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"Stars" of the Xiphophorus Genetic Stock Center The Future of the Xiphophorus Genetic Stock Center
enetic stock centers are priceless resources in scientific research. Genetic experiments often require that special strains of genetically identical animals or plants be used to assure that results can be repeated in any laboratory and that differences are not due to environmental factors or to random variation found in almost any population of animals. Among vertebrates, 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. It comes as a surprise even to most scientists that one of the oldest and best defined groups of genetic strains consists of livebearing fishes of the genus Xiphophorus , the platyfishes and swordtails familiar to the tropical fish hobbyist.
In the 1920s, the American biologist Dr. Myron Gordon and German biologists Haussler and Kosswig independently discovered that hybrids of a particular strain of the platyfish Xiphophorus maculatus and the swordtail Xiphophorus helleri developed cancers virtually identical to malignant melanomas in man. 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 which had replaced both copies of a platyfish regulatory gene with swordtail forms which could not control proliferation of the platyfish pigment cells. 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 identify precisely the genes responsible for development of cancer, genetically identical platyfish and swordtails would be needed. 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 in San Marcos was completed. During its 55+ years, the stock center has been directed by Dr. Gordon and Dr. Klaus D. Kallman in New York, and currently by Dr. Ronald Walter (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. 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 Drs. Kallman and Morizot continue to discover new species, and are critical in future preservation of the increasingly valuable genetic resources of the genus. No fewer than eight species are confined to extremely small geographic areas and are potentially threatened by human habitat destruction; one species already has been listed as endangered and another until recently was thought to be extinct. The Xiphophorus Genetic Stock Center, by providing fish to the international scientific community for study, at the very least reduces collecting pressure on wild populations, and could in the future 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.
enetic Stock Center provides fish from more than 70 genetic strains to scientists and aquarists around the world. Scientists in more than 30 laboratories in the United States, Canada, Mexico, Japan, and Germany work on Xiphophorus genetics and depend on strains available from the stock center. In addition to supplying strains and consultation on husbandry and genetic questions, the stock center makes custom hybrids for a variety of projects, producing hundreds of such fish each year at very affordable costs. Unlike most mammals, where fertile hybrids between species are difficult or often impossible to produce, Xiphophorus hybrids are almost always fertile and are extremely valuable for their genetic variability and their very specific susceptibilities for many different cancers. Extensive use of these hybrids for gene mapping has made theXiphophorus gene map the fifth largest among vertebrates, exceeded only by maps of man, mouse, rat, and cow in numbers of genes assigned. The beauty of many Xiphophorus strains has attracted tropical fish fanciers for decades. The demand among cognoscenti for extraordinary fish like X. montezumae with a sword longer than its body, far exceeds the production capabilities of the stock center, but surplus fish are made available to aquarists when possible. The present stock center facilities hold some 1200 aquaria, balanced between 900 tanks for genetic stock perpetuation and 300 for hybrids for research projects. While this number may seem large, each new species or newly identified genetic strain necessitates some redesign of the overall management plan. Quality control at the stock center must be impeccable to maintain integrity of the large numbers of genetic strains (even fish manage to escape, and a "jumper" can mean disaster). Most strains have been purposely bred to carry diagnostic morphological traits such as red and black pigment patterns. Each strain possesses a unique genetic "signature" which can be checked for assurance of stock purity. Individual broods are never mixed, and males and females are separated prior to sexual maturity. All parents of broods are preserved for later reference in the event of later questions. All these precautions take a great deal of time, but have been essential in maintaining stock integrity sometimes for 50+ years. "Stars" of the Genetic Stock Center
ach of the 70+ strains of swordtails and platyfishes has some distinctive attribute, whether it is the only representative of a species, carries genes for susceptibility to a particular cancer, or produces all male or female offspring in certain matings. Only a few can be illustrated here to illustrate the amazing scope of Xiphophorus genetic research and the fascinating variety of its subjects.
The Future of the Genetic Stock Center
hough Xiphophorus fishes are among the best studied vertebrates genetically, largely due to availability of well characterized genetic stocks, maintenance of large numbers of genetic stocks requires constant effort and substantial funding. Texas State University - San Marcos is providing excellent facilities and support for the stock center, but State funds for this institution are inadequate to ensure maintenance of the center. Federal grants have helped support the stock center through purchase of fish for research, but an increasing share of the budget must be generated from tax-exempt donations. Operating a genetic stock center is a continuing program; lack of funding for a year would result in the loss of more than six decades of carefully controlled breeding. For security of these priceless genetic resources, the goal of the center is to raise $3 million of endowed funds to support stock center maintenance for many years into the future. This endowment would support all day-to-day operations and provide funds for gradual expansion to include newly discovered species and construction and maintenance of new strains. Only such a liberation from year-to-year funding crises can assure continued availability of these fascinating fish to scientists and aquarists of the future.
ontributions to the XGSC are always welcome and appreciated. Such funding helps insure that the center will continue to maintain a high standard of excellence in stock purity and species/strain preservation. Such contributions can be made by submitting checks to "The Xiphophorus Genetic Stock Center". The director of the XGSC can be directly contacted regarding such donations (see below). A formal receipt is promptly provided to the contributor(s) for tax purposes.
equests for live Xiphophorus fish or questions regarding the Xiphophorus Genetic Stock Center, its operation and services should be forwarded to the Director, Dr. Ronald Walter (rw12@swt.edu) Download printable PDF (Acrobat 4.0) version of this page.
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