STABLE ASSOCIATION OF A PIGMENTATION ALLELE WITH AN ONCOGENE - NONHYBRID MELANOMAS IN XIPHOPHORUS-VARIATUS

Sex-linked genes in several species of the fish genus Xiphophorus caus e macromelanophore pigmentation patterns on the flanks of the fish. So me, but not all, of these patterns can develop into melanomas. The tum origenic alleles are tightly linked to a supernumerary oncogene sequen ce, Xmrk. The data show that the association of Xmrk with two of the t umorigenic alleles of X. variatus, P-2 and Li, holds over a broad geog raphic area. From the distribution of the fish and the geology of the area, it is probable that this association is older than the late Tert iary. The persistence of this association suggests that Xmrk confers s ome benefit on P-2- and Li-bearing individuals to offset the deleterio us effect. The nature of this benefit remains unknown.