CHAPTER 19
THE IMPORTANCE OF MATING SYSTEMS IN THE CONSERVATION OF ANIMAL GENETIC RESOURCES

by

Yukio Yamada
National Institute of Animal Industry
Tsukuba Science City
Japan

Summary

To preserve animal genetic resources, appropriate genetic variability should be retained both within and between populations, because firstly the demand of the future is not easily predicted and secondly vulnerability of genetic resources stems from genetic uniformity.

Regardless of the shifts in philosophy and values in the future, resistance to various diseases and environmental stresses must be given high priority. Productivity of the animal should be reevaluated on the basis of energy and protein input/output.

For maintaining a small population for preservation, adoption of an appropriate mating system is very important. Subdivision of a population into several lines and practising circular group mating among these lines is better than conserving a whole population as one unit by random mating. Each subline could be kept in a different station} thus the cost of maintaining the population would be minimized.

19.1 Introduction

One of the important and serious questions about genetic resource preservation is what sort of genetic variation should be maintained in a population and what gene or genes must be retained in that population. Although we might anticipate some modifications of agri­cultural technologies in the future, it is very difficult to define them specifically. The future is not what it used to be. The energy crisis, food shortage and economic recession in recent years have made all predictions of future requirements more uncertain than ever before. This implies that the present policy of genetic resource preservation should be promoted. Genes useful at the present time are likely to be equally important in the future. Furthermore, genes controlling resistance to environmental stresses such as high and low temperatures and diseases like tick infestation, viral and bacterial infections, must be retained. Adaptability to local harsh conditions may be essential in some circumstances. Fertility traits should also be emphasized. In order to improve feed efficiency in all kinds of animal and poultry, the ability to digest nutrients and to consume feed should be studied more precisely from a genetical viewpoint. The usefulness of such characteristics of local breeds or races should be evaluated before they disappear.

The increase in the frequency of a good gene in a population due to selection, reduces the frequency of a bad gene in the same population. Consequently, the genetic variation in the population decreases. Selection for a trait of high heritability causes the loss of genetic variation at higher rate than selection for a trait of low heritability; whereas selection for a trait of intermediate heritability causes loss of genetic variation only if selection intensity is high due to genetic drift. Good genes as well as bad genes will be lost from the population at the same rate. It is well known that the loss of hetero­zygosity per generation is proportional to l/2N_e' where N_e is the effective population size. It is believed that the critical value of N_e in livestock populations lies approximately in the range of 10-50 (Maijala, 1974)• The loss of genetic variation would be 1% per generat­ion for N_e=50, and 5% for N_e=10. Owing to the development of national breeding plans in dairy cattle by use of AI, the effective size of dairy cattle populations in most developed countries lies in the range of N =10-50; this needs special caution. Effective and pertin­ent systems for exchanging frozen semen on an international scale must be established so as to increase the effective population size in dairy cattle.

Our efforts to increase the efficiency of animal production may decrease the number of kinds of animal feed and thus feeding methods may change accordingly. As a result, only the animal which fits these particular conditions would he propagated. Reduction of 'breeds or raoes would then take place. Changes in housing and management as well as control of environment would also lead to the reduction in the number of breeds. For example, the development of the milking machine required a change in the characteristics of dairy cows. The extension of the cage system in poultry required adaptation to this new environment. Consequently, the breeds fitted to these new environments expended their numbers and those which did not fit were eliminated.

Changes in the dietary habits of human beings sometimes require qualitative changes of animal products. The requirements of mass production systems also contribute to the reduct­ion in genetic resources.

It is well known from our previous experiences in animal and plant breeding that the introduction of a special gene in a population is essential for breeding a new breed or variety which fits a new environment. The preservation of a seed stock in animals requires enormous space, labour and cost, which are not comparable to the needs for plants. At this stage when future changes are not predictable, nearly all native races and varieties must be preserved and evaluated. Characteristics of these local varieties or land races are however not fully evaluated in most cases and a lot of time and high cost are needed to test them in a variety of environments. Perhaps the most realistic and practical solution would be to maintain all varieties or breeds presently available in the developed countries until better alternatives are available.

The most practical method for preserving animal genetic resources is to store the germ-plasm of animals with good performance in the deep frozen state. Techniques are already available for cattle semen and are expected to "be applicable to other animals before long. Attempts at preserving frozen fertilized ova in various mammals are now under investigation, and appear successful. Preservation of frozen germ-plasm is almost permanent, and space and cost are negligible. Preserved germ-plasm can be shipped anywhere in the world according to the demand.

