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Essential Conservation Considerations The collection, transfer, and use of animal germplasm are influ- enced by several fundamental considerations. In contrast to plant germplasm collections, little or no collecting for breed- ing purposes is done from free-ranging or wild animal populations. Almost universally, individual domestic animals or their gametes or embryos must be purchased. The maintenance of adequate numbers in representative live populations will generally not be economically feasible for livestock species kept solely for preservation. When cryopreservation is used, semen or embryos must be processed im- mediately after collection to ensure their survival. Regeneration of cryopreserved germplasm requires an established protocol in which host females are available, and in the proper physiological condition, for embryo transfer or artificial insemination. Internationally, the movement of animal germplasm may be limited by strict quarantine regulations. These constraints must be addressed in planning and instituting conservation programs. This chapter examines the technical elements of managing the genetic resources of livestock including preservation criteria, sampling strategies, and preservation methods. CRITERIA FOR CONSERVING POPULATIONS An animal germplasm conservation program will require deci- sions on the populations, which can be a particular breed or stock, to be preserved and the methods to be used. Factors that will influence ~9
50 I Livestock those decisions include the status of a particular population and its uniqueness or economic importance, both currer~t and potential. Status and Vulnerability of the Population To evaluate accurately a population's status, it is necessary to have a census of its total number and an understanding of its dynam- ics, that is, whether the population size is stable or changing and the rate of any possible change. Rough estimates of the size at which a population in a developed country should be considered endangered have been provided by Maijala et al. (1984) and are summarized in Table 2-1. A more detailed scheme for assessing populations and determining appropriate conservation or management actions has re- cently been published by the Food and Agriculture Organization (FAO) of the United Nations (Henson, 1992~; it is depicted in Figure 2-1. In He United States the American Minor Breeds Conservancy (AMBC) uses the categories rare, minor, watch, and feral to classify the status of a livestock breed that may become endangered based on the num- bers of individual registrations per year (Henson, 1985~. (See Table 2-2.) These categories are defined as follows: · Rare cattle and horse breeds with less than 200 registrations per year; sheep, goat, and pig breeds with less than 500 registrations per year. · Minor cattle, sheep, goat, and horse breeds with less than 1,000 registrations per year. Pig breeds with less than 2,000 registra- tions per year. TABLE 2-1 Recommended Sizes at Which Livestock Populations in Europe Should be Considered Endangered Livestock Species Size of Population Direction of Change Number of Breeding Females Number of Breeding Males Cattle 1,000-5,000 Declining <1,000 <20 Sheep and goats 500-1,000 Declining <500 <20 Pigs 200-500 Declining <200 <20 SOURCE: Maijala, K., A. V. Cherekaev, J.-M. Devillard, Z. Reklewski, G. Rognoni, D. L. Simon, and D. E. Steane. 1984. Conservation of animal genetic resources in Europe. Final report of an EAAP (European Association of Animal Production) working party. Livestock Prod. Sci. 11:3-22. Reprinted with permission, @)1984 by Elsevier Science Publishers.
Essential Conservation Considerations / 51 Number of Breeding Females ~ E~1 1 1~00~5~000| 1 5,00~10,0001 ~ · ~ / ~ / ~ / ~ / _ ~ / ~ / ~ / ~ / ~ 1 / 1 / 1 / 1 / 1 _~ ~ ~ ~ ~ Irk 1 / 1 1 1 / 1 / 1 _ 1 / 1 / 1 / 1 / ~ _ 1 / 1 / 1 / 1 / 1 - 1 / 1 / 1 / ' 1 / ~ I ~ I ~ / ~ / Monitor 4 [~1: 1 ~1 evauate FIGURE 2-1 Proposed scheme for classifying livestock breeds and popula- tions identified as in need of preservation. Note: Greater detail and proto- cols for determining specific actions can be found in Henson, E. L. 1992. In Situ Conservation of Livestock and Poultry. FAO Animal Production and Health Paper No. 99. Rome, Italy: Food and Agriculture Organization of the United Nations. · Watch-breeds whose registrations over a 25-year period have shown a steady decline or where registrations are less than 5,000 per year. · Feral stocks known to have been running wild for at least 100 years with no known introductions of outside blood. The information contained in Tables 2-1 and 2-2 is based on the numbers of breeds in developed countries and reflects subjective judg TABLE 2-2 Categories Used to Classify the Status of Livestock Breeds by the American Minor Breeds Conservancy Maximum Number of Breed Registrations Per Year Status Cattle Horses Sheep GoatsPigs Rare 200 200 500 500500 Minor 1,000 1,000 1,000 1,0002,000 Watcha 5,000 5,000 5,000 5,0005,000 aA breed may also be placed in this category if a steady decline in registrations has occurred over the preceding 25 years. SOURCE: Henson, E. L, 1985, North American Livestock Census, (Updated May 1988.) Pittsboro, N.C.: American Minor Breeds Conservancy. Reprinted with permis- sion, O1985 by American Minor Breeds Conservancy.
