The Amazon molly Poecilia formosa is a gynogenetic fish that reproduces through the development of ameiotic diploid eggs triggered by insemination by males of related species without following karyogamie. This leads to clonal offspring. In rare cases, however, this gynogenesis is leaky, and paternal DNA in the form of small supernumerary chromosomes is included into the maternal genome. We have obtained a clone where one such microchromosome contains a pigmentary locus, resulting in macromelanophore pigmentation of the carrier. Approximately 5% of these fish spontaneously develop exophytic nodular or papillomatous pigment cell tumors. The tumors display considerable differences with respect to growth characteristics and invasiveness, despite the genetic uniformity of the affected animals. Following transplantation to syngeneic hosts, a remarkable clonal variability was observed. Oncogenes that are involved in tumorigenesis in hereditary melanoma of the closely related fish Xiphophorus appear not to be instrumental for induction of the P. formosa pigment cell tumors. Moreover, a new genetic locus is defined that mediates susceptibility to pigment cell tumor development and leads to transformation of chromatoblasts.

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This work was supported by grants from the Deutsche Forschungsgemeinschaft through Sonderforschungsbereich 172 (to M. Scha.) and Grant Pa 148/10-3 (to J. P. and M. Scha.), the European Commission through Contracts Bio2-CT93-0430 and CI1*-CT94-0021 (to M. Scha.), and AIR2-CT93-1543 (to M. Schm.), and the Freistaat Bayern by a Wiedereinstiegsstipendium (to A. S.).

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