Cloning, transgenesis, and genetic variance in animals.

251 THIS SYMPOSIUM, “Methods for Manipulating the Embryonic Genome In Vitro,” provides much material for further discussion. Simply to review these reviews would constitute failure to learn from the observation that there are nearly as many reviews of transgenesis as original papers (Wall, 2001). Paradoxes abound when considering the subject matter of the symposium; one example is that one of the most important tools for transgenesis, which can increase genetic variance, will likely be cloning by nuclear transplantation/cell fusion, which has the reputation of decreasing genetic variance. Another paradox is that genetic knockouts produced by transgenic techniques are the ultimate in decreasing genetic variance, resulting in deleting the function of an entire gene. To the knowledgeable scientist, the papers in this symposium constitute a collection of technologies for making genetic modifications in germ-line cells (Seidel, 2000). Cloning by nuclear transfer, as suggested above, can serve as a mechanism for moving genetically modified embryonic or somatic genomes into the germ-line. The seminal paper by Wilmut et al. (1997) on producing offspring via nuclear transfer using cells derived from somatic fetal fibroblasts as nuclear donors provided strong evidence that this would be possible; subsequent publications have substantiated this supposition. The realization that it is possible, albeit with relatively low efficiency, to move genetic material from somatic cells to germ-line cells constitutes a paradigm shift, much as occurred when the central dogma of molecular biology (DNA RNA protein) was shaken when the discovery of reverse transcriptase demonstrated that the first arrow was reversible. In the same way as reverse transcriptase has become an everyday tool in molecular biology, cloning animals from somatic cells is becoming a laboratory resource for researching other questions rather than just an end in itself. This is illustrated in the paper by Campbell et al. (2001), which describes various possible applications of cloning, including rescuing cell lines from senescence (Denning et al., 2001). This recovery mechanism that resets cellular youth is yet to be characterized, and it may or may not be as simple as regulation of telomerase genes (Oback and Wells, 2002). If electronic databases of biological research are searched using the keyword “cloning,” one comes up with many more papers on cloning DNA than making genetic copies of living organisms, and only a small proportion of the latter concern mammals. But cloning DNA illustrates a point nicely: for most rational transgenic experiments, despite recent advances (e.g., Maga, 2001), millions of identical copies of the transgenic construct are required. Similarly, for genetic manipulation of populations of animals, more than one genetic copy often will be useful, particularly if the copies can be of different ages. Several concepts of genetic variability will be described briefly. There are many ways of thinking about genetic variance, some of which are listed in Table 1 for farm animals (although most of these concepts apply to any species). In practical terms, genetic variance often is most relevant at the level of the herd or flock, since that is the economic unit being managed. The success of the herd, of course, depends on individual animals