52 / Livestock meets about the necessity for concern. For developing countries, other factors may affect the risk of survival and must be considered when evaluating risk. For example, a population may be at serious risk, even if it numbers several thousand, because of a drought or disease epidemic. Geographic isolation or fragmentation may also affect risk. Extensive crossbreeding, particularly if artificial insemi- nation is used, can rapidly change the genetic composition of a given population. Unique populations can be genetically diluted through such activities. The FAO recommends that populations in develop- ing countries be evaluated as candidates for preservation when the number of breeding females drops to about 5,000 within a total popu- lation size of about 10,000 (Hodges, l990b). Uniqueness and Importance of the Population Populations that have developed in isolation may possess impor- tant genetic traits not found elsewhere. Many indigenous breeds are found only in the region in which they originated. They should receive special consideration for inclusion in a national conservation program, particularly if the environmental conditions encountered in the region where they were developed are unusual. Alternatively, if THE SIGNIFICANCE OF EFFECTIVE POPULATION SIZE The size of a population is simply the number of individuals in it. However, scientists are more concerned with the flow of genes within the number of individuals contributing gametes to the next genera- tion the "effective" parents in the population. The relative number of effective parents of each sex in a population is important to assess effective population size (Ne). If, for example, there are few breeding males in a population, then the effective size will be much smaller than its actual population size. In general, any- thing that affects the breeding capacity of individuals in a population or the survival potential of their offspring can alter effective popula- tion size. Thus, selection, whether natural or part of a breeding pro- gram, can be important. The importance of Ne to livestock is seen in a hypothetical example of world populations of black and white cattle the familiar Holstein (American) and Friesian (Europe) dairy cattle (Goddard, 1991~. While providing significant benefits in terms of wide access to the best pro- ducing animals, international trade of the germplasm of these animals through the exchange of cryopreserved semen and embryos has the
Essential Conservation Considerations / 53 a breed is also found in neighboring countries, or if several breeds appear equally well adapted to some environmental niche, the need for protective measures may be lower. Measures of genetic distance may help identify candidates for preservation. For example, the close relationship of an endangered breed to another nonendangered breed could lessen the urgency to preserve it over another, more distantly related breed. STRATEGIES FOR SAMPLING POPULATIONS Once a population has been identified for preservation, the next step is to determine how to sample it. Sampling strategies must take various factors into consideration, including geographic distribution of the population and subpopulation structure, if known. Sampling biases can be minimized by using a stratified system of sampling, identifying populations (breeds) and subpopulations (herds or stocks), and taking samples (of unrelated individuals and at random) in a systematic manner. The preserved samples should reflect the pro- portional balance of subpopulations present. With stocks that are rare or endangered, there may be little choice for sampling, and es- sentially all individuals available may be included. (Factors that af potential to greatly reduce their effective population size. Goddard notes that the potential exists to manage these cattle as a single global breed. Under this scenario, the Ne for the global population of black and white cattle could be as low as 80, even though the estimated world population would include several million milking cows. A glo- bal program would likely use sperm from very few bulls and, through embryo transfer, limit the number of females contributing to future generations. The global breed scenario may be an extreme example, but it illus- trates the potential impact of widely distributing the genes of a limited number of individual animals. Attainment of small global Ne will be moderated by even slight variation in breeding objectives or environ- ment because breeders in different countries would then need to use somewhat different breeding animals to meet their goals. Likewise, some would argue that an Ne of 80 may be sufficient to allow reason- able long-term selection responses. Yet long-term propagation of a population at such modest Ne will almost certainly result in some loss- es in genetic variability. The future importance of such diversity can- not now be quantified, but programs to maintain a reserve of genetic diversity seem warranted in populations being maintained at low Ne.
