A close relationship between androgen levels and eumelanogenesis in the teleost red seabream (Pagrus major): Quantitative analysis of its seasonal variation and effects of oral treatment with methyl-testosterone

摘要

Male and female teleost seabream (Pagrus major) were examined for seasonal variation of eumelanin, pheomelanin, 11-ketotestosterone (11KT, fish androgen), lightness (L* value) and Gonad Somatic Index (GSI: gonad mass / body mass × 100). In males, levels of pyrrole-2,3,5-tricarboxylic acid (a marker of eumelanin), 11KT and the GSI increased sharply from September and plateaued in March and April when the fish are sexually mature. These results are consistent with the lightness of their body color. Using the data from males, a high correlation was observed for all combinations of those four variables (PTCA, 11KT, lightness and GSI). In females, little change was observed in those variables except for the GSI. 4-Amino-3-hydroxyphenylalanine (a marker of pheomelanin) was also analyzed, but it was below the detection limit at all times. Oral treatment of juvenile red seabream with synthetic androgen methyl-testosterone for 2 months induced eumelanin accumulation about 3 times higher than the control. These data show that there is a close relationship between androgen levels and eumelanin accumulation in teleosts. This is the first report that androgen affects melanin accumulation in a dose-dependent manner.