Piscine Stars of the Stock Center
Each of the 70-plus strains of swordtails and platyfish have some distinctive attributes. Perhaps 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. We provide only a few examples here to illustrate the amazing scope of Xiphophorus genetic research and the fascinating variety of its subjects.
Xiphophorus maculatus, from the Rio Jamapa, Veracruz, Mexico.
Several of the Rio Jamapa (designated Jp) platyfish strains are direct descendants of fish collected by Dr. Gordon in Mexico in 1939. Two sublines, Jp 163 A and Jp 163 B, were derived from offspring of a single female and have been inbred for over 100 generations. Females carry spotted dorsal, spotted side, dorsal red, and shoulder spot pigment pattern genes on their X chromosomes, while males additionally carry striped side and anal red genes on their Y chromosomes. Both strains develop malignant melanomas in hybrids with swordtails; the melanoma from Jp 163 A spotted dorsal pigment cells is spontaneous, while the melanoma from Jp 163 B spotted side cells develops very rapidly only after irradiation with ultraviolet light. Several other strains, including a recently collected wild Jp strain without red or black pigment patterns but with a beautiful blue iridescence, have been developed at the center. These strains have been central in genetic studies of cancer for more than 50 years.
Southern X. hellerii strain Cd, from Rio Jamapa, Mexico.
The magnificent Cd swordtail is the oldest stock of the center, derived from fish collected in Mexico as early as 1930. Because swordtails usually mature much more slowly than platyfish and reach much larger sizes, Cd now is nearing only it's 50th generation of inbreeding. The genetics of its remarkable red, yellow, and green color patterns still are unknown, but in closely related strains orange or green sword coloration is controlled by a single gene on the Y chromosome. A peculiarity of the Cd strain is the occurrence every few generations of almost all-male or all-female broods, a source of constant anxiety in its keepers. No cause for this deviation from normal sex ratios in this genetically uniform fish has yet been identified.
Pygmy swordtail, X. multilineatus, Rio Coy, San Luis Potosi, Mexico.
Many notable discoveries in ecological and behavioral genetics have resulted from studies of the pygmy swordtails. A sex-linked polymorphism at a sex hormone-related gene produces four sizes of males maturing at different ages, from 6-9 months old for the smallest gold males to more than one year old for the largest blue males. Behavioral studies have shown that females prefer the large males that exhibit elaborate courtship displays, rather than the small males that do not court but mate by sneak attacks upon females. This complex polymorphism also occurs in other swordtail and platyfish species. Several northern swordtail species, including X. multilineatus, are found only in one short river system and could easily become endangered through habitat destruction.
Montezuma swordtail, X. montezumae, Rascon strain, Rio Gallinas, San Luis Potosi, Mexico.
For many years, the Montezuma swordtail of the aquarium hobby was actually a sister species, X. nezahualcoyotl, with a much shorter sword and body. The "true" Montezuma swordtail was rediscovered by stock center directors in Mexico in 1980, and has become a favorite among aquarists because of its spectacularly long sword. The black spotting of X. montezumae, unlike that of its sister species, is not due to a sex-linked gene and does not lead to pigment cell cancers in hybrids.