CHAPTER 25
MANAGEMENT OF POULTRY GEMBTIC RESOURCES

by

 R.D. Crawford

Department of Animal and Poultry Science
University of Saskatchewan
Saskatoon, Saskatchewan S7N OWO, Canada

Summary

Poultry resources kept by producers at the village, middle, and industrial levels are described. Conservation of these resources should be preoeded by cataloguing or préparation of an inventory. Catalogues hâve been prepared in Canada, U.S.A., Australia and Czechoslov-akia. Experience gained in an inventory of Canadian poultry stocks indicates that érosion of resources at ail production levels has been extreme; if stocks are to be preserved, a public agency must assume the task; in Canada, resources at the middle-level are most seriously endangered. A similar situation prevails in some other countries.

Biological and genetic aspeots of poultry resource conservation are discussed. Stocks at présent must be held as living flocks since techniques for storage of gametes, zygotes, and embryos bave not yet been adequately developed. Stocks should be reproduced when reproductive fitness is optimum; in most poultry species, annual reproduction is needed. The breeding programme adopted should minimize loss of genetic variability. Procedures which have been developed for breeding experimental control strains of poultry can be adapted easily. Stocks can be kept as purebreds, or a number of breeds can be combined in a gène pool. Expérience in poultry conservation indicates that gène pools may be useful in maintain-ing variability for a far-future need; but since retrieval of single genes or combinations of gènes from a pool can be a very laborious procédure, maintenance of purebreds may be preferred where a near-future need is foreseen.

Under the influence of the industrial poultry sector, erosion of resources has prooeeded very rapidly. It is urgent that official agencies recognize the need for conservation of variability and ensure that it is maintained.

25.1 Poultry resources defined

The term "poultry" refers to ail those avian speoies which reproduce freely under the care of man and provide him an economic service; it includes the usual agricultural species along with pigeons, peafowl, ostriches, Canaries, and others.

As a human food source, chickens are by far the most important species of poultry. They are cultivated throughout the world, and in most areas they contribute a large proportion of animal protein through eggs and meat to the human diet. Other species have much less import­ance globally, but local production and consumption can be significant. Turkeys are a large contributor in North America, geese in Europe, ducks and Coturnix quail in South-east Asia; pigeons still have importance at the village economy level of some developing countries. Certain species are gaining favour as specialty products - guinea fowl and muscovy ducks in France, pheasants and quail in Australia, but they contribute little to world food production. The remaining species, while they may have potential as food sources, are usually kept only for ornament.

In the fourth FAO expert consultation report (FAO, 1973). three broad groups of producers were recognized - village or subsistence level farmers, middle—level producers with moderate-sized flocks under good management, and large-scale industrial poultry producers. Related to these groups, the report identified various reservoirs of poultry genetic resources:

(a) Stocks maintained by large specialized breeders for use in industrial poultry production.

(b) Native or indigenous stocks, such as the desi chickens of India, which are valued at the village level.

(c) Local populations, such as Partridge Greenlegs of Poland, used in non-commercial or small-scale farming at the middle-level.

(d) Gene pools purposely established to preserve genetic materials.

(e) Native stocks purposely collected and maintained for future use in breeding programmes for village, middle-level or industrial producers.

The first three of these are of major importance when discussing a classification or invent­ory of existing poultry genetic diversity. The last two are of importance when considering ways to preserve that diversity.

Breeding of stock for industrial poultry production is in the hands of a very few primary breeders, most of them based in North America. They produce and sell breed crosses or strain crosses only, distributing them to many countries. Parent and grandparent stocks may be strains of recognized pure breeds or they may be synthetic lines developed for partic­ular purposes. They are the exclusive property of the breeding company. White egg chickens are almost exclusively White Leghorn. Brown egg chickens are derived from several breeds -notably Rhode Island Red, New Hampshire, Barred Plymouth Rock and Black Australorp. Meat-producing chickens are derived mainly from White Plymouth Rock, and White Cornish. Most commercial turkeys are white-feathered and broad-breasted. Commercial ducks are derived almost exclusively from White Pekin. Most primary breeders are believed to be holding extensive "libraries" of resource stocks for future needs of the company, but they are understandably reluctant to reveal information about these resources since they are private property.

In some parts of the world there exist native or indigenous poultry stocks. Most of these have very low productivity, partly due to genetics, but also because of inadequate feed­ing and management. They subsist mostly as scavengers in a village economy but still provide an important source of animal protein through eggs and meat. Indigenous stocks have received little attention from research workers and very little is known about them. Notable except­ions are the desi chickens of India and Fayoumis of Egypt; refer to Animal Breeding Abstracts volumes 39 to 48 for references to many research reports pertaining to them. An interesting description has recently been published on an isolated population of Ingie chickens in Japan, which have an unusual feathering condition which has not been reported previously (Hashiguchi et al., 1978). There are few reports in the technical literature describing other indigenous stocks.

The third category listed in the FAO expect consultation report (FAO, 1973) is that of local populations used in non-commercial or small-scale farming at the middle-level. Many of these have evolved into well-defined breeds and with that identity some have achieved world­wide distribution. They have achieved some form of official recognition, something is known of their biological characteristics, and a breed standard has been written. The American Standard of Perfection (1974) is a major reference source in North America. In Great Britian, standards for breeds recognized there are published in British Poultry Standards (1971). Similar volumes exist in some other countries. Most stocks in this category are now kept only by hobbyists for exhibition. But some have been used and some are still in use at middle-level farming, and the stocks now used for industrial production were originally derived from this category. Also included in this category are the miniature or bantam stocks which have wide popularity with hobbyists; although not presently utilized as a food source, they continue to hold that potential, particularly now that the industrial sector is interested in dwarfing genes to reduce feed and housing costs.

