Why produce transgenic fish? There are two chief reasons for introducing novel genes into animals. The first is as a means of increasing knowledge of gene regulation in that particular group of organisms. The second is that transgenic induction may involve some economic benefit from the modified organism in terms of its increased growth potential, disease resistance, or other desirable genetic trait. Both reasons are of importance in the context of transgenic fish. Fish are good candidates for transgenic induction for several reasons. They lay large numbers of eggs, and both fertilization and development are external to the body of the female (except in a few species of mouth-brooding fish and ovoviviparous species, such as the guppy [Poecilia]). Also, the eggs are usually quite large and may be readily pierced by a suitable glass needle.

A third reason is that the piscine genome is readily manipulated, since no mechanism of gender imprinting of genes, such as occurs in mammals, has been observed in fish. Thus, gynogenetic manipulation is fairly straightforward. A further plus for Salmonid fish is that, since they are intensively and extensively farmed, eggs and milt are widely available, at least for certain periods of the year. Additionally, some trout farms have brood stock that are maintained in conditions of artificially controlled day length, thereby shifting the onset of sexual maturity and allowing egg and milt production over periods of the year not normally covered by the breeding cycle. Thus, rainbow trout eggs are available from some farms for up to 9 mo of the year. Tilapia species are widely cultured as food fish in Asia, Africa, and Israel, and are easy to culture in the laboratory. They mature in 4-6 mo in farm conditions, and females lay eggs every 3-4 wk. Transgenic induction in fish is, however, not without some difficulties. Eggs of some fish species are enclosed within a very stout chorion, and this may prove hard to pierce with a small microneedle. A more major objection is the long generation time of many fish species. Under normal laboratory conditions, this is likely to be at least 2 yr in the rainbow trout and perhaps 9 mo in Tilapia species. Some small species, such as zebra fish (Chapter 7) and medaka, have shorter generation times, but offer no economic dimension to the experimental program.

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