that in general the tropical race is able to be cultured without difficulty in the temperate zone. The multivoltinism among the most representative characteristics of the tropical race is easily adaptable to some monovoltinisms, from the multivoltine to the tetra-, triand bivoltines and rarely to the univoltine, in the temperate zone. In addition, this biological event is only concerned in the phenotypic changes of the gene function controlling the multivoltinism, but that is independent from the essential changes of the gene structure. In this part, some possible mechanisms on the adaptation of the tropical multivoltine race including the strain Cambodge to the temperate climatic zone will be discussed from a viewpoint of ecogenetics.