25.2 Inventories of resources

Because of very rapid and dramatic changes that have occurred and continue to occur in the world poultry industry, any consideration of germ-plasm maintenance and preservation should be based heavily on a current inventory of genetic resources.

Thus far, a world inventory has not been attempted. This was recommended in the fourth FAO expert consultation report (FAO, 1973). Cataloguing has been accomplished or is in progress in only a few countries. One of the existing inventories includes information from several countries.

In Canada, a catalogue of poultry stocks held at research and teaching institutions has been revised and issued each year since 1967 (Crawford, 1980). It is believed to be oomplete and current in is intended coverage. The Canadian inventory is now being broadened to assess the holdings of exhibition stocks kept by hobbyists, to determine what resources remain available to seasonal hatcheries for sale to middle-level producers, and to estimate the extent of resources held by primary breeders catering to the industrial poultry sector.

An inventory of chicken stocks in the United States was first published in 1972; it was expanded in a 1975 edition to include Japanese quail and turkeys, and included listings from the Canadian catalogue; a third edition (Somes, 1978) added listings from research stations in Great Britain, Prance, Holland, and Spain. A fourth edition is being prepared for public­ation in 1981, and it is anticipated that information from additional countries will be included (Somes, 1980), personal communication). This catalogue contains information on most of the stocks kept by public institutions in the United States, but since it relies on voluntarily contributed material its coverage of privately-owned stock is necessarily incomplete.

In Australia, an Expert Panel on Conservation of Poultry Genetic Resources has issued a report (Standing Committee on Agriculture, 1979) which urges preparation of a register of poultry genetic stocks including commercial, institutional and fancier holdings. An invent­ory of stocks has been prepared for the State of Victoria (Sheldon, 1980, personal communic­ation)} based on this as a pilot study, it is expected that other states in Australia will soon begin inventory preparation.

Reference has been found to a Czechoslovakian catalogue of poultry genetic resources (Grom, 1974), which includes 80 egg-producing lines and 41 meat-producing lines of chickens. There is also a catalogue in the U.S.S.R.

Encouragement should be given to cataloguing of stocks in other countries. Only when the total poultry genetic resources are known can satisfactory decisions be made on which stocks to maintain and preserve.

25.3 Resources to be preserved

A survey of all poultry genetic resources in Canada, conducted by the author, is nearing completion. Experience with this inventory may be applicable to situations elsewhere in the world.

The industrial poultry sector in Canada is supplied by most of the world's major primary breeders. The breeder retains the purebred parent or grandparent strains, and has a contract­ual agreement with owners of multiplier flocks, who provide hybrid commercial stook to the producer of market eggs and meat. It is evident that nearly all productive egg and meat strains of chickens and turkeys which formerly supplied the commercial industry in Canada have been collected and are now held exclusively by primary breeders. The only Canadian-based major breeder of both egg and meat chickens has very extensive genetic resources in addition to those currently used to generate its commercial products; primary breeders of chickens elsewhere, probably hold similar reserves. Canada's only large-scale breeder of turkeys apparently has no reserve resources except for sub-lines derived from its commercial parent lines. Although the primary breeders are producing stocks with superb productive performance for industrial poultry sector, it is disturbing to learn that for "business reasons most breeding companies are planning only about ten years in advance. Hence, public agencies may have to accept a role in preservation of egg and meat stocks for far future needs of mankind.

Only a few purebred poultry stocks remain on the commercial middle—level market in Canada. They are held by small seasonal hatcheries and will not likely survive much longer. There are only two purebred egg—producing stocks remaining, neither of them White Leghorn. Eleven dual-purpose chicken lines have been located - three Barred Plymouth Rock, two Light Sussex, four Rhode Island Red, one Columbian Plymouth Rock and one New Hampshire. One strain of bronze turkeys and one of white survive. There is only one source of Muscovy ducks, two of Rouen and two of Pekin ducks. All of Canada's goose production is based on three breeder flocks of Emden geese. The extreme paucity of genetic resources in a country such as Canada is shocking. Demand for poultry stock for middle-level and subsistence farming is small but rapidly increasing because of growing interest in self-sufficiency and home production of food. Most of this demand is being met by birds from the major primary breeders. Their stock is bred and selected for industrial conditions is not necessarily suited to extensive production environments. Unfortunately, the old-time dual-purpose stocks which once filled this niche very satisfactorily have almost disappeared. If they are to survive, a public agency may have to keep them.

Poultry hobbyism is flourishing in Canada, keeping a wide array of fancy breeds for exhibition. These breeds are a reservoir for genes representing immense variation. Little of that variation has been studied by research workers or evaluated for use in food-producing stocks. It might be expected that this diversity would be secure in the hands of hobbyiste who would treasure variation for its sake. Although there are a few dedicated breeders who hâve spent a lifetime in selecting a particular breed, these are the exception. Most hobby-ists seem content to buy stock of a breed, exhibit it and perhaps reproduce it, and then replace it with another. It has been very disturbing to find that Canadian hobbyists are not breeders; instead they rely very heavily on a few dealers and supply houses as stock sources instead of patronizing local hobby breeders. Hence, it is probable that the genetic base for fancy breeds is far narrower than expected, and losses of entire breeds becomes inevitable. The Chantecler, a breed developed in Canada, seems to hâve suffered that fate; it was synthesized about 1910. enjoyed brief popularity as a commercial breed, and then became a hobbyist stock; it disappeared from shows a few years ago and probably became extinct. The Chantecler is again very popular among hobbyists but all evidence indicates that the present-day stock is a new synthesis.