54 / Livestock feet the collection and storage of embryos, including estimates of the numbers required, have been presented by Springmann et al. [1987~.) The number of individuals to be sampled need not be large to prevent loss of genetic variation. For example, the committee sup- ports the conclusions of the Council for Agricultural Science and Tech- nology (1984) and of Smith (1984b) that, if animals are properly sampled, semen from 25 males or embryos or live progeny of 25 males and 50 females would reduce the potential loss in genetic variation in the initial sample to less than 1 percent. With frozen stores, no loss of variation occurs until the store is used, whereas with living popula- tions of small sizes, a continual decline of genetic variation can occur through inbreeding or genetic drift. For rare stocks, all males and females should be sampled if possible. METHODS FOR PRESERVING LIVESTOCK GERMPLASM Three primary methods have been used by public and private programs to preserve livestock germplasm: maintaining live popula- tions, cryopreserving gametes and embryos, and establishing DNA (deoxyribonucleic acid) stores. In some cases, keeping breeding popu- lations may be the most practical method. Cryogenic techniques will continue to be improved and developed and, thus, should be consid- ered the best strategy for long-term preservation. Cryopreservation technology can be applied to most domestic species. However, im- portant exceptions are semen from swine and embryos from swine and avian species. Embryos from pigs have been cryopreserved at expanded blastocyst and early hatched stages and pig semen can also be frozen. DNA is being stored in gene libraries and as an addition to cryo- genic stores. Although long-term storage of DNA is likely to be more important in the future, it is not yet a viable method for preserving and using livestock germplasm. Managing Live Populations For many countries the most practical way to preserve animal germplasm is to mair~tain live breeding animals at as many locations as feasible. The herds and flocks maintained by private owners often suffice, but if a given breed becomes endangered, publicly supported efforts may be necessary to develop breeding programs that will con- trol inbreeding and ensure breed purity. The disadvantages of preserving small living populations include costly facilities and supervision and exposure to several hazards. These
Essential Conservation Considerations / 55 hazards include loss due to disease, the increased potential for in- breeding that may result in a greater frequency of deleterious traits, contamination from other stocks, and changes due to natural selec- tion (Smith, 1984b). As an alternative to individual breed or population preservation, animals representing several different breeds could be maintained in a single interbreeding population. Although this method is suitable for preserving the constituent genes of the various breeds, particu- larly if many breeds merit saving but financial or other restrictions exist, it has significant drawbacks. The population would serve to maintain single alleles, particularly if they confer a selective advan- tage, but the gene combinations associated with specific breeds would be broken up and lost. Isolation of the individual gene combinations that were characteristic of a breed would be difficult after they have been combined in a single interbreeding population. Also, special mating programs are needed to avoid loss of some genes through negative selection. Thus it is preferable to preserve gametes and embryos from purebred lines rather than from a pooled population. Public and Private Initiatives to Manage Live Populations The conservation of animal genetic resources can involve both public and private initiatives (Crawford, 1990a; Setshwaelo, 1990; da Silva, 1990; Wezyk, 1990~. Many governments have established in situ herds in public parks set aside for wildlife conservation. Deple- tion of the Longhorn breed of cattle in the United States, for example, was prevented by keeping semiferal herds in state parks in Okla- homa. In 1932, the South African government collected and pre- served the Nguni cattle in the Natal Parks (Setshwaelo, 1990~. Simi- lar approaches are used in France with Rove goats and Camargue cattle and horses, among others. At the Hortobagy National Park in eastern Hungary, government resources and initiatives ensure preser- vation of Hungarian Grey Steppe cattle, Mangalica pigs, Racka sheep, and poultry (Alderson, 1990a; Henson, 1990; Wezyk, 1990~. Many individuals and private groups are interested in maintaining and propa- gating unique types of animals (Alderson, 1990a; Office of Technol- ogy Assessment, 1985~. For example, private farm parks, such as those supported by the Rare Breeds Survival Trust (RBST) in the United Kingdom, have proved successful (Alderson, 1990b; Henson, 1990~. These working farms keep rare breeds and are financially supported by the public, who pay to enter. Public subsidies have been used to encourage private on-farm maintenance of rare breeds. In some cases government subsidies
