Ch. 6— Maintaining Animal Diversity Offsite • 141 



Photo credit: Zoological Society of San Diego 



The frozen zoo: Cryogenic storage of cell strains, gametes, and embryos is being undertal<en 

 as part of the conservation activities of zoos. 



properly, the procedures should allow regener- 

 ation of the original population without the 

 genetic changes inherent in the maintenance 

 of captive breeding populations. 



The long-term genetic stability of frozen em- 

 bryos and sperm is a matter of some concern. 

 Freezing and thawing does not appear to in- 

 crease the mutation rate in these tissues, but 

 long-term exposure to low levels of radiation 

 could be a problem, especially because DNA 

 repair mechanisms would be inoperative at 

 — 196° C (1). Mouse embryos and semen have 

 been kept frozen for at most 10 and 30 years, 

 respectively. However, frozen mouse embryos 

 also have been exposed to augmented levels of 

 radiation equivalent to that experienced in 



2,000 years of normal storage without appar- 

 ent ill effects (16). Normal progeny were 

 produced. Thus, risks of genetic damage from 

 background radiation appear negligible. 



Just as captive breeding programs reinforce 

 rather than replace natural populations, cryo- 

 preservation efforts reinforce rather than re- 

 place captive breeding programs. In wild spe- 

 cies, females must still be available to gestate 

 frozen embryos or to provide female gametes 

 in matings involving frozen semen. In domes- 

 tic species, breeding populations selected for 

 biologically or economically important traits 

 may still be required, but cryogenic storage of 

 individuals from the original population pro- 

 vides a valuable measure of insurance. Periodic 



