Juvenile hormone control of larval colouration in the silkworm, Bombyx mori: Characterization and determination of epidermal brown colour induced by the hormone

Abstract

Larval colouration in the silkworm, Bombyx mori, is under hormonal control. Juvenile hormone injection at certain critical times during the moulting cycle induces a colour change in larval epidermis from greyish white to yellowish brown at ecdysis. This epidermal brown pigment was isolated from larvae injected with the juvenile hormone analogue (JHA) methoprene. Based on redox behaviour, visible and near ultraviolet absorption spectra and R f values after thin layer chromatography in various solvents, the brown pigment was identified as xanthommatin. Epidermal xanthommatin was assayed by the use of high pressure liquid chromatography (HPLC). 3-Hydroxykynurenine, the direct precursor of the pigment, and other epidermal pigments such as sepialumazine and riboflavin were also determined by HPLC. The higher the dose of injected hormone, the greater were the quantities of xanthommatin and 3-hydroxykynurenine found in larval integument. However, the amounts of sepialumazine and riboflavin were little affected by the hormone. These facts indicate that JHA acts on tryptophan metabolism and that xanthommatin is yet another pigment which causes characteristic colouration of silkworm larval integument.