56 / Livestock Gloucester Old Spot pigs are an endangered native livestock breed in Great Britain. They are hardy and could be crossed with other breeds to produce commercial pigs that can be kept outdoors. Credit: Cotswold Farm Park. have been provided to farmers to maintain animals of a specific breed. For example, the regional government of the Province of Quebec, Canada, pays farmers to rear purebred Canadienne cattle (Henson, 1990~. The methods of payment and the amount of the subsidy may be · Per head. The system of paying a per head rate on each ani- mal raised is simple, but it can lead to problems of overgrazing and mismanagement of flocks or herds. · Production linked. These subsidies seem to be the best con- trolled and most effective. The subsidy is determined by evaluating the production of the rare breed under normal management and com- paring it with the potential production of the replacement breed un- der the same management system. The farmer is paid a subsidy
Essential Conservation Considerations / 57 equivalent to the difference. With production-linked subsidies it is still essential that the farmer continue to manage the animals well to realize the maximum value from them. · Male only. This is probably the simplest form of subsidy. Farmers are paid for rearing males and making them available for semen col- lection and for use in contract matings. It has been used successfully in Britain for maintaining rare strains within cattle and pig breeds, and it demonstrates the feasibility of close cooperation among indi- vidual farmers, breed societies, private conservation groups, and farmer- funded organizations. Cryopreservation Cryopreservation of semen and embryos complements the pres- ervation of live populations, providing a safeguard when breed num- bers are dangerously low or when breeds or lines are likely to be replaced or lost. It does not allow for continued adaptation of the population to changes in environmental conditions, but it does offer the potential to store large numbers of genotypes for indefinite peri- ods of time. Generally, the costs of collecting and processing samples for cryopreservation are not excessive for domestic animals, particularly for semen (Smith, 1984a). However, they may vary considerably, depending on the particular collection conditions. Once the material is collected and processed, the cost of maintaining it in cold storage is much lower than the cost for maintaining live animals. Further- more, cryopreserved samples kept in liquid nitrogen can be moved long distances with relative ease. The successful cryopreservation and banking of animal germplasm requires special expertise. First, the behavior and reproductive physiol- ogy of the animal must be known and suitable for control and ma- nipulation, and the cryopreservation procedures must be adapted to the gametes or embryos of the particular species being preserved. Second, the breed must be sampled with care to ensure preservation at the desired level of diversity. Third, females must be available to serve as recipients of cryopreserved germplasm. Finally, secure, long- term storage facilities with trained personnel must be established. Ideally, duplicate samples should be kept at other locations. Finally, the health status of the donor animals must also be checked and records maintained. Appendixes B and C discuss the risks of disease transmission that can be associated with the collection, stor- age, and transfer of embryos and semen, respectively. New tech- niques for washing embryos (see Appendix B) will greatly advance
58 / Livestock capabilities to store germplasm that is free of pathogens. Before cryopreserved samples are regenerated, the health of the recipients should be determined to ensure that healthy germplasm is not con- taminated. Cryogenic technology may not be routinely applicable to all breeds or in all regions. Embryo collection involves maintaining donor fe- males at suitable facilities and having adequately trained personnel. Hormonal profiles vary among breeds so that hormonal treatments for embryo transfer technology must be developed for individual breeds. Factors such as climatic or dietary stress may affect hor- monal response. In Britain, for example, a failure to collect cattle embryos of one breed was apparently due to differences in the physi- ological responses of the rare breed cows to hormonal superovula- tion treatment (E. Henson, Cotswold Farm Park, United Kingdom, personal communication, July 1987~. Finally, some animals produce gametes that have higher viability rates than others after freezing. For example, much success has been achieved with cattle semen. In contrast, the cryopreservation of pig semen is feasible but' conception rates are poor, resulting in higher regeneration costs. These individual differences may lead to unin- tended selection pressures on the population. Genomic Libraries In some cases the rationale for preserving a particular breed of animals is solely to preserve one or a few traits