IMPACTS OF CONTEMPORARY BIOTECHNOLOGY-ANIMAL SCIENCE 75

the qualitative similarity between transgenic and traditional breeding and
they should at the same time remember that contemporary stock animals
are the products of gene pools which have been intensively manipulated
since the dawn of domestication.
 Two different approaches have been employed to produce transgenic
animals. The Palmiter/Brinster approach involves direct intervention
shortly after the egg is penetrated by sperm. At this time, the sperm's head
rounds up to form the male pronucleus which contains the genetic contri-
bution of the male to the offspring. At approximately the same time, meio-
sis is completed in the egg and the female pronucleus forms. The two
pronuclei then fuse to form a diploid nucleus which is the first nucleus of
the offspring. By the original Palmiter/Brinster approach, the recombi-
nant DNA was microinjected into the male pronucleus immediately before
fusion. Brinster et al. (1985) have subsequently shown that injection into
the female pronucleus is nearly as efficient but injection into the cytoplasm
surrounding the two pronuclei or injection into one of the two diploid
nuclei immediately after the first division is considerably less efficient.
That the DNA must be injected into the pronuclei gives rise to the only
apparent limitation of the Palmiter/Brinster approach to creating
transgenic animals: the pronuclei must be visualized to be microinjected.
The pronuclei of sheep ova could be seen only by special microscopy and
the pronuclei of swine ova were visible only after the egg's cytoplasm had
been stratified by centrifugation (Hammer et al., 1985b). These authors
found that integration of a human growth hormone gene was about 10
percent efficient in pigs but only about one percent efficient in sheep.
 The second approach to producing transgenic animals employs re-
 troviruses, which can be integrated into the genome of an infected cell and
 inherited by progeny of that cell as a stable genetic trait. This property can
 be exploited by exchanging a portion of the viral genome for the gene of
 interest as was done by Jahner et al. (1985). These researchers infected
 mouse embryos at the four to eight cell stage with a construct of the re-
 trovirus Moloney murine leukemia virus bearing the E. coli gene for the
 enzyme Xanthine (guanine) phorphoriboryl transferase. Of seven mice
 which were derived from infected embryos, one male was transgenic for
 the bacterial gene and his progeny were crossed to produce a strain of mice
 homozygous for this locus.
 In a similar experiment, Souza et al. (1984) replaced the Src gene of the
 avian retrovirus, Rous Sarcomca virus, with the gene for chicken growth
 hormone and then infected nine-day-old chick embryos with the recombi-
 nant virus. After hatching, 50 percent of the chicks had circulating GH
 levels three- to ten-fold higher than normal but did not grow more rapidly.