In Canada, there are no stocks that could be classified as native or indigenous. If they existed, and no doubt they did in the era of first settlement, they have gone to extinction.

Hence public agencies in Canada muet actively engage in collection and preservation of its remaining poultry genetic resources if it wishes to provide for far-future needs of its people. Industrial poultry stocks are ourrently safely held by primary breeders but there is danger of genetic loss inherent in the short-term advance planning of the breeding companies. Stocks traditionally used by middle-level farmers are already nearing extinction. Unless the hobbyist group changes from emphasis on showing to emphasis on breeding and sel­ection, stocks held by that sector may undergo rapid loss of variability. Native stocks are gone now.

A similar situation prevails in Australia. The Expert Panel on Conservation of Poultry Genetic Resources (Standing Committee on Agriculture, 1979) observed that genetic variability in Australia has already eroded to the extent that its industry has poor potential for genetic advance without the assistance of new importations.

Indigenous stocks in countreis without a highly sophisticated and modem poultry indus­try may not be seriously endangered. But continuous monitoring is needed to ensure that they are not rapidly supplanted. A student from Latin America described to me how native stocks of his country were eliminated by diseases introduced with high-producing stocks from abroad. If indigenous stocks become endangered, representative samples should be gathered for main­tenance by a public agency.

25.4 Biology and genetics of poultry germ-plasm preservation

In preserving poultry stocks, existing expertise on biology and genetics should be fully utilized to ensure that time and resources are used optimally. Certain biological attrib­utes peculiar to poultry species must be considered. Genetic effects and consequences of maintenance over many generations must likewise receive careful attention.

At the present time, it is necessary to maintain poultry stocks in living flocks. Tech­niques for storing gametes, zygotes, embryos and tissues are under development but they are not yet routinely useable in a germ—plasm preservation program.

Hatching eggs cannot retain viability during long-term storage. Under adequate condit­ions of temperature and humidity, chicken eggs can be accumulated or held in storage for up to three weeks without serious loss of hatchability. If the eggs are stored in sealed containers in an atmosphere of nitrogen, storage can be prolonged for one week. Storage of eggs from other poultry species has received little study, but it appears that their storage duration is shorter than that of chickens. There seems to be little hope of acquir­ing technology for long-term storage of eggs in the immediate future.

Freeze preservation of avian sperm cells is an active area of research. Technology now exists for deep-freezing of cells while maintaining fertilizing ability (Buckland, 1979, personal communication). There appears to be no limit to storage time; the current record is fertility following four years of storage but much longer time records are anticipated in the future. Fertility of 50-70% can be expected using pooled semen from several males. There is extreme variation among males in ability of their sperm to withstand freeze-storage; heritability for this trait is about 0.1 permitting some response to selection. It is not yet certain whether hatchability of embryos from frozen semen is reduced. Access to the research literature can be gained through papers by Lake and Stewart (1978), Mitchell et al., (1977), Oderkirk and Buckland (1977), Sexton et al., (1978), and Watanabe et al., (1977).

Although freeze-storage of avian sperm cells holds great promise for the future, at the present time it is not a useable field technique for preserving germ-plasm. The laboratory procedures require great expertise and elaborate facilities. Buckland (1979, personal com­munication) has advised that in his experience semen from less than 50% of the males tested will withstand freezing, a selection pressure hot likely to be acceptable in a germ-plasm preservation programme. Only genetic material from one sex can be preserved making it impossible to retain a pure breed. And Y or W chromosome is lost since in birds females are the heterogametic sex.

Lyophilized and deep-frozen preservation of living embryos and of somatic tissue may be possible in the future. They are in current use with lower forms of life but hold no imm­ediate promise for preserving poultry genetic stocks.

Hence, conservation and preservation of poultry germ-plasm requires maintenance of living flocks.

Frequency of regeneration of flock replacement must also be considered. To avoid genetic loss due to natural and artificial selection, flocks should be regenerated when reproductive fitness is optimum (Gowe et al., 1959). For most species, this requires replacement at yearly intervals. Fecundity of geese is poor until their second year and then remains good until they are five years of age. Coturnix quail should be reproduced by six months of age. Frequent replacement can be very costly. In Australia, Polkinghorne (1980, personal com­munication) is reproducing his large collection of chicken stocks at two-year intervals as a means of reducing the drain on meagre financial resources; it will be of interest to learn whether this procedure will in fact impose selection for longevity on the stocks being maintained. Unless precautions are taken, short generation interval can also lead to rapid inbreeding with consequent loss of genetic variability. This danger is reduced for a long-lived species such as geese. Polkinghorne's procedure in reproducing chickens at two years of age should likewise reduce rate of inbreeding. Fortunately it is much less difficult to maintain large numbers of individuals, particularly of males, in poultry than it is in large animal stocks} generally, as population size increases, inbreeding rate declines as does the extent of genetic drift.

