Animal Breeding and Transgenic Animals
Transgenesis offers considerable opportunity for advances agriculture. In livestock, the ability to insert new genes for such economically important characteristics as fecundity, resistance to or tolerance of other environmental stresses would represent a major breakthrough in the breeding of commercially superior stock. Another opportunity that transgenic technology could provide is in the production of clotting factors in the milk of domestic livestock. The genes coding for these proteins have been identified and the human factor IX construct has been successfully introduced into sheep and expression achieved in sheep milk (Clark et al., 1990). Moreover, the founder animal has been shown to be able to transmit the trait to its offspring (Niemanet al., 1994). To date, the majority of genes transferred into
sheep have been growth hormone encoding gene constructs. Unfortunately, in most cases the elevated growth hormone levels have resulted into a clinical diabetes situation leading to an early death of the transgenic sheep (Rexroadet al., 1990). The first reports of the production of transgenic animals created a lot of excitement among biological scientists. In the field of animal breeding, there were diverse opinions on how the technology might affect livestock genetic improvement programmes. Some (Ward et al., 1982) believed that it would result in total re-organization of conventional animal breeding theory while others (Schuman and Shoffner, 1982) considered the technology as an extension of current animal breeding procedures which, by broadening the gene pool, would make new and novel genotypes available for selection. Application of the technology in animal improvement is still far from being achieved. However, consideration needs to be given to its potential role in this field. Smith et al (1987) presented a comprehensive evaluation of strategies for developing, testing, breeding and disseminating transgenic livestock in the context of quantitative improvement of economic traits. An important contribution of transgenic technology is in the area of basic research to study the role of genes in the control of physiological processes. The understanding of the molecular control of life processes has important implications for both medicine and agriculture. For example, the generation (through mutation of an endogenous gene) of an organism which lacks a specific gene is a powerful tool to investigate the function of the gene product. This type of genetic analysis has been facilitated by the availability of invitro cultures of embryonic stem cells from mice (Bradley et al., 1984).
Recent advances in in vitro technology (in vitro fertilization and maturation) will increase the number of zygotes available for gene transfer purposes. This, plus the utilization of embryonic stem cell (Sticeet al., 1994) and primodial germ cell technologies should enhance the efficiency of gene transfer in cattle and sheep considerably.