A key element of the enhanced production of domesticated species is the development of genetically superior breeding stocks tailored to their maintenance conditions and to the marketplace. Characteristics that are generally desirable in all species include improvement of growth rates, feed conversion efficiency, disease resistance, and a capacity to utilize low-cost or nonanimal protein diets.
The techniques for gene transfer into fish have focused on direct transfer of DNA into gametes or fertilized eggs and include DNA microinjection, electroporation, retroviral vector infection, and biolistic methods.
Stem-cell-based technology is not available for farmed fish. The making of transgenic fish is different from gene transfer in mammals or birds because:
- fish usually undergo external fertilization and no culture or transfer of eggs into recipient females is required;
- the eggs of many fish have a tough chorion requiring special methods for delivering the gene constructs; and
- DNA delivery, including by microinjection, is usually into the cytoplasm.
Probably because of the cytoplasmic nature of DNA delivery, many founder transgenic fish are mosaic. Germ line mosaicism seems also to occur because frequencies of transgene transmission to F1 are clearly less than at Mendelian ratios.
Transmission of the transgenes to later progeny occurs at Mendelian frequencies, indicating stable integration of the transgenes. A variety of inducible and targeted transgene strategies developed for mammals are now available to be tested and explored in fish.
Aquaculture is still in its infancy compared with the farming of mammals or poultry. Growth rates of the many fish species used are naturally slow but are currently being enhanced by traditional methods of domestication and selection.
Programs for growth-promoting gene-transfer into fish usually use GH-based gene constructs. Because of the lack of available piscine sequences, the first experiments were conducted with mammalian GH gene constructs.
The effects on growth performance were, however, either not detectable or very small. Gene transfer using fish GH sequences driven by nonpiscine promoters has resulted in growth stimulatory effects in transgenic carp, catfish, and tilapia; weight increases were approximately twice those of controls. These experiments provided the first consistent data demonstrating that growth acceleration in fish can be achieved by transgenesis.
Subsequent use of all-piscine gene constructs produced fish with up to fortyfold elevated circulating GH levels and five- to elevenfold increased weight after one year of growth. Pleiotropic effects in the GH-transgenic fish included altered body composition (50% reduced fat levels), unpredictable variations in food consumption and conversion, and some pathological side-effects.
Comparative gene-transfer programs demonstrated that GH- transgenes dramatically enhanced the growth of wild but not domesticated fish. Thus in domesticated and selected farm animal species the capacity for further growth enhancement by GH may be restricted by limitations in other physiological pathways. In mammals this is reflected by dramatic growth stimulation in GH-transgenic mice but not in domestic livestock that have undergone many centuries of genetic selection. Genetically engineered fish with enhanced phenotypic traits have yet to be implemented in commercial applications.
In addition to the technical issues described, this is partly because of the difficulties in reliably predicting the ecological risk of transgenic fish should they escape into the wild. The ecological consequences of the phenotypic differences between transgenic and wild-type fish, as determined in the laboratory, can be uncertain, because of genotype-by-environment effects.
Salmonids are fish of high economic value which are unable to survive in waters characterized by ice and subzero temperatures. Antifreeze proteins (AFP) are produced by several fish that inhabit extremely cold waters. One possible way of solving the problem of overwintering salmon in sea cages in the northern hemisphere is the transfer of antifreeze protein genes. The AFP-transgenic salmons produced so far express the transgene at levels insufficient to confer freeze resistance.
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