Current genetics thought is that the ideal breeding programme to minimize loss of genetic variability is one of random breeding. In preserving poultry stocks the objective should be to maintain them from generation to generation without genetic change. This objective applies also to experimental control populations where it has received extensive study (Hill, 1972a, 1972b). Procedures for breeding control strains have been described by Gowe et al., (1959). The same procedures would apply in a conservation programme. Two breeding plans have been considered:

(a) Random breeding flock - unpedigreed chicks are chosen at random from those avail­able at hatching and are reared to maturity. Parents are taken at random from the survivors. All breeding is by artificial insemination to avoid natural selection of mates. Individual males are assigned to females at random, and are reassigned to different females at each subsequent insemination. Several consecutive settings of eggs are made to yield unpedigreed chicks for the next generation.

(b) Pedigreed flock - matings are randomly assigned to a fixed number of females per male and they are repeatedly mated to him through the breeding season, preferably by artificial insemination. Pedigree information is retained for each resulting chick. Sufficient chicks are reared to ensure that each sire contributes one son and each dam contributes one daughter to the next parental generation.

The data in Table 25.1 illustrate effects of these two breeding plans on rate of inbreeding and genetic drift. Gowe et al., (1959) concluded that for most oircumstances, the pedigreed flock procedure has sufficient advantages to justify the extra costs of pedigree maintenance.

A matter frequently overlooked in maintaining resource stooks is the beneficial effect of using very many sires to inhibit inbreeding change and genetic drift (Table 25.1 ). For instance, the same change in inbreeding coefficient per generation and the same maximum expected drift in egg production result when using 25 sires and 25 dams compared with a more traditional ratio involving 20 sires and 200 dams, fortunately, keeping large numbers of chicken males is far easier than keeping large numbers of bulls or boars.

Under certain circumstances, the breeding plans advocated by Gowe et al., (1959) cannot be followed precisely, and compromises must be made. In the poultry germ-plasm collection maintained by the author, natural mating is used in flocks of 50 females using 15 males shifted thrice weekly in groups of five, and replacement chicks are taken at random from successive settings of eggs. This procedure has been used for ten generations or more in some stocks with no apparent decline in fitness. A similar procedure is used in the South Australian Department of Agriculture poultry stock collection, using 100 females and 30 males per strain; these stocks are reproduced at two-year intervals (Polkinghorne, 1980, personal communication).

A further matter to consider is that of keeping stocks in a purebred form or combining a number of them for reproduction in a gene pool. Both methods are in use and both can satisfy particular needs. The first method seeks to maintain identity and integrity of a breed, the second to maintain all the genes comprising the pool. The pooling procedure is probably the most economical method for holding numbers of stocks, particularly for an extended time. And it is also a simple means to make maximum use of limited resources. For instance, the author is considering gathering samples of the remaining three strains of Canadian Barred Plymouth Rocks and combining them in a breed gene pool; individual strains would lose their identity but the within-breed variability being preserved would be greatly increased; present facilities do not permit keeping the strains separately.

Table 25.1

Effective number of parents for two mating plans and flocks of various sizes (Modified from Gowe et al.,1959)

* R and P refer to random breeding flock and pedigreed flock.

At some teaching institutions in North America, large gene pools have been constructed to include major marker genes from many breeds to be used as both a research and teaching resource. Although these pools have had some use, the general finding has been that retrieval of single genes or combinations of genes from them can be a very laborious proced­ure. Accordingly some institutions have discarded these segregating populations. However where the objective is maintenance of variability for far-future use, pooling of stocks is an attractive procedure. Maintenance of purebreds may be the preferred system where a near-future need for specific gene combinations characterizing the breed can be foreseen.

25.5 Need for official support

It is imperative that regional, national, and international agencies recognize the need for maintenance of poultry genetic resources and that they provide continuing support for these resources in the very near future.

Extinction of poultry stocks can and does occur with extreme rapidity under the influ­ence of a highly efficient and sophisticated industry dealing with species having low unit value and very short generation interval. The severe erosion of resources that has occurred in Canada and in Australia is perhaps indicative of situations prevailing elsewhere. In Canada there is concern about maintenance of industrial stocks for far-future needs; stocks for middle—level farming are nearly extinct now; hobbyist stocks are being bred from a dangerously narrow genetic base; indigenous stocks, if they existed in Canada, disappeared long ago. At least in Canada, the inescapable conclusion is that if resources are to be preserved, public agencies must assume the task.

Unfortunately, it appears that nowhere is poultry stock preservation work securely established. There are hobbyist groups whose main interest is rare breed conservation -for instance the Society for Preservation of Poultry Antiquities in the United States, and the Rare Breeds (Poultry) Society in Great Britain - but the extent of their activities is difficult to measure. Present activity among public agencies is mainly a reflection of the personal interest of a few individuals; they have obtained a little financial aid from granting agencies and governmental sources but there is no assurance of continuing support. In the United States, Somes (1980, personal communication) has been able to fund his catalogue through periodic grants from the U.S. Department of Health, Education and Welfare; nowhere in that country are there large collections of poultry resources although collections exist for some laboratory animal species. In Canada, the author's cataloguing activity has received modest but intermittent support from national and provincial governments; the stock collection which he has assembled is presently maintained by the University of Saskatchewan but continuing support is not guaranteed. The very large collection of production stocks at Parafield Poultry Research Centre in Australia is financed by the South Australian Depart­ment of Agriculture (Polkinghorne, 1980, personal communication), but there is continuous pressure for those funds to be applied instead to research of immediate practical applic­ation. It has been encouraging to find a brief committee report (Mérat and Hann, 1979) indicating that public agencies is some European countries are playing a role in preservation of specialized lines; a list of these lines is given. The report notes that some breeds are being maintained at public research centres in Spain and Norway but no details are given.

25.6 References

American Standard of Perfection, 1974. American Poultry Association Inc., Cushing, Oklahoma, U.S.A.

