In many countries the development of agriculture and breeding programmes has resulted in serious changes in cattle breeding stocks during the last decades. The establishment of herd-book societies in conjunction with intensive breeding activities has led to a pronounced supra-regional propagation of certain cattle breeds due to gradual improvements in performance. Since export is facilitated by new developments in biotechnology, such as cryoconserved semen and frozen embryos, these cattle breeds also dominate internationally. As a consequence the contingent of local breeds has decreased continuously to the same extent as the preferred high performance breeds have expanded. It can be assumed that the world cattle population (ca 250 million) will consist predominantly of only twenty different breeds at the turn of the millennium. Most of the currently existing 500 different cattle breeds are actuely threatened either by complete extinction, or by grading-up with other breeds.
There are two approaches to deal with the loss of cattle breeds which is expected to increase. The first approach is to accept the situation on the basis that economically unsuccessful breeds are of no concern and because man may dispense whith breeds which have lost their original value and appear no longer worthy of conservation. The other approach is to acknowledge the cultural and genetic importance of these breeds and to take suitable measures to preserve them. This could be achieved by establishing small populations or by cryogenic storage of germplasm.
This monograph has several aims. It will present a brief synopsis of reasons in favour of preserving breeds, and review the modern biotechnological techniques allowing conservation of the genomes, or individual genes, of these breeds for future generations. A discussion of the possibilities of reactivating the genes of livestock as well as the genetic consequences will be included. The problems of feasibility including a discussion of the practicalities and potential costs will also be addressed.
Man has been keeping domestic cattle, at least in Asia minor and the Near East, for approximately 8500 years. We may start from the assumption that domestication of cattle began independently at several locations. This is very interesting from a breeder's point of view, because domestication at different locations thus led to the collection of various populations differing in their gene pools. Since only a small fraction of the wild-type population became domesticated, early domesticated cattle already differed from their wild-type counterparts with respect to their gene pools. Subsequent breeding has resulted in new combinations not found in the wild-type population and eventually has led to the establishment of early breeds from the extraordinary range initially present in the wildtype population (Röhrs, 1980).
Within a species individual breeds are distinguished by marked differences in morphological and physiological characteristics of performance and production potential. It should be noted that the classification of species into different breeds is not included in the zoological nomenclature.
Breeds are groups of animals of the same species which belong together, because they have the same origin, share certain physical and physiological characteristics, and are of commercial value. In view of their biological behaviour such breeds are not constant entities. Unfortunately, the term has been defined in various ways, by making, for example, the following distinctions:
breeds defined by herd-book entries which are subject to strict breeding programmes and constantly monitored for performance, and
breeds just constituting populations.
Kräuβlich (1981) has defined cattle breeds as genetic entities developed over a long period of time by either following certain mating schemes, or by selection for certain characteristics. Names given to different breeds are arbitrary. Frequently the presence of a single gene in a particular population has given rise to a new genetically defined group with a unique breed name. Subgroups of these breeds are known as varieties or strains.
A breed is said to be endangered if the number of animals available for further breeding has decreased below a certain minimum. Individual animals kept at different locations do not guarantee continuation of a breed if they are not allowed to take part in the process of reproduction.
The Committee for Genetic and Statistical Techniques in Animal Breeding of the German Society for Breeding Research has issued a statement on the generation of gene reserves in animal breeding (Züchtungskunde, 1979). According to this statement a breed is said to be threatened in its continuation if the number of sires available for breeding falls below ten.
Maintenance of genetic variability and hence the ability to survive as a breed is not only determined by the number of animals capable of reproduction, but also by the ratio of dams and sires. A decisive factor is the so-called effective population size, Ne, which can be calculated according to the following equation:
|Ne = [4 × Nm × Nf] / [Nm + Nf]||(1)|
with Nm representing the number of male animals and Nf the number of available female animals.
The effective population size, Ne, can be used to calculate the increase of inbreeding, ΔF, per generation according to:
|ΔF = 1/2Ne||(2)|
Inbreeding effects may be reduced slightly by taking precautions that mating is non-random, i.e. by selectively mating animals that are as unrelated as possible.
In order to be able to select successfully for quantitative traits such as size, weight etc., an effective population size of more than 100 animals is required. If selection and hence breeding progress can be dispensed with, an effective population size of Ne = 50 may be sufficient. The concomitant increase in inbreeding of 1 percent appears to be acceptable. The data presented in Table 1 show that it would be most appropriate to keep equal numbers of dams and sires although this may not always be possible for practical reasons. Under normal circumstances the number of sires will usually be lower than the number of dams, but even then 20 sires and 50 dams will be sufficient. The effective population size is not increased significantly by further raising the number of dams for example to 80.
|Effective population size||Increase in inbreeding after one generation (%)*|
* random breeding
Table 1: Breeding Parameters: Number of Animals, Effective Population Size, Increase in Inbreeding
The problem of decreasing genetic resources of domesticated animals has been discussed at least since the First World Congress of Applied Genetics in Madrid in 1974. A plethora of statements has been issued, giving reasons for preserving endangered breeds. Only two publications will be cited to illustrate the issues.
The Committee of the German Society of Breeding Research (Züchtungskunde, 1979) gives the following reasons for preserving endangered breeds:
Endangered breeds or breeding populations may contain hitherto unrecognized or disregarded genetic traits. These may have advantages over traits existing and/or predominating in populations currently available in view of environmental changes, altered market situations, or cross-breeding programmes with other populations. Superiority may result from the presence of single genes which would be lost inevitably with the disappearance of a particular breed. It may also result from the interaction of several genes. It would be very time-consuming to restore this particular genetic make-up, if needed, by specifically selecting these genes from the predominant population.
