Appendix II C— Animal Fertilization Technologies • 313 
n n HK 
Kapiil progrt's.s in restMirh is anticipated and 
main of th»* (xitential applications of in \ itro fertiliza- 
tion to animal breeding should become practical 
VMthin the next ft) to 20 years V\ ith furtber develop- 
ment of in vitrt) fertilization methoilolo^v . along with 
storage of unfertilizetl (kh v fes (gamete banking), fer- 
tilization of desiretl crosses slunilil become possible. 
In the more distant fiiturtv genetic engineering and 
sjH*rm >e\ing along with in vitro fertilization may 
iHH ome fxissible 
t*itrtherutfivncsis 
i)t:i IM noN 
The initiation of dev(>lopmi*nt in the absense of 
sjx'rm 
STMKOI niK \HT 
Parthenogenesis has not been satisfactorily dem- 
onstrattxl or describetl for mammalian species. The 
lx*st available information leads to the conclusion 
that maintenance of parthenogenetic development to 
prixluce normal offspring in mammals approximates 
imfxissibilitv 
(Inning: pradiictian of identical tit ins 
nUFIMTIOV 
The protiuction. using a variety of methods, of 
genetically identical indiv iduals. 
ST \TE OF THE ART 
There are several ways to obtain genetically iden- 
tical livestock. The natural way is identical twins, 
although these are rare in sp€*cies other than cattle 
and primates. Both natural and laboratory methods 
depiend on the fact that the blastomeres of early em- 
bryos are totipotent (i.e.. each cell can develop into a 
complete individual if separated from the others.) 
For practical purposes, highly inbred lines of some 
mammals are already considered genetically iden- 
tical: F, crosses of these lines are also considered 
genetically identical and do not suffer from the 
depressiv e effect of inbreeding. 
AD\ A.VTAGE 
•An advantage of identical twins is the e.xperimen- 
tal control provided by one animal through which 
two sets of environmental conditions can be com- 
pared for effects on certain end points, e.g., native v. 
surrogate uterine environments for gestational de- 
velopment, nutrition on milk production, etc. 
Cloning: nuclear transplantation 
DEFIMTION 
The production of genetically identical mammals 
by inserting the nucleus ot one cell into another, 
before or after destroying the original genetic com- 
plement. These occur by separation of embryos or 
parts ot embryos early in development but well after 
fertilization has occurred. 
STATE OF THE ART 
h.xpeM'imentalists have found in certain amphibia 
that transplantation of a nucleus from a body cell of 
an embryonic (tadpole) stage into a zygote following 
destruction or removal of the normal nucleus can 
lead to development of a se.xually mature frog. 
FllTDRE 
The ideal technique for making genetic copies of 
any giv'en outstanding adult mammal would inv^olve 
inserting somatic (body) cell nuclei into ova, which 
may take years of work to perfect if indeed it is possi- 
ble. There is some evidence that adult body cells are 
irreversibly differentiated. 
How identical will clones be? They can be ex- 
pected to be fairly similar in appearance. They would 
be less similar than identical twins, however, which 
share ooplasm and uterine and neonatal environ- 
ments. Furthermore, certain components are inher- 
ited exclusively from the mother, e.g., the mitochon- 
drial genome and perhaps the genome of centrioles. 
The random inactivation of one or the other of the X 
chromosomes may also limit similarities. Other dif- 
ferences among clones would result from the pre- 
natal environment; in litter-bearing species even 
uterine position can affect offspring. In single-bear- 
ing species the maternal effect may be pronounced. 
Environmental differences in later life may greatly 
affect certain traits, even if those traits have a strong 
genetic component. 
Serious technical barriers must be overcome 
before realistic speculation of possible advantages in 
animal production can be foreseen. 
Cell fusion 
DEFINITION 
The fusion of two mature sex cells or the fertiliza- 
tion of one ovum with another. An analogous scheme 
for the male would be accomplished by microsurgi- 
cal remov'al of the female pronucleus and substitu- 
tion of nuclei from two sperm. Combining sex cells 
from the same animal is called “selfing.” 