Monitoring, cataloguing and collating the information on animal genetio resources should be made by an international organization such as FAO. This body should act as a link with other organizations, both national and international or private and governmental, and should be independent from politics.

An alternative to the above, when freezing of germ-plasm is not possible is to establish a mixed population as mentioned in the following section. The foundation population would be formed by mixing several strains, each possessing some unique characteristics, and there­after maintaining the population as a unit.

Very little has been worked out theoretically for establishing principles of conservation and management of animal genetic resources. Thus, there is no sure way of maintaining a population in a given genetio state (Hickman, 1979)• In any breed or population which is confronted with the danger of extinction, the increase of the size of the population is crucial for maintaining the indispensable genetic variability. Except in the case where new techniques such as the use of frozen embryos are available, conservation efforts must be focused on finding appropriate breeding techniques to prevent a loss of desirable genes present in the population.

19.2 Theoretical basis for maintaining a mixed population

One of the most important problems encountered in conBerving a small population is to avoid the hazards caused by inbreeding, which is inevitably brought about by the restriction in the number of breeding animals. For this reason, a computer simulation study was under­taken to find out how mating systems and subdivision of a population into several lines affect the degree of inbreeding and consequently influence the chance of extinction of a desirable allele from the population.

The following assumptions were made: (1) No overlapping of generations; (2) constant population size; (3) pair mating; (4) equal sex ratio; and (5) no relationship among members of the first generation.

Methods; (1) Genetic relationships among all members of the population were calculated and coefficients of inbreeding of the progeny produced by designated matings were obtained for every generation. (2) Expected inbreeding coefficients in the n^th generation in a random-mating population of size N, based on f_t=1/2N + (1-1/N)^f _ -1, where t is the gener­ation number, were fitted to the result of simulation and the effective size, N_e, of the population was estimated.

Three experiments were performed as follows:

Experiment 1. A single random-mating population.

System 1. The number of progeny per breeding individual follows the Poisson distribution with the mean equal to one in both sexes.

System 2. Each pair mating produces one male and one female in the next generation.

Experiment 2. A population of size N_t was subdivided into N_p sublines, each consisting of N_SUb individuals. 

System A. Repeatedly subdividing the entire population and mixing those sublines at every t_m generations.

System B. Circular group mating proposed by Kamura and Crow (1963a) and System 2 men­tioned in Experiment 1 were combined. In the two systems, A and B, random mating was practised within sublines.

Experiment 3. Probability of loss of a gene with and without selection.

Conditions of the experiment were N = 10 and 20, initial gene frequency =0.10 and selection intensity = 50%.

Results: When a population was subdivided, the chanoe of extinction decreased as the number of sublines increased. This is because the chance of fixation tends to increase as the number of the sublines increases, whereas the direction of fixation toward plus or minus genes is random in each line and thus the subdivision makes it possible to maintain the gene more effectively in the population as a whole. Table 19.1 shows the calculated probab-ability of loss of a gene from a population when the population was subdivided, provided N_t=60, N = 1 to 6, p = 0.5 and 0.2 or 0.8. In the extreme case where many inbred lines originating from a population were maintained by sib—mating, the probability of extinction of the gene from the population would be negligible.

In a random pair-mating population of fixed size, if one male and one female contributed to the next generation (System 2 in Table 19.2), the effective size of the population could increase approximately twofold, compared with the case where the number of progeny was random (System 1 in Table 19.2). This is illustrated in Figure 19.1, together with the increase of inbreeding coefficient by random pair-mating with varying population size. Sub­division of a population into several sublines enhances inbreeding of the members of the population more rapidly, in comparison with the case of a random mating population of the same size. Unless the sublines were mixed, the larger the number of sublines the higher the rate of inbreeding. By mixing these sublines at a certain interval, the coefficient of inbreeding of the entire population remained at a rather low level (Figure 19.2 when mixing was made at every 10 generations).

Doubling of the effective number of a population by imposing the restriction of equal offspring per mating had been reported previously by Kimura and Crow (1963b).

