Transgenic fish have been launched with optimistic hopes that the fish will grow rapidly, use food efficiently and pose little threat to the native stocks. Prof. Joe Cummins reveals why those hopes don't hold water.
To prevent gene escape from the patented fish to native stocks, the fish-farming industry has been making fish "sterile" by increasing the normal complement of chromosomes from two sets (diploid) to three (triploid). The theory is that such triploid stocks will not be able to produce germ cells (egg or sperm) properly, and so will not reproduce.
Triploids have been created in a wide variety of fish. Triploids are frequently produced by suppressing the second meiotic division - cell division involved in producing germ cells - using shock treatments, such as heat, cold or pressure . A related technique, uses irradiated sperm to fertilize normal eggs followed by inhibition of the first or second meiotic division of the egg , enabling triploids to be mass produced.
Sterile triploids have been suggested as a means to contain transgenes in transgenic stocks released to the natural environment, as if the method were "fool proof". In reality, a number of studies suggest that sterile triploids are "leaky", and some fertile gametes are sometimes produced. Even though sterile triploid grass carp are used extensively in North America to control lake vegetation, there have been few studies on the fertility of feral grass carp. Among feral grass carp recovered from the Chesapeake Bay waterway, gonad development was found in two female and one male triploids . A study of transgenic Tilapia found that triploid females produced no eggs while males had poorly developed gonads which nevertheless produced sperms . And triploid oysters were found to have irregular numbers of chromosomes . Extensive studies on sterile triploid leakiness in producing gametes should be done before any transgenic fish are released to the environment.
Triploids themselves may pose special problems. For example triploid Atlantic salmon had a high prevalence of skeletal deformity and reduced gill surface area . Families of triploid salmon were found to have much greater variability in growth than fertile diploids . Ocean migration and recoveries of triploid Atlantic salmon were between 12% and 24% of diploid siblings .
Turning to transgenic fish, a number of studies indicate that release of such fish to the environment is premature. For example, growth-enhanced transgenic Atlantic salmon were found to be bad at avoiding predators in comparison to normal fish . The likelihood of growth-enhanced Atlantic salmon achieving maximum growth or even surviving outside intensive culture conditions was lower than non-transgenic salmon . Growth-enhanced transgenic Arctic charr were found to partition nutrients in a manner that resembled domestic rather than wild rainbow trout . Fish that are growth-enhanced with human growth hormone has been attempted frequently, for example, human growth hormone enhanced carp was reported to have higher growth rates and food utilization efficiency than non-transgenic carp . The use of human genes, especially those coding for hormones active in human beings, to modify animals meant for human consumption, bears special scrutiny. Finally, the impact of fertile transgenic organisms on natural populations need to be thoroughly considered, as studies suggest that transgenes which reduce viability can lead to extinction of the natural population .
In conclusion, peer reviewed studies on induced triploidy in fish and on transgenic fish indicate that the technology is fraught with uncertainties and problems that have not yet been resolved. The available evidence indicates that the risks to the natural fish stocks may be far too great to allow transgenic fish to be released to the environment. The production of transgenic fish must be confined to inland facilities rather than fish pens in open waters.
Article first published 30/07/02
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