In the silkworm, many mutations involved in the lethality at the egg stage have been reported (Banno et al., 2005; 2010). Embryonic lethality is seen in most of these mutations, such as lethal 19 (l-19; Doira et al., 1977; Ito et al., 2018), brown egg lethal (l-be; Nishikawa, 1925), blastokinesis lethal (l-bk; Umeya, 1927), lethal brown (l-br; Doira et al., 1975), lethal non-diapausing egg (l-n; Ooi, 1968), red egg lethal (l-r; Nishikawa, 1925), and lethal white-rot egg (l-w; Nakayama, 1958). In contrast, there are few lethal mutations other than at the embryonic stage; lethal 10 (l-10; Simodaira, 1947), dark-spotted egg lethal (l-d; Nishikawa, 1925), and dark-spotted egg lethal b (l-db; Nishikawa, 1930) show lethality at the head pigmentation stage, and bluish egg lethal (l-bl; Umeya, 1927) becomes lethal at body pigmentation stage. The analysis of the genes responsible for the lethal mutations will provide useful findings on insect embryology and physiology; however, most of these genes are largely unknown. We recently isolated a novel lethal mutant during the maintenance of the Sj strain. This phenotype is similar to that of the sex-linked chocolate lethal mutant (sch; ch121.5 cM; Ohnuma, 2001); the skin color of the mature embryo became a light orange, and the embryo died before hatching. In this study, we performed morphological observation and genetic analysis of the lethal mutation.