British Poultry Standards, 3rd edition, 1971. C.G. May, ed. Newnes-Butterworths, London, England.

Crawford, R.D. 1980. Catalogue of poultry stocks held at research and teaching institutions in Canada, 13th edition. University of Saskatchewan, Saskatoon, Saskatchewan, Canada.

FAO, 1973. Report of the fourth FAO expert consultation on animal genetic resources (poultry breeding). Food and Agriculture Organization of the United Nations, Rome.

Gowe, R.S., Alan Robertson, and B.D.H. Latter, 1959. Environment and poultry breeding problems. 5. The design of poultry control strains. Poultry Science 38:462-471.

Grom, A., 1974. (Characteristics of genetic resources of egg-producing and meat-producing types of fowl in the Comecon countries). Chov Hospodarskych Zvierat 34s29-33. Seen in Animal Breeding Abstracts 44:328, #2926, 1976.

Hashiguohi, T., T. Fujisaki, Y. Maeda, and M. Taketomi, 1978. Genetics of the hypoplasia of feathers of the tail region in. the Ingie fowl. British Poultry Science 19:487-490.

Hill, W.G., 1972a. Estimation of genetic change. I. General theory and design of control populations. Animal Breeding Abstracts 40:1-15.

Hill, W.G., 1972b. Estimation of genetic change. II. Experimental evaluation of control populations. Animal Breeding Abstracts 40:193-213.

Lake, P.E., and J.M. Stewart, 1978. Preservation of fowl semen in liquid nitrogen - an improved method. British Poultry Science 19: 187-194.

Mérat, P., and C. Hann, 1979. A critical review of genetic and selection research in Western and Northern Europe (Group 3). World's Poultry Science Journal 35:257-262.

Mitchell, R.L., R.B. Buckland, and B.W. Kennedy, 1977. Heritability of fertility of frozen and fresh chicken semen and the relationship between the fertility of frozen and fresh semen. Poultry Science 56:1168-1177.

Oderkirk, A.H.P., and R.B. Buckland, 1977. A comparison of diluents and cryopreservatives for freezing turkey semen. Poultry Science 56:1861-1867.

Sexton, T.J., R.B. Buckland, and R. Lopez, 1978. Comparison of two procedures for freezing semen from cocks of high and low fertility with frozen semen. Poultry Science 57:550-552.

Somes, R.G., Jr., 1978. Registry of poultry genetic stocks. Bulletin 446, Storrs Agricul­tural Experiment Station, University of Connecticut, Storrs, Connecticut, U.S.A.

Standing- Committee on Agriculture, 1979. The conservation of genetic material. Report of the Expert Panel appointed by the Animal Production Committee of Standing Committee on Agriculture, Canberra, Australia.

Watanabe, M., T. Terada, and Y. Shirakawa, 1977. A diluent for deep freezing preservation of fowl spermatozoa. Journal of the Faculty of Fisheries and Animal Husbandry, Hiroshima University 16:59-64.

Aménagement des ressources génétiques avicoles
Résumé

Définition des ressources avicoles. Les termes "volaille" et "aviculture" s'appliquent & toutes les espèces aviairea qui se reproduisent librement soue la surveillance de l'homme et lui rendent un service économique; ils englobent les espèces habituelles de l'agri­culture, ainsi que les pigeons, paons, autruches, serins et autres. Celles qui revêtent le plus d'importance pour l'agriculture sont les poulets, les dindons, les canards et les oies; les pintades et certaines espèces de faisans et de cailles sont d'une importance moindre mais qui tend & s'accroître.

Le rapport de la quatrième consultation d'experts de la PAO (PAO, 1973) recense divers réservoirs de ressources génétiques avicoles. On pourrait choisir une classifica­tion quelque peu différente pour discuter de la préservation des souchest

1. Race et variétés homologuées. En maintes régions du monde, des races nette­ment établies se sont développées. Elles ont acquis une certaine forme de reconnaissance officielle, on en connaît assez bien les caractéristiques biologiques et l'on a pu rédiger une norme pour chaque race. L'Amerioan Standard of Perfection (1974) est une source de référence importante en Amérique du Nord. Certaines de ces souches ont été utilisées et quelques-unes sont encore utilisées pour la production commerciale des oeufs et de la viande; la plupart d'entre elles ne sont désormais élevées que par des amateurs qui les montrent dans des expositions.

2. Souches de production appartenant aux principaux sélectionneurs. On pense que la plupart des principaux sélectionneurs qui vendent sur le marché international détiennent des "catalogues'' importants de souches. Ces souches peuvent correspondre & des races pures homologuées ou bien il peut s'agir de lignées synthétiques conçues à des fins particulières. Elles sont la propriété exclusive de l'entreprise procédant a la sélection.

3. Souches indigènes. Il existe encore probablement dans beaucoup de pays des souches locales de volaille "non améliorées". Elles ne sont pas classées parmi les races proprement dites et il n'en existe aucune description écrite.

Inventaire des ressources. Compte tenu des modifications très rapides et spectaculaires qui ont eu lieu et se poursuivent dans l'aviculture mondiale, toute considération relative au maintien du patrimoine génétique doit s'appuyer essentiellement sur un inventaire à jour des ressources génétiques. Jusqu'à présent, on n'a pas essayé d'établir un inventaire mondial. Au Canada, un catalogue des souches de volaille détenues dans des établissements àe recherche et d'enseignement a été révisé et il paraît chaque année depuis 1967 (Crawford, 1980). On élargit actuellement la portée de l'inventaire canadien pour y inclure des souches détenues par des amateurs et celles qui appartiennent à des sélectionneurs commerciaux, la catalogue des Etats-ltois d'Amérique (Somes, 1978), dont la quatrième édition est en cours de préparation, est très bien connu. En Australie, une Commission d'experts sur la con­servation des ressources génétiques avicoles s'emploie actuellement à dresser un inventaire et à préserver les souches. Le catalogage dans d'autres pays devrait bénéficier d'une haute priorité.

