Genetic studies on the novel lethal mutant, light orange lethal, in the silkworm, Bombyx mori

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.