condition for their stage of production, exhibited puberty at the expected age, have high conception rates after artificial insemination, and show no clinical or immunologic abnormalities.
It is difficult to determine which problems are due to cloning (nuclear transfer) per se, to embryo culture or transfer methods, or to some combination of cloning and culture/transfer methods (Wilson et al., 1995; Kruip and den Dass, 1997; Van Wagtendonk-de Leeuw et al., 1998). There is considerable variation among studies in rates of early embryonic death, perinatal mortality, LOS, and dystocia (Kruip and den Dass, 1997; Cibelli et al., 2002). The incidence of these problems actually is sometimes lower in animals produced by NT than is typical for animals produced by IVP. Varying levels of expertise and proficiency with the relevant techniques certainly could be contributing factors. Because of their economic value, cloned animals would be expected to receive a high level of veterinary oversight and intervention, which could contribute to the higher postnatal survival of cloned animals in some studies. In cases where there are neonatal problems, they might resolve within a few days of birth (Garry et al., 1996).
One possible contributing factor to the high prenatal and neonatal mortality seen in cloned animals is improper epigenetic reprogramming (Young and Fairburn, 2000; Rideout et al., 2001). Cloned animals have abnormal methylation patterns, although the significance of this for embryo development and survival in livestock is unclear. The longer-term effects of cloning and/or improper epigenetic reprogramming on animal welfare have yet to be thoroughly evaluated; as the number of surviving cloned livestock increases, such assessments will be possible. There still is a need for detailed behavioral studies of cloned livestock, since cloning has been shown to result in the impairment of mice in learning and motor tasks, although this impairment is transient (Tamashiro et al., 2000).
Clones produced by fusion of nuclear donor cells with unfertilized eggs are not identical twins, but “genetic chimeras,” since almost all cloned livestock studied to date have mtDNA from the recipient egg but not from the donor cell (Evans et al., 1999; Takeda et al., 1999). Whether or not there are potential adverse effects on health and welfare due to having nuclear DNA from one source and mtDNA from another are unknown, although mitochondria are responsible for important cellular functions and mitchondrial type theoretically could affect relevant production traits as well. Of course, each time normal fertilization occurs, nuclear genes from the sperm are introduced into a different genetic mitochondrial environment than existed in the cells of the male providing the sperm, so the mixing of nuclear and mitochondrial genes is ubiquitous in nature.
During normal aging, telomere lengths shorten, and this phenomenon has been associated with cell senescence (Chapter 2). Normal reproductive processes restore telomere lengths in newborns, but there has been concern