Fig. 1.

Cocoons from Gb strains contain 5-O-glucosylated quercetins that emit bright yellow fluorescence under UV light. (A) Silkworm cocoons of various colors. Green color is produced from flavonoids (arrows), whereas other colors, yellow, orange and pink, are from carotenoids. (B) Cocoons from several silkworm strains under white light (upper) and UV-B (302 nm) (lower). Strain names and their presumptive genotypes (in parentheses) are indicated above. Cocoons emit yellow fluorescence in the presence of the Gb allele (detailed in Fig. S4). (C) The structural formula of quercetin. (D) Quercetin emits yellow fluorescence when its 5-O position is glucosylated (Q-5-G), but does not do so when other positions, 3-O, 7-O, 3′-O or 4′-O, are glucosylated (Q-3-G, Q-7-G, Q-3′-G, or Q-4′-G, respectively). Fluorescence was also observed in quercetin 5,4′-diglucoside (Q-5,4′-G), the major constituent of the cocoon flavonoids in strain 939 (Gb/+) (Table S1). Concentration of quercetin glucosides in each vial was 100 nmol/mL.