Food producers and aquarium enthusiasts alike are interested in preserving rare breeds of fish. Fish such as platinum arowana and large bluefin tuna fetch a high market price because they are highly desirable for customers. But rare means their populations are small. Scientists are interested in finding ways to preserve some of their most valuable traits – like disease resistance, rare color patterns, or fast growth. One way is by saving reproductive tissue from recently deceased fish.
If fish die as adults, their sperm and eggs can be frozen and saved for in vitro fertilization. If the fish are too young, their sperm cells and eggs wouldn’t be viable, but their genes would. A team from the Tokyo University of Marine Science and Technology ran an experiment to see if they could transplant these rare genes into a less rare variety of the same species. But this is not easy.
Previous studies have shown that when transplanting immature sperm or egg cells from one fish to another, the donor cells migrate through the body and settle on what are called “genital ridges” which later become ovaries or testes. Some of the offspring are then related to the donor fish and not the physical parent. (This does not work in humans because we have an innate immune system that would attack the foreign reproductive cells.) The scientists thought this could be used to rescue the genes of rare fish populations that may have been killed by a disease outbreak or system failure
The researchers euthanized a group of nine-month-old male rainbow trout to collect their cells. These trout were bred to produce a protein in their reproductive cells that lights up green under a microscope after being exposed to certain chemicals. After euthanizing, they either removed the trout immediately or let them sit in flowing water for 6, 12, or 24 hours at 50°F (10.5°C). To show how much the fish had decomposed, the scientists measured the amount of a molecule called ATP in their muscle tissue. ATP is used by the cell for energy, and declines after death.
The scientists then harvested the fish’s testicular cells and looked at them under a microscope. Even 24 hours after death, most of the cells appeared intact. They transplanted these cells into 30-day-old male and female trout. After 25, 90, and 150 days, they took samples of the fish’s reproductive organs to see what percentage of them glowed green, like the donor fish cells.
The researchers found that the transplants from zero and six hour post-death fish were quite successful, with 91% and 82% of fish respectively showing reproductive tissue that glowed green. The 12-hour post-death cells were less successful, with a 73% positive rate, and the 24-hour fish barely had any success with a rate of 7%.
The researchers then grew the eggs and sperm from the receiving fish in a culture dish. They found that they carried the glowing genes at rates of 78%, 78%, 55%, and 5% for the 0, 6, 12, and 24-hour post-death fish respectively. Helpfully, the transfer can occur between two males, two females, or between male and female, making this technique more flexible than they thought.
The researchers hypothesized that the drastic reduction in success for the 24-hour fish is caused by the lack of ATP available. ATP dropped to near undetectable levels after 12 hours in these fish. The cells use ATP to migrate to the genital ridges, so while the cells from 24-hour fish looked intact visually, they may have been unable to reach the genital ridges where they needed to settle to influence the tissue there.
This experiment showed that even after twelve hours, the genes of male fish can be saved by transplanting their reproductive cells. But there are still many questions left unanswered, like whether or not this would work with female fish as donors. Furthermore, there are many desirable and expensive varieties of tropical fish that live in much warmer water than rainbow trout. Warmer temperatures mean faster decomposition, so their cells wouldn’t last as long. Lastly, stress from disease would reduce the starting amount of ATP available in the fish. It is important to find out how long cells from sick fish would be viable compared to those of healthy fish. Nevertheless, the demonstrated success of this method gives conservation an additional tool to preserve important fish genes.