Normal Black pigment cells stop growing, but in some mixed Xiphophorus there are modifiers which allows the Black pigment cells to never stop growing. That's what causes the melanoma.
Much of what is known about the genetics of melanoma in Xiphophorus sp. comes from the work of Manfred Schartl and his colleagues. They identified a gene called Xmrk that stimulates black pigment cells (melanocytes) to grow uncontrollably, producing melanomas. Darrell is correct that a modifier gene plays an important role in this process, but in this case the modifier gene (R) acts to prevent melanoma formation. Wild platyfish (X. maculatus) have both Xmrk and R so even though they may have black spots, they don’t get melanomas. When they were crossed and then backcrossed to wild swordtails which lack Xmrk and R, eventually black offspring arise that have Xmrk, but not the R melanoma-suppressor gene. This results in uncontrolled melanocyte proliferation and cancer. Most of the colorful domestic swordtails available today have platy ancestors, so almost all contain the Xmrk gene and are therefore prone to developing melanoma. Other genes and gene modifiers that control pigment pattern (e.g. tuxedo) may also affect this process. The good news is that Schartl found fish in which the Xmrk gene was lost or inactivated by mutation, and these fish were melanoma-free. It may therefore be possible to produce all black and red jet swordtails that do not develop cancer.
What you explained, explains why Frank Grainer's Albino Swordtails with large amounts of Black body coloration are cancer free. Some people (myself included), used to believe that the combination of Albino and Black on Xiphophorus always resulted in melanoma.
Have a 20 gallon right now set up with 2 showa males and 6 velvet red albino females.So Darrell and Roy do you think I can eliminate this problem with the showa line developing cancer, with a cross to an albino swordtail ?
I wish it were true that crossing blacks or red jets with albino swordtails could prevent melanoma formation. Unfortunately, that is probably not the case. The gene that is responsible for albinism is tyrosinase which is needed to make black pigment. Albinos still have melanocytes, but they are usually not black. Albinos can develop amelanotic tumors in which the non-black melanocytes keep growing. It’s possible that Frank Grainer's albino swordtails with black pigmentation cannot develop melanomas, but it’s probably not just because they are albinos. The albinos with black pigmentation that I’ve raised often develop melanomas. In my previous posting I optimistically wrote that it might be possible to obtain healthy blacks and red jets. I left out the pessimistic part that inactivating the Xmrk gene is probably a rare event and that we’ll have no way of knowing by looking at a fish whether Xmrk is inactivated or if its offspring will be melanoma-free.
Another possibility is that other genes or pigment patterns might suppress melanocyte growth. I’ve noticed that my black swordtails containing a large number of blue-iridescent scales throughout the body and especially near the base of the caudal fin, where my fish frequently develop melanoma, don’t seem to develop cancer. This might be worth exploring.
During the 1980's in an attempt to develop a strain of Lyretail Platy Ib successfully crossed a Milk & Ink Platy male with a Hifin/ Lyretail Green Swordtail female which had an Orange dorsal fin. In the F1 offspring were some striking Black bodied Lyretails and Hifin Lyretails with bright Orange dorsals and/ or tails. I sold or gave most of these away and kept two nice females which I tried to breed to several male Platies both Maculatus and Variatus type. Neither female ever became gravid so since I had several other projects going at that time in a very small set-up, I decided to trash the project. I gave both of these to a friend of mine hoping that he'd continue to try getting the backcross with Platy, but he decided to breed them to a Red Tuxedo Swordtail male which produced very pretty Red & Black offspring which were loaded with melanoma.
Hey Darrell,the first cross with my showa line was to my high-fin black eyed red sword line.The female never really got large, so I was not prepared for her to drop young ,as a result I was only able to save 4 young swords.Out of these 1 has developed a malanoma.Next I crossed my showas into my high-fin tuxedo line.Have 3 drops already of around 25 young per drop.They are still quite young but I have already 3 with melanomas.I'm now working with an red Albino line,no young yet. I have something like you described,a gold sword with a red dorsal,only wished it was a high fin.
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