Ressources à protéger. L'expérience acquise avec l'inventaire canadien reflète peut-être la situation qui règne ailleurs dans le monde. Il est clair que la presque totalité des souches de poulets et de dindons productrices d'oeuf» et de viande sont actuellement détenues par les principaux sélectionneurs. Comme ces entreprises n'établissent leurs plans, penae-t-on, que pour les dix années à venir, il se peut que les organismes publics aient à assumer un rô1e cour la préservation des souches productrices d'oeufs et de viande en vue de, faire face aux besoins à long terme. Il ne subsiste sur le marché que quelques races pures bi­valentes. Elles sont détenues par de petits centres d'incubation saisonniers et il est peu probable qu'elles survivent encore longtemps. Si l'on veut les conserver, c'est aux organismes publics qu'il appartient de s'en occuper. L'aviculture chez les amateurs est florissante et permet d'entretenir tout un éventail de souches d'agrément; toutefois, on a constaté aveo inquiétude que les amateurs sont fortement tributaires de quelques négociants et maisons d'approvisionnement comme sources de souches: le fondement génétique des races élevées par les amateurs est donc, peut-être beaucoup plus étroit qu'il n'apparaît à première vue. Abstraction faite de quelques entreprises de production de canards fortement intégrées, les autres espèces de volaille sont peu exploitées au Canada et ne comptent que quelques élevages fort peu nombreux et sans sélection.

Il est probable que les souches indigènes dans les pays où il n'existe pas d'aviculture hautement complexe et moderne ne sont pas sérieusement en danger. Toutefois, une surveillance peraanente s'impose pour garantir qu'elles ne soient pas rapidement supplantées. Si cela devait se produire, il faudrait alors recueillir des échantillons représentatifs dont la préservation serait confiée à un organisme public. Le catalogage est nécessaire et il doit comprendre le relevé des caractéristiques.

Biologie et génétique de la préservation du patrimoine génétique avicole. Si les souches doivent être détenues par les organismes publics, il faudra faire appel aux experts en biologie et génétique avicoles pour garantir une utilisation optimale des ressources dans un délai raisonnable.

La préservation de souches de volaille comme ressources génétiques soulève des diffi­cultés propres à cetto espèce. A l'heure actuelle, il n'est pas possible de stocker des oeufs viables pendant une longue durée; les spermatozoïdes peuvent survivre à la surgélation pendant au moins plusieurs années dans les conditions de laboratoire, mais cette technique de stockage n'est pas encore utilisable d'une manière fiable sur le terrain (Buckland, 1979); aussi les volailles doivent-elles être conservées sous forme d'élevages vivants. L'autre difficulté est due & la fréquence de la régénération. Pour éviter toute perte génétique, il faut que les troupeaux se reproduisent quand leur aptitude est optimale à cet égard. Chez la plupart des espèces, cela exige la reproduction à des intervalles d'un an. Ches les oies, la fécondité est médiocre jusqu'à la deuxième année, puis elle demeure satis­faisante jusqu'à, l'âge de cinq ans* Les cailles Cotornix doivent se reproduire dès l'âge de six mois. A moins qu'on ne prenne certaines précautions, un intervalle court entre les générations peut conduire à une rapide oonsangulnité entraînant une perte de variabilité génétique pendant le déroulement d'un programme de préservation à long terme. Port heureuse­ment, la conservation d'un grand nombre de sujets, en particulier de mâles, est beaucoup moins diffioile pour la volaille que pour le gros bétail.

A l'heure actuelle, les généticiens pensent que le programme de sélection idéal pour réduire au minimum toute perte de variabilité génétique dans les souches de volaille doit être fondé sur la reproduction au hasard, qui doit comprendre la sélection par ascendance afin de conserver toutes les familles de coqs et de poules. Il faut aussi incuré l'insé­mination artificielle, technique simple ches les principales espèces de volailles, afin d'éviter les effets d'une sélection naturelle. Des précisions sont données ft ce sujet dans le rapport de Gowe et al. (1959). Pour la conservation des souches, on a parfois tendance ft négliger l'avantage résultant de l'utilisation d'un très grand nombre de coqs afin d'empêcher les modifications dues & la consanguinité et la dérive génétique. Par exemple, on constate la même modification du coefficient de consanguinité par génération et la même dérive maximale prévue de la production d'oeufs quand on utilise 25 coqs et 25 poules, en comparaison avec le rapport plus traditionnel de 20 coqs pour 200 poules.

Bu fait que la reproduction au hasard tenant compte des ascendants est très laborieuse et coûteuse, il peut s'avérer nécessaire d'opter pour un programme moins précis. Peut-être est-il possible d'obtenir à l'intérieur d'un élevage une approximation de la reproduction au hasard dans des conditions d'accouplement naturel en procédant au choix aléatoire des oeufs ft incuber pour plusieurs séries consécutives et en changeant souvent de coqs. On applique ce système dans une collection de souches en utilisant 15 coqs et 5° poules par génération.

