Because fish lay large numbers of eggs, in which both fertilisation and development is external, they are rather suitable organisms for transgenic induction. To date fish of many different species have been used in such work - rainbow trout, atlantic salmon, channel cat fish, tilapia, medaka, zebrafish, goldfish, carp and loach. Normally the induction of transgenics has involved injection of multiple copies of the chosen novel gene into the egg cytoplasm soon after fertilisation, followed by assays for subsequent gene integration, expression and transmission to progeny.
Genes which have been utilised for transgenic induction in fish include somatotrophin (growth hormone) of both mammalian and piscine origin, metallothionein, 'antifreeze' protein, and crytallin coding sequences. A number of laboratories have demonstrated by Southern blotting of restricted DNA fragments that genomic incorporation has been achieved, although in others there appears to be a continuing problem of long term persistance of extrachromosomal DNA material. Expression of the injected sequences tends to be at a low level and in some cases not to be detectable. This is probably due to the inefficiency of mammalian gene promoters in fish cells and will hopefully be overcome by the use of genes of piscine origin. Germ line integration and transmission to progeny of the novel sequence has been established in a few laboratories.
Transgenic fish may also offer some potential as expression systems for valuable pharmaceutical products. Problems of containment and planned release of transgenic fish are also briefly discussed.