144 • Technologies To Maintain Biological Diversity 



tion, often lacking for threatened breeds within 

 developing countries, can be extremely impor- 

 tant. In the absence of a formal evaluation, bib- 

 liographic databases may provide some needed 

 information (12). Following the evaluation, 

 breeds can be put in one of four categories: 



1 . Useful under current economic conditions. 

 Such stocks should be integrated into the 

 production system in a way that uses their 

 genetic material in pure lines, crosses, or 

 selected gene pools. Pure lines should be 

 maintained with selection for net merit in 

 production systems that are characteris- 

 tic of commercial production within the 

 country of origin or preserved cryogeni- 

 cally if maintenance as a pure line is im- 

 possible. 



2. Viable under current economic conditions 

 in relation to other indigenous types, but 

 inferior (in pure lines or in crosses) to im- 

 proved types; no obvious biological ex- 

 treme or major gene. Germplasm preser- 

 vation in such populations could have two 

 rationales: preservation of frozen semen 

 or embryos to prevent total loss of the germ- 

 plasm and as insurance during a period of 

 breed replacement with the improved 

 types, or maintenance as pure lines for 

 their cultural-historical value at the option 

 of local governments and producers. A 

 dual philosophy exists here— a unique pop- 

 ulation should not be discarded until its 

 inferiority is documented, but preservation 

 should not hinder use of improved breeds. 



3. Not competitive under current economic 

 conditions; possesses an extreme pheno- 

 type for one or more traits or carries a ma- 

 jor gene. Such breeds should be conserved 

 cryogenically or as pure lines. Research use 

 should be encouraged, and selection to in- 

 tensify the extreme phenotype should be 

 considered. 



4. Not competitive with existing adapted 

 types; not a biological extreme; no major 

 genes for production traits. No particular 

 efforts should be made to conserve such 

 breeds unless they can be documented as 

 unique in their genetic origin. Stocks could 

 move from the second category to this one 



as more productive breeds prove them- 

 selves. 



Preservation and Collection 

 Considerations 



The number of individual animals required 

 to initiate a captive population or a cryogenic 

 store will depend on the nature and extent of 

 the genetic diversity to be maintained, on the 

 population structure in nature, and on the rate 

 at which the captive population reproduces. 



Tiie Nature and Extent off 

 the Cenetic Diversity 



Both natural and artificial selection reflect 

 different fitness or reproductive success for in- 

 dividuals carrying different genes and lead to 

 changes in the frequencies of those genes in 

 a population. The diversity of genes in a large, 

 interbreeding population may be quite exten- 

 sive, with different individuals possessing a 

 somewhat different genetic composition. It is 

 this diversity that enables populations to adapt 

 to environmental changes. Indeed, preserving 

 the evolutionary potential of the species re- 

 quires the maintenance of these possibly use- 

 ful genes. 



The objective in sampling a source popula- 

 tion, then, should be to obtain a group that rep- 

 resents the bulk of its genetic diversity. Fewer 

 animals are required to obtain an adequate ini- 

 tial sample of a population's diversity than are 

 required to ensure continued maintenance of 

 that diversity over time. Thus, 20 to 30 founder 

 animals should provide an adequate sample of 

 the genetic diversity in most interbreeding pop- 

 ulations (6,43), but much larger subsequent pop- 

 ulation sizes are required to prevent erosion 

 of this diversity over time. 



In terms of cryopreservation, enough frozen 

 semen to produce 10 live offspring from each 

 of 25 sires, which would require 50 to 100 units 

 of semen per sire, would constitute a good sam- 

 ple of an interbreeding source population (40). 

 The Council for Agricultural Science and Tech- 

 nology recommends production of 40 to 80 off- 

 spring from frozen embryos representing 20 or 



