Production of transgenic livestock: Overview of transgenic technologies

Transgenic livestock came into existence in 1985 with the production of the first transgenic pigs and sheep (Hammer et al., 1985). Since then, various technologies have been developed for the generation of transgenic livestock (Fig. 6.1). Sometimes entirely new technologies emerged, seemingly superseding an established technique, while occasionally new improvements to older methods saw them brought back to the forefront. Nevertheless, most approaches are still in use today albeit in combinations with other developments enhancing the original methods by making use of improvements. Each will offer a unique set of advantages and disadvantages. Hence, the choice of technique will be dependent on the specifics of the application and not least on the species of livestock considered for transgenesis. The following will provide an overview of available technologies for the genetic engineering of livestock species. The chapter will describe the main approaches and their advantages and disadvantages and will portray the technical advancements that today allow for the efficient and precise engineering of livestock genomes almost without limitations. Generally, transgenic technologies can be divided into embryo-mediated and cell-mediated approaches. The first strategy introduces the genetic modification into an embryo (Fig. 6.2a), while for the latter, the genetic information is introduced into a cell which is subsequently used to generate an entire animal based on the genetics of this cell (Fig. 6.2b). The following overview will describe the various enabling technologies that have been successfully used for the generation of transgenic livestock and will discuss their relative strengths and weaknesses. The order in which the different methods are described is intended to provide an overview and does not reflect the chronological order of their development (Fig. 6.1).