EVOLUTIONARY ORIGIN AND MOLECULAR-BIOLOGY OF THE MELANOMA-INDUCING ONCOGENE OF XIPHOPHORUS

Melanoma formation in platyfish/swordtail hybrids of genus Xiphophorus is due to overexpression of the receptor tyrosine kinase oncogene Xmr k, This gene is the molecular equivalent to the Tu-locus of platyfish, formerly identified by Mendelian genetics. The supposed evolutionary origin of the Xmrk oncogene is a nonhomologous recombination event in the 5' region of the corresponding Xmrk protooncogene with an anonymou s sequence, D. This event led to a gene duplication of Xmrk, whereby t he new copy obtained a novel promoter derived from D. Inactivity of th is promoter in parental fish warrants lack of tumorigenicity of the Xm rk oncogene in wild playfish. In hybrids, however, the promoter is act ive. This leads to the pigment cell transforming overexpression of Xmr k.