Deux méthodes sont utilisées pour la conservation contrôlée du patrimoine génétique des volailles: soit le maintien des élevages à l'état de race pure, soit la combinaison d'un certain nombre de souches pour la reproduction dans un pool de gènes. La première méthode vise à conserver l'identité et l'intégrité d'une race et la seconde à conserver tous les gènes composant le pool. Cette méthode de pool est probablement la plus économique pour maintenir un grand nombre de souches, en particulier pendant une durée prolongée. Toutefois, la récupération de gènes isolés ou de combinaisons de gènes à partir de ce pool peut se révéler très laborieuse. On accordera la préférence à la conservation de races pures quand on peut prévoir pour un proche avenir la nécessité de disposer de combinaisons spécifiques de gènes caractérisant la race.

Nécessité d'un soutien dea pouvoirs publics. Il est impératif que les organismes régionaux nationaux ou internationaux reconnaissent la nécessité de conaerver les ressources génétiques avicoles et fournissent un appui permanent dans un très proche avenir. L'extinction des souches de volailles peut se produire et se produit effectivement avec une extrême rapidité sous l'influenoe d'une industrie avicole hautement efficace et complexe qui élève des espèces de faible valeur unitaire et chez lesquelles l'intervalle entre les générations est trêa court. En l'abaence d'un inventaire mondial des ressouroes, les efforts déployés en faveur de la préservation ne sauraient être que sporadiques. On constate que nulle part la pré­servation des souches de yolailles n'est garantie d'une manière sûre. C'est ainsi que l'activité déployée en Amérique du Nord reflète seulement 1' intérêt personnel de quelques individus; ils ont obtenu d'organismes publics une modeste aide financière, mais la pour­suite de cet appui n'est nullement garanti.

Manejo de los recursos genéticos avícolas
Resumen

Pefinición de los recursos avíoclas. El término "avicultura" se refiere a todas las especies avícolas que se reproduoen libremente bajo el cuidado del hombre y proporcionan a éste un servicio económico; además de las especiea avícoías más conocidas también comprende las palomas, pavos reales, aveatrucea, canarios y otras variedades. Aquellas que presentan más importancia para la agricultura son los polios, pavos, patos y ganaos; la gallina de guinea y algunas eapeoies de faisanes y codornioes tienon una menor pero creciente importancia.

En el Informe de la Cuarta Consulta de la FAO de expertos (FAO, 1973), se identification diversas fuentes de recursos genéticos de las aves de corral. Podîa emplearse una clasi-ficación algo diferente para estudiar la conservación de las poblaciones:

1. Rasas y variedades reconocidas. En muchas partes del mundo se han desarrollado razas hoy sólidamente estableoidas. Hesta cierto punto r'econocidas oficialmente, se conocen en parte sus caracterfsticaa biológicas e incluso se ha desorito el tipo de la rasa. Eh América del Sorte, la American Standard of Perfection (1974) constituye la mejor referencia. Algunas de estas aves se han utilizado y otras se emplean todavïa en la producción comercial de huevos y oarne} actualmente, la mayor parte de ellas son criadas únicamente por afioiona-dos y para presenterlas en exposioiones.

2. Pie de crîa perteneciente a grandes criaderos comerciales. Se cree que la mayor parte de eatcs oriaderos con ventas internacionales poseen extensas "bibliotecas" de pie de crfa. Este pie de crîa puede ser las razas puras ya reconocidas 0 también las lineas desa-rrollados con fines espeoificos, las cuales son de propiedad exolusiva de la compania de crianza.

3. Poblaciones indfgenas. Probablemente, en muchos paísee existen aún poblaciones locales de aves de corral no mejoradas". Las cuales no están reconocidas como razas y no se ha hecho una desoripoión escrita de allas.

Inventerio de recuraos. Debido a los cambios muy rápidos e impresionantes que han tenido y siguen teniendo lugar an la industria avícola, todo proyecto de mantenimiento y conserva-oifin del germoplasma deberá basarae fundamentalmente en un inventario actualizado de los recursos genéticos. Hasta ahora, ni siquiera se ha intentado hacer un inventario mundial. En el Canadá, desde 1967 (Crawford, 1980) se reviaa y publica anualmente un catálogo de las poblaciones de aves de corral que poseen las instituciones de investigación y de snsenanza. Actualmente, el inventario canadiense se esté ampliando para inoluir poblaciones mantenidas por aficionados y las criadas por empresas comercides. Es muy conocido un catálogo americano (Somes, 1978) cuya cuarta edición está actualmente en preparación. En Australia, un Grupo de Expertos en Conservación de Recursos Genéticos de Aves de Corral se ocupa de la prepara-ci6n de un inventario y de la conservacion de las poblaciones. Se debería prestar también en ctros países una prioridad importante a la elaboración de catálogos.

Recursos que deben protegerse. Ia experiencia del Inventario canadiense puede servir de ejemplo en otras partes del mundo. Resulta evidente que casi todas las variedades de polios y pavoe productores de huevos y de came están hoy en manos de empresas de crianza. Como se tiene enténdido estas empresas programan sólo con diez anos de adelanto de manera que los or ganismos públicos deben aceptar ocuparse de la protección del pie de cria productor de huevos y carne para atender necesidades futuras. Sólo permanecen en el mercado unos cuantos ejemplares de pura raza de doble finalidad; se tienen en pequeñas incubadoras de teniporada y no es probable que sobrevivan mucho tiempo. fera conservaxse deberán de ser protegidas por los organismos públicos. la afición por las aves de corral está floreciendo y gracias a ella se mantiene una amplia colección de poblaoiones de adorno o de lujo. Sin embargo, se ha observado con inquietud que los aficionados sólo pueden adquirir ejemplares en unos pocos establecimientos - la base genética de las razas pare aficionados puede por consiguiente, resultar mucho más reducida de lo que parece a primera vista. Exrcepción hecha de algunas operaciones bien llevadas a cabo para la producción de patos, otras especies de aves de corral ee explotan poco en el Canada y su conservación se basa en el aprovisionamiento de pocas bandadas no seleccionadas.

