Xiphophorus Genetic Stock Center
The Xiphophorus Genetic Stock Center
In the 1920's, while studying hybrids between the different species of Xiphophorus, scientists observed that the hybrids were viable and could produce F2 hybrids. The hybrids, in some cases, showed traits from both parent species, and were intermediate in appearance including several dominant pigment patterns. In other cases, pigment patterns from one species became enhanced in the hybrids. Further, some of these hybrid fish developed melanoma, one of deadliest skin cancers. Scientists found that these melanomas were derived from improperly regulated melanistic pigment patterns and realized that they had discovered a useful animal model to study cancer.
This research continues today, with research focusing on melanoma and other cancers that form in hybrid fish. We have also extended our research program to include other model species such as Medaka and Zebrafish.
Why do we need genetic stock centers?
Genetic stock centers are irreplaceable resources for scientific research. Genetic experiments often require specially created strains of genetically-identical animals or plants. To produce genetically-identical animals centers, such as XGSC, must inbreed animals for many generations between brothers and sisters. The outcome is that in each generation, about half of the genetic differences between the parents are lost, which increases genetic identity among siblings with each inbred generation. The inbred or isolines, with a high degree of genetic identity are important for the following reasons:
- Reproducible Results: Genetic stock centers track the ancestry of each organism maintained at the center, and intentionally breed individuals to control and enhance genetic identity among different lines. This allows scientists to control for genetic differences between organisms that could result in differences in biological outcomes that can’t be controlled in experiments resulting in non- reproducible results.
- Clearer Connection Between Gene and Characteristic: Research on isolines makes it easier and quicker to link genes with function and/or disease by limiting the amount of variation in an experimental model system.
Xiphophorus as the Gordon-Kosswig melanoma model: the start of the XGSC
In the 1920s, three biologists, Drs. Myron Gordon, G. Haussler and C. Kosswig independently discovered that inter-species hybrids of the platyfish, Xiphophorus maculatus, and the swordtail, Xiphophorus hellerii, developed cancers that were effectively identical to malignant melanomas in human (reviewed by Walter & Kazianis, 2001 here). They found that pigment cells of a platyfish color pattern consisting of black spots on the dorsal fin resulted in these tumors. Based on genetic studies they found that melanomas developed only in hybrids where both copies of a platyfish regulatory gene were replaced by regulatory genes from the parental swordtail and this gene could not control the proliferation of the platyfish pigment cells (reviewed here). Research with these fish provided some of the first evidence that some cancers were inherited diseases; these fish continue to be used for cancer research 70 years later in the United States, Germany, Canada and Japan.
In 1939 Dr. Myron Gordon established the Xiphophorus Genetic Stock Center, at the American Museum of Natural History and the New York Aquarium where it was maintained until 1993, before being transferred to the stock center at Texas State University.
A genetic cross between two Xiphophorus species that leads to the development of melanomas.
The importance of the XGSC in conservation of both inbred model organisms and threatened/endangered wild species
Several of the virtual genetic clones that originated from the strains of platyfish and swordtails developed by Dr. Gordon in the 1930s are still available today. These clonal strains are the products of over 100 generations of brother-to-sister matings, in some cases. The XGSC is one of the oldest live-animal resource centers in the world and these model platyfishes and swordtails are the same species familiar to the tropical fish hobbyist.
When Dr. Gordon began his genetic studies and field work in Mexico and Central America, only 12 species of Xiphophorus fishes were described. Today, 26 species have been described. The stock center maintains representatives from of all but a few species. While ongoing field studies continue to discover new species, preservation of existing species is critical to conservation. At least 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 others are extinct in the wild. The Xiphophorus Genetic Stock Center reduces collecting pressure on wild populations, by providing fish to the scientific community for study. The XGSC may end up ultimately preserving the only living representatives of some species. Breeding strategies of the stock center have veered away from generation of new inbred strains to maintenance of maximal natural variability in newly originated genetic stocks for conservation purposes.
Services of the Stock Center
The Xiphophorus Genetic Stock Center provides fish from more than 70 genetic strains to scientists from over 30 laboratories in the United States, Canada, Mexico, and Germany. These laboratories depend on strains available from the stock center along with custom hybrids. The stock center produces hundreds of such fish each year at affordable costs and aids with husbandry and genetic questions. Extensive use of these cancer producing hybrids for gene mapping has made the Xiphophorus gene map the fifth-largest among vertebrates, exceeded only by maps of man, mouse, rat and cow in numbers of genes assigned.
Tropical fish hobbyists have appreciated the beauty of many Xiphophorus for decades. Fish like X. montezumae, with a sword longer than its body, are in strong demand, and the requests far exceeds the production capabilities of the stock center, but surplus fish are made available to aquarists when possible. The XGSC facilities contains about 1,400 aquaria, balanced between 1200 tanks for genetic stock perpetuation and 200 for hybrids for research projects.
Quality control is required to maintain integrity of the large numbers of genetic strains (some 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. These precautions take a great deal of time, and are essential for maintaining stock integrity.
The Future of the Stock Center
Maintenance of large numbers of genetic stocks require constant effort and substantial funding. Texas State University provides excellent facilities and support for the stock center, but these funds are insufficient to ensure maintenance of the center. The stock center is also supported by federal grants, but tax-exempt donations are required to round out our budget. Loss of funding for a year would result in the loss of more than six decades of carefully controlled breeding. For safekeeping of these irreplaceable genetic resources, the XGSC is striving to raise $3 million of endowed funds to support stock center maintenance for many years into the future. Such an endowment would support all daily operations and provide funds for potential expansion including newly discovered species and construction and maintenance of new strains. An endowment would assure continued availability of these fascinating fish to scientists and tropical fish hobbiests of the future.