Genetics of Melanomas in Fishes V. The Reappearance of Ancestral Micromelanophores in Offspring of Parents Lacking These Cells*

In previous papers in this series (5, 8, I3) it has been shown that spontaneous malignant melanomas are produced in the hybrid offspring of the black-spotted platyfish, Platypoecilus maculatus, and the swordtail, Xiphophorus hdlerii. The melanomas are evoked genetically in hybrids by the interaction of the sexlinked, heritable factor for macromelanophores, Sp, of the platyfish with a series of Sp modifiers, d and B, of the swordtail. These melanomas may be said to be produced experimentally because the platyfish and the swordtail probably do not crossbreed under natural conditions in Mexico. The author ( io) has collected nearly Io,ooo specimens of these two species in their native habitat without finding a single natural hybrid, despite the fact that both species were often found living side by side.