Probablemente no se encuentra en grave peligro el pie de cria indîgena en los países que careoen de una industria moderna de producción de aves de corral. Pero se requiere una vigilancia continua para que tal pie de cria no se vea reducido rápidaraente. En el caso de que esto occurriera, los organismos públicos deberán conservar ejemplares representatives para mantenimiento de la especie. Se necesita elaborar un catálago de esos ejemplares incluyendo el registro de sus caracteristicas respectivas.

Biología y genética de la conservación del germoplasma en avicultura. Si se desea que los organismos públicos se hagan responsables del mantenimiento del pie de cria seria conveniente utilizar los servicios de especialistas en biología y genética de las aves para estar seguros que el tiempo y los recursos se aprovechan al máximo. El mantenimiento del pie de cría aviar como recurso genético presenta dificultades peculiares para las especies. Actualmente, no es posible el almacenamiento a largo plazo de huevos fértiles; los espermatoz.oides con-gelados pueden conservarse viables durante varios anos en condiciones de laboratorio, pero esta téenica no es todavía fiable en el terreno práctico (Buckland, 1979); por consiguiente, las aves de corral deben conservarse en bandadas vivas. La segunda dificultad que se presenta es la regeneración constante. Para evitar pérdidas genéticas, las bandadas deben reprodusirse cuando sea óptima lasptitud a la reproduoción. En la mayor parte de las espeoies la reproducoión debe efeotuarse anualmente. La fecundidad de los gansos es baja hasta el segundo ano de vida, pero después llega a un buen nivel hasta les cinco años. Las codornices deben reproduoirse a los seis meees de edad. Si no se toman precauciones, los intervalos de generación cortos pueden llevar a una rápida consanguinidad, con la consiguiente pérdida de variabilidad genética en un programa de conservación a largo plazo. Afortunadamente, resulta mucho menos difíoil mantener un gran número de aves, particularmente machos, que cuando se trata de hatos de grandes espeoies animales.

De aouerdo a los últimos adelantos en genética, el programa ideal de cria para reducir al minimo las pérdidas de variabilidad genética en poblaciones avîcolas es el de la seleccién al azar. Este procedimiento deberá efectuar el apareamiento según la aacendencia genealógica para conaervar todas las familias de los padres y de las madres. Tarabién se tomará en cuenta la inseminaoión artificial, téonica simple en la mayor parte de las especies avîcolas, para evitar los efectos de la selección natural. En el informe de Gowe y colaboradores (1959) se ofrecen detalles al respecto. Una cuestión que a veces se pasa por alto en el mantenimiento del pie de cria es el efecto benéfico que resulta del empleo de muchos padres para impedlr los inconvenientes de la consanguinidad y la fluotuación genética. Por ejemplo, se obtiens el miemo cambio en el coefioiente de consanguinidad por generación y se prevee la misma fluotuaoión máxima en la producción de huevos, empleando 25 padres y 25 madres que usando una proporción más tradicional que comprenda 20 padres y 200 madres.

Debido a que la selección al azar sobre la ascendencia resulta muy laboriosa y costosa, puede ser necesario un programa Minos precise La seleoción al azar puede lograrse quizá en oondiciones de apareo natural en una bandada por la selección al azar de los huevos para incubación de varias puestas consecutivas, y por rotaoiones frecuentes de machos de cría. Este es el sistema que se sigue en un pie de cría utilizando 15 padres y 50 madree por generación.

Se emplean doa métodos para mantener el control del germoplasma de aves: conservar bandadas de pura raza, o bien combinar varias poblaciones para reproducir un banco genético. El primer método tiene por objeto mantener la identidad y la integridad de una raza; el segundo mantener todos los genes presentes en el banco. Probablemente, este último es el método más económico para mantener varios pie de cría, especialmente por un período prolongado. Sin embargo, la reouperaci6n de genes individuales o de combinaciones de genes a partir del banco genético puede Ber un procedimiento muy laborioso. Posiblemente sea preferible mantener razas puras para una necesidad a corto plazo, de combinaciones de genes espeeíficoe de una determinada raza.

Necesidad de apoyo oficial. Es imperativo que los organismos regionales, nacionales 0 internacionales reconozcan la necesidad de mantener los recursos genéticos de las aves de corral y otorguen un apoyo continuo en el futuro inmediato. La extinción del pie de crla aviar puede ocurrir y de hecho oourre con extrema rapidez, bajo la influencia de una induBtria altamente eficaz y compleja que se ocupa de especies de bajo valor unitario y de intervalos de generacion muy cortos. Sin un inventario mundial de los recursos, los esfuerzos de con-servación sólo pueden ser esporáicos. Parece ser que actualmente la conservación del pie de cría aviar no constituye en ninguna parte una actividad sólidamente establecida. Por ejemplo, la actividad que se desarrolla en América del Norte refleja el interés personal de sólo unos cuantos individuos que han obtenido escasa ayuda financiera de organismos donantes, pero no hay ninguna seguridad de un apoyo continúo.