Problems and prospects of transgenic fish production

Abstract

The ristricted scope of cytoplasmic introduction of transgene, and the limited availability of fish cDNA sequences and promoters, are the major hurdles in the production of transgenic fish. Although integration frequency is low, expression is usnally detected in transgenic fish, into whose eggs transgene of fish origin is introduced. Long-term researches are desired to produce drought-resistant(aestivating) transgenic carps. To ensure survival and transgenicity of the fish, a volume of 10-20 nl with a DNA concentration of 10-15 μg/ml containing 1-2 million copies is usually injected into the fish egg at its 1-cell stage. Catfish and tilapias are too sensitive to microinjection and hence must be subjected to alternate methods of gene introduction like electroporation and sperm-mediated transfer. The quantity of transgene is decreased as functions of injection dose and age of the transgenic fish. A most common observation is the mosaicism in transgenic fish; the delayed delivery and/or integration of the transgene is implicated as the causative factor. Much of available information on transmission is based on zebrafish, whose generation time is short. The number of copies of the transgene inherited by the offspring belonging to F₁ and F₂ varies from tissue to tissue, and individual to individual. Though effictive viral and metalothionein promoters are now giving way to fish promoter genes and ‘all-fish’ gene sequences comprising endogenous fish promoters (e.g-β-actin,AFP) have been shown to be more effective. The tedious procedures involved in Southern and Western blot techniques have made some scientists to use reporter genes, whose expression can be detected by simple and rapid methods. The use of sterile triploid eggs is recommended for initial production of transgenic fish. Techniques for cryopreservation of sperm and androgenesis are urgently required for many Asian fish species, whose wild strains have to be preserved.