Adult-type pigment cells, which color the ocular sides of flounders at metamorphosis, localize as precursor cells at the proximal parts of the dorsal and anal fins in early larvae

Flounders form left-right asymmetry in body coloration during metamorphosis through differentiation of adult-type melanophores and xanthophores on the ocular side. As the first step in investigating the formation of flounder body coloration asymmetry, in this study, we aimed to determine where the precursors of adult-type chromatophores distribute in larvae before metamorphosis. In Paralichthys olivaceus and Verasper variegatus, GTP cyclohydrolase 2 (gch2), a common marker of melanoblasts and xanthoblasts, was found to be transiently expressed in cells located along the bilateral skeletal muscles at the basal parts of the dorsal and anal fins of premetamorphic larvae. When V. variegatus larvae were fed with a strain of Artemia collected in Brazil, this gch2 expression was abolished and the differentiation of adult-type melanophores was completely inhibited, while the density of larval melanophores was not affected. In a cell trace test in which the cells at the basal part of the dorsal fin were labeled with DiI at the premetamorphic stage, adult-type melanophores labeled with DiI were found in the skin on the ocular side after metamorphosis. These data suggest that, in flounder larvae, adult-type melanophores are distributed at the basal parts of the dorsal and anal fins as unpigmented precursor cells.