The yellow gene is associated with black pigmentation in several body parts
Previous studies focused mainly on pigmentation in fifth‐instar larvae and adults (Table 1). It was, therefore, unknown whether these observations were applicable to first‐instar (embryonic) larvae. To confirm the previous reports, we used a Δyellow mutant of B. mori (k12), which exhibits a splicing deficiency in the yellow gene (Futahashi et al., 2008). We compared wild‐type and Δyellow silkmoth larvae at three embryonic stages to determine which pigmented body parts require the activity of the yellow gene (Fig. 1). In the earliest stage, there were no differences between wild‐type and Δyellow silkmoth larvae (Fig. 1A, D). We then observed that as the embryonic stages progressed, not only did the differences between them increase, but black pigmentation was absent in the trachea in Δyellow larvae (Fig. 1B, B’, E, E’). In the next embryonic stage, we found that black pigmentation was absent in the head capsule (Fig. 1C, F), the hair and the thoracic legs (Fig. 1C, C’, F, F’) in Δyellow larvae. In summary, we found that several body parts of B. mori are pigmented through the effects of the yellow gene.
Figure 1  Melanin pigmentation in Bombyx mori embryonic larvae. (A–C) B. mori wild‐type (WT) yellow (p50 strain). (D–F) B. mori mutant yellow (k12 strain). Each species is presented in three developmental stages [stage 1 (A, D); stage 2 (B, B’, E, E’); stage 3 (C, C’, F, F’)]. B’ and E’ show the trachea magnified from B and E. C’ and F’ show the hair (left) and thoracic legs (right) magnified from C and F. Development proceeds in order from stage 1 to 2 to 3. Dotted red lines indicate the position slightly below the tracheas (B’, E’; note the tracheas in B and E are not illustrated by colours). Arrowheads indicate one of the hairs (white arrowheads in C and F; black arrowheads in C’ and F’). Arrows indicate the thoracic legs (white arrows in C and F; black arrows in C’ and F’). Asterisks indicate the head capsules (C, F). (A–F) Scale bars = 500 µm. (B’, C’, E’, F’) Scale bars = 125 µm.