Consequences of manipulating gametes and embryos of ruminant species

During the past 12 years, ruminants have provided a focus for some significant advances in mammalian reproductive biotechnologies. Lambs were the first offspring generated after nuclear transfer of fetal or adult cells to enucleated oocytes, and many calves of pre-determined gender are today the result of commercialized semen sexing. In 1990, the birth of one calf provided living proof that even 'dead' spermatozoa can be paternal, whereas, more recently, a short-lived gaur calf and viable mouflon lamb represented a novel option for conservation of endangered species. As well as highlights, hazards have emerged, resulting in setbacks or developmental anomalies, such as those associated with the large offspring syndrome which encompasses a range of adverse fetal, placental and post-natal phenomena expressed in ruminants. In this review, the developmental and other consequences of applying manipulative procedures, such as assisted fertilization, semen sexing, cloning and gene transfer, to gametes and embryos from bovine, ovine and caprine species are considered. Although assisted fertilization techniques can overcome mammalian infertility, they also usurp natural gamete selection safeguards, but not always with impunity. In the case of manipulations such as cloning, and to a lesser extent gene transfer, it is evident that nuclear-cytoplasmic interactions and nuclear-mitochondrial DNA interdependences are at least partially damaged or destroyed with a view to reconstruction. Therefore, among surviving zygotes and embryos it is inevitable that the legacy is frequently one of altered genetic, epigenetic or cellular programmes and processes.