It is well-documented that certain black pigment patterns in the many species of Xiphophorines (platies and swordtails) can produce melanoma cancers in hybrids. I have seen melanomas on individuals of the "bumblebee" strain and I expect that is is eventually fatal as in other affected hybrids. Most of the commercially produced swords & platies are complex hybrids originally produced from inter-species crosses. Cancers are not unknown in black spotted hybrids. Here is a link to an historical article by Myron Gordon with an extensive bibliography, containing references to production of melanoma in these fishes.