Status of Cloning Domestic Animals
Live young have been produced by nuclear transfer in all major domestic mammalian species: cats, dogs, horses, cattle, goats, sheep, and pigs. Cloning in cats appears to be repeatably successful; oocytes may be obtained using tissue recovered from clinical ovariohysterectomies. Interspecies cloning of closely related nondomestic cat species using domestic cat oocytes has produced live young.
Cloning in dogs is complicated by several factors. Mature canine oocytes must be obtained from the oviduct after ovulation, because effective methods for in vitro maturation of canine oocytes have not been developed; additionally, bitches are in estrus only about once every 6 mo, so oocyte availability is low and synchronization of recipients is problematic. Cloning in horses results in a low blastocyst development rate (3%–10%), but viability after transfer is among the highest reported; ~30% of transferred embryos produce live foals, and postnatal survival is high (>85%).
Cloning in food animal species is facilitated by the ready availability of oocytes from slaughterhouse tissue. Cloning in sheep and cattle is inefficient (5%–15% of transferred embryos result in live young), and 30%–50% of live neonates die by 4 yr of age. Cloning in sheep and cattle is associated with frequent placental abnormalities, especially low numbers of atypically large cotyledons. There is a high incidence of large offspring syndrome in cloned calves and lambs (ie, fetal overgrowth with related abnormalities) and, in live calves, of transient metabolic abnormalities such as hypoglycemia or poor renal function. Cloning in goats has similar efficiency in production of live young per embryo transferred, but cloned kids tend to have greater viability. Goat oocytes are typically obtained by follicle aspiration ex vivo, and this may increase viability of resulting clones. In pigs, 1%–5% of transferred recombined oocytes produce live young, but cloning is made practicable by the ready availability of large numbers of oocytes and the ability to transfer hundreds of recombined oocytes to the oviducts of a single recipient. Cloned piglets are generally healthy; they have a higher than normal incidence of stillbirth and of some abnormalities but do not develop large offspring syndrome.