Did not read most of this thread because once I hit this from Colin
Swordtails and platies were hybridized 40 years ago to get more colour, as were guppies and Endler's livebearers, and different species of molly were crossed for more colours.
I had to correct it. The platy-xiph hybrid created one of the most important animals in medicine and and animal with one of the most complete gene mappings we have. This did not begin just 40 years ago nor did it have anything to do with producing more colorful ornamental fish. Here are the facts which got its roots almost 100 years ago and it's formal start in 1939 some 80 years ago. You will see why it was/is so important to all people not just fish keepers. Here comes a long quote that will amaze most who read it. It goes against everything many of us assumed about hybrids and incestuous breeding:
Xiphophorus as the Gordon-Kosswig melanoma model: the start of the XGSC
In the 1920s, the American biologist Dr. Myron Gordon and German biologists Haussler and Kosswig independently discovered that inter-species hybrids of a particular strain of the
platyfish, Xiphophorus maculatus, and the swordtail, Xiphophorus hellerii, developed cancers virtually identical to malignant melanomas in man (reviewed
here). They traced the origin of these tumors to pigment cells of a platyfish color pattern consisting of black spots on the dorsal fin. Genetic studies demonstrated that melanomas developed only in hybrids that had replaced both copies of a platyfish regulatory gene with swordtail forms that could not control proliferation of the platyfish pigment cells (reviewed
here). This animal model was one of the first to prove that some cancers were inherited diseases; after 65 years, these fish are still used in cancer research in the United States, Germany, Canada and Japan.
Dr. Gordon realized that to precisely identify the genes responsible for development of cancer, scientists would require genetically identical platyfish and swordtails for research. Therefore, in 1939, he established the
Xiphophorus Genetic Stock Center, housed at the American Museum of Natural History and the New York Aquarium until 1993, when transfer of the stock center to Texas State University
-San Marcos was completed. Since its inception more than 70 years ago, the stock center has been directed by first, Dr. Gordon and Dr. Klaus D. Kallman in New York, and currently by Dr. Ronald Walter at Texas State University.
The importance of the XGSC in conservation of both inbred model organisms and threatened/endangered wild species
Several of the original genetic strains of platyfish and swordtails developed by Dr. Gordon in the 1930s are still available today; they are virtual genetic clones, the products in some cases of more than 100 generations of brother-to-sister matings. The XGSC is one of the oldest live-animal resource centers in the world. It surprises even many scientists that one of the oldest and best-defined groups of model organisms are livebearing fishes of the genus
Xiphophorus, the platyfishes and swordtails familiar to the tropical fish hobbyist.
If you are more curious about the above, the site is here
https://www.xiphophorus.txstate.edu/about/introduction.html
One personal observation. Nature finds its own way. Some species can interbreed but most cannot. However, one thing is different between nature and fish keeping. There is no way fish which never meet in nature can hybridize. We do not tend to respect disconnected borders in our tanks more often than not. Fresh water bodies of water tend to be isolated and do not mix often, especially when from different continents.
