Genetic stock centers are priceless resources in scientific research. Genetic experiments often require that special strains of genetically-identical animals or plants are used. Among vertebrate stock centers (like XGSC), genetically-identical animals most often are produced by inbreeding for many generations between brothers and sisters; in each generation, about one-half of the genetic differences between the parents are lost, increasing genetic identity among siblings with each inbred generation. The high degree of genetic identity in model organisms is scientifically important for a couple reasons:
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 helleri, 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 still are 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-San Marcos.
Several of the original genetic strains of platyfish and swordtails developed by Dr. Gordon in the 1930s still are available today; they are virtual genetic clones, the products in some cases of more than 80 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.
When Dr. Gordon began his genetic studies and field work in Mexico and Central America, only a half-dozen species of Xiphophorus fishes were known to science. Today some 23 species have been described; representatives of all but one are maintained at the stock center. Ongoing field studies by Dr. Kallman continue to discover new species, and are critical to the preservation of the increasingly valuable genetic resources of the genus. No fewer than eight species are confined to extremely small geographic areas and are threatened by human habitat destruction; several species have already been listed as endangered, and another was thought to be extinct until recently. By providing fish to the international scientific community for study, the Xiphophorus Genetic Stock Center reduces collecting pressure on wild populations, and may ultimately preserve the only living representatives of some species. As the science of conservation genetics has evolved, so have the breeding strategies of the stock center, away from generation of new inbred strains to maintenance of maximal natural variability in newly originated genetic stocks.
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