Studies on the differentiation of pigment cells with use of transgenic medaka fish bearing the gene for mouse tyrosinase

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Transgenic fish bearing the gene for mouse tyrosinase are produced by microinjecting it into the oocyte nucleus of or by introducing it by electroporation into the fertilized egg of an orange-colored variant of medaka (Oryzias latipes). We successfully obtain 41 transgenic fish exhibiting brown skin pigmentation, 3 fish by the former method and 38 by the latter. Brown skin pigmentation in these transgenic fish is found to result from melanogenesis in "amelanotic melanophores", which are distributing in an orange-colored variant of this species. immunofluorescence using the anti-mouse tyrosinase antibody discloses and apparent distribution of mouse-type tyrosinase in these melanophores. Electron microscopy on the skin of transgenic fish discloses that the melanophores are installed with numerous, unevenly sized melanosomes having a lattice-like internal structure, which is different from "multivesicular" ones common to fish melanophores, and that little melanin deposition is detectable in xanthophores and leucophores. All these findings strongly suggest that the gene transfected is expressed under a cell-type specific manner in medaka. Crossing between a transgenic fish and a fish from an orange-colored variant yields offsprings having wild-type brown and orange-colored pigmentation at an approximate rate of 1 : 1, suggesting transmission of the transfected gene according to the Mendelian law.