unique to it. Because variation within species is molecularly based, the phenotype may originate from a single gene or a few genes. It may be possible and advantageous to preserve the desired genetic traits by preserving genes in addition to maintaining live or cryogenically stored semen or em- bryos. In the future, it is expected that techniques will exist for preserved genes to be transferred into embryos of the same species or into other species, thus permitting the desired traits to eventually be returned to a living animal population. For these reasons, genomic libraries may have an important place as a supplement to frozen semen and embryo banks in a long-term strategy for preserving ani- mal genetic resources. The value of genomic libraries will be continually enhanced as more is learned about the molecular physiology of animal species. For example, in cattle the gene products of the major histocompat- ibility complex, a region of the genome that is involved in immune responsiveness, are being characterized (Benyo et al., 1991; Maguire et al., 1990~. Differences in the gene products are being noted among
Essential Conservation Considerations / 59 breeds and are being associated with differences in production traits. With further understanding of these products, it may be feasible and desirable to transfer genes among groups of animals independently of the rest of the genome. This would be facilitated by the existence of genomic libraries. The screening of a genomic library might also provide the basis for a decision to thaw and transfer embryos from a specific population for further study as live animals. RECOMMENDATIONS With few exceptions, mostly in developed countries, little is known about breeds that have become extinct. Similarly, for most endan- gered stocks, no objective assessment has been made of their relative merits or unusual qualities. In marry cases there may not be time to characterize endangered breeds before they disappear. Although loss Scottish Highland cattle are very hardy and withstand long periods of cold, wet weather. Credit: Cotswold Farm Park.
aged. 60 ~ Livestock of breeds suggests a decline in genetic diversity, the extent of actual losses is not understood. Nevertheless, the general concern is that some unique and potentially useful germplasm is vanishing germplasm that at some point could play a valuable and necessary role in in- creasing the efficiency of livestock production (Alderson, 1990a; Wiener, 1990~. The committee considered assuring the availability of these resources to meet unseen needs as a strong argument in support of conservation efforts, especially in view of the contributions of landraces in improving agricultural crops in both developed and developing countries (National Research Council, in press). Sampling strategies for populations must be structured to prevent in- advertent loss of genes through the use of improper methods or an inad- equate number of samples. The development of collection strategies must include an assess- ment of the size of a population, its genetic structure, and its geo- graphic distributions. This information must be available to scien- tists and policymakers who are developing collection strategies. Given that sampling 25 males and 50 females will reduce loss in genetic variation at any one locus to less than 1 percent, the committee con- cluded that the following standards for population sampling should be adopted: for semen, 25 sires, each sufficient for 200 to 500 insemi- nations; for embryos, 25 sires and 50 dams, each with 10 to 25 em- bryos; for live animals, 25 breeding sires and 50 breeding dams. Private efforts to preserve animals in breeding herds should be encour Private individuals and interest groups, typically in developed countries, maintain a wide array of livestock for reasons not neces- sarily linked to genetic conservation. These efforts, however, can provide an important supplement to public efforts to manage herds and flocks. The RBST and the AMBC are examples of successful private efforts. The presence and activities of such groups enable a broader diversity of populations to be preserved than would other- wise be possible with limited resources. The degree of government involvement in private conservation efforts may vary from exchange of information to direct subsidy of private conservation efforts. The preservation and management of endangered and unique popula- tions as breeding herds or flocks should be supplemented by cryopreserva- tion of their germplasm. Cryogenic technologies are an important supplement to main- taining live herds. However, many region-specific populations in the
Esse~fi~' Co~ser~ffo~ Co~sfJer~ffons / 6I most immediate danger of exOnchon may not be bow widely studied breeds Omened 19907 It may be difficult to save some of these popu- labons using current cryogenic technology Research and develop- ment are also needed to refine held techniques and to evaluate ~ithin- and bet~een-breed responses to collection and the viability of semen and embryos after cryogenic storage @