Endangered breeds may acquire an important status as replacement populations serving as genetic reservoirs if the effective and utilizable genetic variation in the predominant population decreases.
Intensive selection within a predominant breeding population has been characterized by a trend towards a reduction in the utilizable genetic differences between individual animals. In the long run this will have restrictive effects on further genetic improvement and adaptation of farm animals to environmental changes. The risks may be spread to a certain extent by parallel breeding programmes pursuing the same goals in differing breeding populations. If needed independently developed populations may be re-united at a later stage as a contribution to restore genetic variety.
Breeds of Domestic Animals as Cultural Heritage
Some breeds of domestic animals have been associated at times with specific historical and technical phases in the development of agriculture. In this respect it may be argued that such breeds should be worthy of preservation with the same degree of consideration that has been given almost universally to architectural monuments or collections of technical tools in museums. Some breeds of domestic animals have predominated traditionally in certain regions. They may therefore contribute not only to local colour, but also to the recreational value of these regions.
In an FAO manual for a training course in conservation and management of genetic resources of domestic animals Bodo, Buvanendran, and Hodges (1984) list the following reasons for preserving genetic resources:
It is impossible to forecast future demands of mankind for animal products or changes in production systems which may be brought about by alterations in price structures. It is therefore conceivable that the specific characteristics of animals may differ considerably in the future from those currently available. This may be of particular importance with factors such as disease resistance or adaptability.
Populations of domesticated farm animals and their morphological and productive characteristics are the result of creative human interference. Such populations therefore are entitled to be preserved in the same way as architectural monuments.
It is conceivable that individual true-breeding populations per se may not be of economic importance; however, they may be economically valuable in view of breeding programmes.
Under rough environmental conditions, e.g. those that have given rise to nomadic grazing management, suitably adapted breeds with low production performance can be kept frequently with minimal expenditure. Under these conditions these breeds may even posses advantages over other exotic breeds, which may require more intensive management and more specialized food-stocks due to their higher production performance.
In most cases indigenous breeds are linked intimately with the history and the character of a certain region. As such they are unique and therefore worthy of being preserved.
Indigenous breeds also demonstrate the historic development of animal breeding and are therefore of educational value.
Indigenous breeds may be very valuable for comparative studies of the genetic and physiological characteristics of current cattle populations.
In certain regions local breeds are attractions and therefore play an important role for tourism.
This list of reasons for the conservation of endangered breeds must be extended by taking into consideration another important aspect, i.e. new developments in animal breeding, in particular experiments designed at producing transgenic and cloned animals. If these new techniques should prove successful in the future, so that they could be employed on a larger scale, it may be expected that genetic alterations will also be introduced into currently existing breeds. This will not rule out the possibility however that exotic breeds in particular may be important starting populations to be employed in the course of gene transfer programmes.
There is an intricate and possibly reversible relationship between the causes and the effects of reducing the varieties of existing breeds and their conservation by new breeding techniques. On the one hand, modern techniques such as artificial insemination, embryo transfer or embryo manipulation are increasingly used especially for high-performance breeds. On the other hand, it is the application of these techniques that makes possible the establishment of banks allowing to preserve entire genomes or individual genes, thus preventing such breeds from becoming extinct.
There are several ways, differing in their efficiency, feasibility and costs, to preserve the genetic information of a particular breed (Brem, Graf and Kräuβlich, 1982). These are the alternatives:
Maintenance of small populations in domestic animal zoos. These attempts may be sponsored by private initiatives and allotment of premiums for keeping the animals:
• male and female animals are kept in one or more herds. Storage of frozen semen or embryos is not projected.
• The breeding population comprises female animals only. Matings are carried out exclusively by artificial insemination with cryoconserved semen. This approach requires cryoconservation and storage of sufficient amounts of semen.
Exclusive cryoconservation and storage of semen without concomitant conservation of live animals. The genetic potential of a particular breed is reactivated by artificial insemination of suitable female animals taken from available populations, followed by genetic grading-up in several subsequent generations to increase the frequency of genes of the initial breed.
Storage of frozen embryos and semen without concomitant conservation of live animals. If needed transfer of the embryos into foster animals is used to restore the original breed.
The techniques described above have already been used for establishing genetic reserves of individual breeds of cattle. Apart from keeping livestock or storing frozen embryos and semen there are also attempts to isolate, preserve and transfer nuclei or chromosomes. These approaches may also allow conservation of genetic resources.
The development of modern molecular biological techniques offers yet other methods. It must be pointed out, however, that these techniques usually do not allow conservation of genomes in a form which can be reactivated in toto at a later stage. Nevertheless they permit individual genes, or a multitude of undocumented and unknown genes to be stored in the form of so-called gene libraries.
Figure 1 gives a summary of the various components of genetic information and the techniques required for their conservation. The next chapters will focus on a discussion of techniques employed for storing and reactivating genetic resources in view of technical, genetic and economic aspects and of feasibility.
|Genetic information||Techniques used for preparation and collection|
|exon||biochemical, genetic engineering|
|gene-cluster||biochemical (in the future), genetic engineering|
|chromosome||genetic engineering, cryoconservation, microinjection|
|semen (haploid)||cryconservation, artificial insemination|
|oocyte (haploid)||in vitro fertilization|
|cells||cloning, gene library|
|adult animals||small populations, zoos|
Figure 1: Techniques Available for Preserving Genetic Resources at Different Levels