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The genetic basis of spontaneous melanoma formation in spotted dorsal (Sd) Xiphophorus platyfishswordtail hybrids has been studied for decades, and is adequately explained by a two-gene inheritance model involving a sex-linked oncogene, Xmrk, and an autosomal tumor suppressor, DIFF. The Xmrk oncogene encodes a receptor tyrosine kinase related to EGFR; the nature of the DIFF tumor suppressor gene is unknown. We analyzed the genetic basis of UV-B-induced melanoma formation in closely related, spotted side platyfish-swordtail hybrids, which carry a different sex-linked pigment pattern locus, Sp. We UV-irradiated spotted side Xiphophorus platyfish-swordtail backcross hybrids to induce melanomas at frequencies 6-fold higher than occur spontaneously in unirradiated control animals. To identify genetic determinants of melanoma susceptibility in this UVinducible Xiphophorus model, we genotyped individual animals from control and UV-irradiated experimental regimes using allozyme and DNA restriction fragment length polymorphisms and tested for joint segregation of genetic markers with pigmentation phenotype and UV-induced melanoma formation. Joint segregation results show linkage of a CDKN2-like DNA polymorphism with UV-B-induced melanoma formation in these hybrids. The CDKN2-like polymorphism maps to Xiphophorus linkage group V and exhibits recombination fractions with ES1 and MDH2 allozyme markers consistent with previous localization of the DIFF tumor suppressor locus. Our results indicate that the CDKN2-like sequence we have cloned and mapped is a candidate for the DIFF tumor suppressor gene.Genetic hybrids between species of the genus Xiphophorus (Teleostei: Poeciliidae) exhibit spontaneous melanoma formation in several different cross types and have been used for decades to investigate genetic factors contributing to melanoma formation (1). The most studied and best understood Xiphophorus hybrid melanoma is the spotted dorsal GordonKosswig platyfish-swordtail model (2-5), represented by genetic hybrids derived from crossing F 1 hybrids between the platyfish Xiphophorus maculatus Jp 163 A and the swordtail Xiphophorus helleri back to X. helleri. Melanoma formation in this tumor model is genetically controlled by inheritance of a sex-linked receptor tyrosine kinase gene (Xmrk), associated with the spotted dorsal (Sd) pigment pattern locus, and segregation of an autosomal locus in Xiphophorus linkage group (LG) V, variously referred to in the literature as DIFF, R DIFF , and R (3-6). The DIFF locus is believed to regulate macromelanophore pigment cell differentiation (4, 6), and behaves in the Gordon-Kosswig model as a classical tumor suppressor for which loss of species-specific alleles in pigmented backcross hybrids results in melanoma formation according to simple, Mendelian inheritance (1, 3-5).Elegant experiments have shown that Xmrk is a duplicated gene, which has adventitiously acquired the promoter from another gene (7,8). It has been postulated that expression of the oncogenic Xmrk gene duplicate is re...

Genetic hybrids of Xiphophorus fishes have been used for decades to study heritable melanoma formation. In these models, overexpression of pigmentation patterns from melanin-producing pigment cells can lead to genetically regulated melanoma formation in backcross hybrids. In the best studied of these models, the Gordon-Kosswig hybrid melanoma, tumors form spontaneously in all individuals of a subset of backcross hybrids between the platyfish Xiphophorus maculatus Jp 163 A and the swordtail species Xiphophorus helleri. Backcross hybrids susceptible to melanoma formation inherit a sex-linked oncogene, Xmrk, associated with the spotted dorsal (Sd) pigment pattern and have lost both copies of an autosomal gene, DIFF, from the X. maculatus parent. Spontaneous melanoma formation conforms to simple, two-gene Mendelian inheritance in which DIFF behaves as a recessive tumor suppressor gene. Recently, Xiphophorus hybrids in which melanomas can be induced by UV and near-UV visible light exposure have been described. We report here results of genetic linkage analysis of one of these Xiphophorus light-inducible hybrid melanoma models, in backcross hybrids between the two platyfish species X. maculatus Jp 163 B and Xiphophorus couchianus. Our linkage results provide the first estimate of recombination between the tumor suppressor locus, DIFF, and glycerate-2-dehydrogenase (GLYDH) in Xiphophorus linkage group V. Also, they demonstrate that DIFF regulates hyperplasia of spotted side (Sp) pigment cells in this hybrid model, analogous to its regulation of hyperplasia of Sd pigment cells in the "classical" Gordon-Kosswig hybrid. Joint segregation analyses of melanoma-bearing fish indicate that segregation of DIFF is genetically linked to melanoma induction by 405 nm light in this model but that induction of melanomas by UV wavelengths apparently does not depend on segregation of the DIFF locus.