Ch. 6— Maintaining Animal Diversity Offsite • 145 



more unrelated parents (3). Assuming a preg- 

 nancy rate of 30 percent and a subsequent sur- 

 vival rate of 80 percent, 167 to 333 frozen em- 

 bryos would be required for each breed. 



For particularly rare breeds, too few individ- 

 uals may be available to comply with these rec- 

 ommendations. Although a viable population 

 can be established with as few as 4 to 10 ani- 

 mals, such a population may differ considera- 

 bly in genetic composition from the unendan- 

 gered source population, and it may have an 

 impaired ability to respond to future changes 

 in environment. Initiation of a captive popula- 

 tion with only a few founders could be justi- 

 fied if a reasonable likelihood exists of obtain- 

 ing additional individuals from the wild at some 

 future point. 



Most domestic and wild populations exist as 

 groups of semi-isolated subpopulations. The ex- 

 tent of this subdivision differs among species 

 and influences the sampling process in devel- 

 oping a captive population. If the population 

 is strongly subdivided, genes present in one sub- 

 population may be absent in others, and sam- 

 pling must attempt to include individuals from 

 all major subgroups. According to one calcu- 

 lation, the recommended 20 to 30 founder ani- 

 mals can be decreased by about one-third if the 

 population exists as a small number (2 to 10] 

 of very distinct subpopulations, but it should 

 be increased by about one-third if 50 to 100 dis- 

 tinct subgroups exist (35). 



Current assessment of genetic diversity 

 among subpopulations must be based on bio- 

 chemical, historical, morphological, and eco- 

 logical criteria. For genes that produce an iden- 



tifiable protein molecule, genetic differences 

 can be identified by the behavior of the pro- 

 teins on an electrically charged (electropho- 

 retic) gel. Electrophoretic testing procedures 

 help identify the existence and distribution 

 of various genes in different subpopulations. 

 Rapid advances in molecular biology also hold 

 promise of DNA probes that would directly as- 

 sess the similarity of DNA molecules among 

 subpopulations. In domestic animals, however, 

 differential selection pressures may result in 

 considerable genetic variation among breeds 

 with similar evolutionary origins. 



Reproductive Rate 



Reductions in diversity are cumulative over 

 generations in small populations, so the losses 

 associated with a single sampling event are 

 much lower than those that would accumulate 

 over time if the population size remained at the 

 founder number. As soon as a captive breed- 

 ing population is started, therefore, it should 

 be expanded to a size consistent with continued 

 maintenance of the available genetic diversity. 

 If the reproductive rate is high, maintenance 

 can be achieved rapidly and with only a few 

 founders. If the reproductive rate is low, sev- 

 eral intervening generations at limited popu- 

 lation size will be required to reach eventual 

 target numbers, and more founders will be 

 needed to assure retention of genetic diversity 

 during this period. Sample sizes for cattle have 

 been suggested to be twice those required for 

 pigs, sheep, and goats, for example (3). One 

 advantage of cryogenic preservation would be 

 that the period of population expansion can be 

 deferred until appropriate facilities and habi- 

 tat are available. 



THE MOVEMENT OF GERMPLASM- DISEASE AND 

 QUARANTINE ISSUES 



For many reasons, effective programs for 

 conservation of endangered populations will 

 require extensive international transfer of 

 germplasm. First, facilities, funds, and institu- 

 tional stability in developing countries may be 



insufficient to allow endangered species to be 

 conserved onsite, and animals may have to be 

 transferred to countries better equipped to sup- 

 port captive breeding programs. Second, with 

 wild animals, effective maintenance of genetic 



