In 1995, a new brown egg mutant was discovered in one race of the domestic silkworm, Bombyx mori, which has been held at the National Institute of Agrobiological Science. The mutant resources were maintained in recessive genes and showed typical maternal inheritance which had been reported in the mutant, b-1. Complementation tests revealed that the mutant gene shared an allele of the well-described brown egg mutant, b-2. Hence, the newly discovered gene was named b-2s , and mapped on the 6th chromosome (29.1 cM). On the other hand, in the reciprocal cross experiment between b-2s and b-2na, F1 and F2 moths produced only brown eggs, while F3 moths produced normal egg color batches separated from the mutant batches. The segregation of the normal eggs may be derived from a recombination of the genes. The recombination rate of between b-2s and b-2na was calculated at approximately 1.85 cM.
An unknown organ was not natural detected in the 2nd abdominal segment of the silkworm larva. We examined this organ anatomically. The organ was round and located near the dorsal vessel and under a crescent mark of the integument. The organ in the male larva contained cells like spermatocytes and bundles of sperms. An oviduct-like tube, on the other hand, was included in the female organ. From these results, we concluded that the organ was a gonad that developed abnormally in the 2nd abdominal segment.
We investigated the effects of day length on the diapause ratio in embryos and larvae of the tropical multivoltine silkworm, Bombyx mori. Our results show that the incubation of eggs at a high temperature (28°C) under a photoperiod of 24L : 0D effectively induced diapause-egg production. We also observed that the incubation affected the hatchability of eggs. Finally, we discovered that low hatchability and diapause ratios were improved by exposing eggs to more moderate temperatures (25°C) under a photoperiod of 16L : 8D for one day before hatching.
A balanced sex-limited lethal (BL) strain was bred from the sex-limited Moricaud for the purpose of setting up a male-raising system, since males excel against females in the silk manufacturing efficiency and cocoon quality. This BL strain was backcrossed using the W7 strain (Chinese race) with good economical characters, reconstructed to recover the balanced lethality and selected for the economical characters comparable to the level of W7 strain. Then, males of thus improved BL strain were crossed with females of a Japanese race to prepare a hybrid, which was committed to the rearing test in a farmhouse. The hybrid exhibited, in the summer rearing season of 2004, remarkable records in terms of cocoon shell percentage, cocoon shell weight, cocoon filament length and so on. On the basis of these results, we concluded that a new BL strain was established and named this “PLATINA BOY.”