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Transgenics in aquaculture
The best genotypes for aquaculture in the future may be developed using traditional selective breeding and the new biotechnologies (Dunham et al., 2001; Hew & Fletcher, 2001). Initial experiments show good potential for this combined approach, using mass selection and cross-breeding, genetic engineering and selection, genetic engineering and cross-breeding, and sex reversal and polyploidy; all work more effectively in combination than alone. Genetic enhancement is an increasingly important com-ponent of aquaculture management and, if used properly, has the potential to enhance aquaculture production. The constraints of property rights, food security and consumer perceptions have, however, to be solved.
Transgenic fishes could be as beneficial as transgenic plants and land animals and could be more effective than those traditionally bred in producing improved fish strains (Hew and Fletcher, 1997). The transgenic technology can be helpful not only in producing faster growing fish, but also in regulating maturation/reproduction, enhancing resistance to diseases (Leong et al., 1999) and to environmental extremes (Fletcher et al., 1988), improving nutritional and other qualities of meat, and modification of metabolic pathways to enhance food conversion efficiency (Doupe and Lymbery, 2003).
The first form of gene transfer has been successfully accomplished in China. Due to lack of fish gene sequences, initial transgenic research employed mammalian growth hormone (GH), which enhanced growth in some species, but not all species were examined. Salmonids showed no effect (Guyomard et al., 1989) in spite of their being very responsive to growth stimulation by exogenously administered GH protein, though gene constructs using fish GH sequence had shown some lower growth enhancement in common carp, catfish and tilapia (Martinez et al., 1996). This is probably the first evidencethat growth enhancement in fish can be achieved by transgenics.
Hew and Fletcher (2001) list 13 instances involving eight fish species (common carp, crucian carp, catfish, loach, tilapia, pike,Atlantic salmon and Pacific salmon), where growth increases have been obtained using GH genes, as evident from published reports between 1986 and 1996. Growth enhancement was in the lowest range for common carp (1.1-fold) and catfish (1.2-fold) and highest for two salmonidstested (Atlantic salmon 10-fold; sockeye 11-fold).
Recently GH gene constructs have been used in obtaining precocious smoltification of Atlantic Salmon (Du, 1992) and of Coho salmon (Devlin et al., 1995a,b). When a gene is transferred with the objective of improving a specific trait, it may affect another trait, causing positive or negative ‘pleiotropic’ effects. So it is important to evaluate all major traits of transgenic fish. Transfer of growth hormone genes have been observed to affect body shape and composition, feed conversion efficiency, disease resistance, reproduction and tolerance of low oxygen concentration, carcass yield, swimming ability and predator avoidance.
Most studies in aquaculture genetics to date are concerned with the improvement of growth rate of selected fishes (see above). It is also possible to genetically improve the food conversion efficiency (FCE, which is another expression of FCR), but there are gaps in knowledge in these interacting aspects of genetics and nutrition in aquaculture, which have to be bridged, taking cues from terrestrial livestock farming.
More recent approaches to inbreeding, cross-breeding, hybridisation, genetic selection, correlated responses, polyploidy, sex manipulation, gynogenesis, androgenesis and cloning, as well as applications to molecular techniques in aquaculture genetics are explained.
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