Expression and Transmission of Wild‐Type Pigmentation in the Skin of Transgenic Orange‐Colored Variants of Medaka (Oryzias latipes) Bearing the Gene for Mouse Tyrosinase

Transgenic fish carrying a reconstructed mouse tyrosinase gene, mg‐Tyrs‐J, were produced by microinjecting the gene into the oocyte nucleus of an orange‐colored variant of medaka (Oryzias latipes). Of 64 oocytes microinjected and subsequently inseminated, 13 embryos developed normally beyond hatching and three of them exhibited brown skin pigmentation in the adult as was commonly observed in the wild type of this species. Light and electron microscopic examination disclosed a ubiquitous distribution of typical melanophores in the skin of these transgenic fish. Judging from their population density and distribution pattern, it was presumed that melanogenesis in these fish was elicited in amelanotic melanophores that resided in the skin of the orange‐colored fish of this variant. Immunofluorescence with use of the anti‐mouse tyrosinase antiserum lacking reactivity to medaka tyrosinase clearly disclosed that the gene introduced was expressed in the melanophores of transgenic fish. Crosses of female transgenic fish and males from an orange‐colored variant yielded offspring exhibiting wild‐type or orange‐colored pigmentation in a ratio of 1:1, thus implying that mg‐Tyrs‐J integrated into the medaka genome behaves like a dominant gene. Little melanogenesis was observed in xanthophores, leucophores and iridophores in transgenic fish, suggesting possible specificity in recognition of teleostean cell types (i.e., melanophores) by the regulatory region of the mouse tyrosinase gene.