If these sublines were mixed repeatedly at a certain interval (t_m), the increase of inbreeding could be reduced considerably. Figure 19.3 shows the effect on inbreeding, of changing the interval between mixing of sublines, provided N_t=60, N = 10 and t_m=5 and 10. Inbreeding within sublines increases steadily, whereas the mixing of sublines either every five or every ten generations reduces the inbreeding nearly to the same level as that in a large random-mating population of N = 60. Circular group mating among 10 sublines shows a slightly higher inbreeding, compared with a large random-mating population. Such a trend of higher rate of inbreeding in a population of ciroular group mating in relation to that in a large random-mating population would be reversed in later generations. From the view­point of genetic resource preservation, we may consider that the two systems are equally effective maintaining the genetic variability of a population.

Effects of selection on the probability of loss of gene from populations of varying size were studied, and the results are given in Table 19.4 and Figure 19.4. The effect of selection is strikingly high. For a favourable allele, serious loss would be encountered only in the case of low initial gene frequency (q = 0.10) or when the population is small, for example, when N = 10 as seen in Table 19.4. The effect of increasing population size is very conspicuous when the gene frequency is low, if selection is made for the favourable allele.

Acknowledgement

The author is grateful to Dr. Shinya Iyama, National Institute of Genetics, Mishima, for performing the simulation experiment.

19.3 References

Hickman, C.G. (1979) Management of animal resource systems (Mimeograph), Rome.

Kimura, M. and J.F. Crow (1963a) On the maximum avoidance of inbreeding. Genetic Research, Cambridge, 4: 399-415.

Kimura, M. and J.F. Crow (1963b) The measurement of effective population number. Evolution, 17: 279-288.

Maijala, K. (1974) Conservation of animal breeds in general. 1st World Congress on Genetics Applied to Livestock Production, Madrid. Vol. 2, 37-46.

Table 19.1

Probability of loss of a gene from a population when the population is subdivided (N=6o)

Table 19.2 

Inbreeding effective number (No. of generations = 100)

Table 19.3

Inbreeding coefficients (f) and effective population size (N ) when the population was sub­divided and mixed at every t_M generations

Table 19.4

Probability of loss of a gene from a population of different size with and without selection. Selection intensity = 50% heritability = 0.1 with no dominance^

Fig 19:2. Coefficients of inbreeding by subdivision and mixing of sublines in a population (N =60)

Fig19:3. Coefficients of inbreeding for different mating systems with or without nixing sublines

Fig19:4. Probility of loss of a gene, with and without selection (h²=0.1 and selection intensity =1/2)

Résumé

Rares sont les travaux théoriques visant á établir les principes régissant la con­servation et 1'exploitation des ressourcee génétiques animales. C'est ainsi qu'on ne dispose d'auoun moyen sûr pour conserver une population dans un Stat génétique donné (Hickman, 1979). Chez toute race ou population menacée d'extinction, il est capital d'augmenter l'effectif si l'on veut conserver 1'indispensable variabilité génétique. Sauf dans les cas où l'on dispose de techniques nouvelles telles que l'utilisation de sperme congelé d'oeufs fécondés congélés, les efforts de conservation doivent être axes eur la recherche de techniques de sélection appropriées afin d'éviter la perte des génes souhaitables qui sont présents dans la population.

L'un des problèmes les plus importants auxquels on se heurte quand on ne conserve qu'une population à effectif récluit, c'est d'éviter tous les risques inhérents à la con­sanguinité que provoque inévitablement la limitation du nombre des animaux d'elevage. A ce point de vue, on a entrepris une étude avec simulation sur ordinateur pour déterminer comment les systèmes d'accouplement influent sur le degré de consanguinity ("inbreeding") et, partant, sur le risque d'extinction d'un allèle désirable au sein d'une population.

Les hypothèses sur lesquelles repose cette étude sont celles que retiennent habituelle-ment les chercheurs, à savoirs (1) pas de chevauchement entre les générations; (2) population à effectif constant; (3) accouplement par paire; (4) nombre égal de sujets des deux sexes; (5) auoune parenté entre les membres de la premiére génération.

Mfithodes: (1) On a calculi les rapports génétiques entre tous les membres de la population et l'on a obtenu pour chaque génération les coefficients de consanguinité de la descendance issue d'accouplements désignés. (2) Les coefficients de consanguinité prevus à la èniéme génération dans une population avec accouplement aléatoire d'effectif N, sur la base de f_n - 1/2N + (1-1/N)fn-1 + (l/2N)f_n_2, ont été compares au résultat de la simulation et l'on a estimé l'effectif réel, Ne, de la population.

Expérience 1. Population avec accouplement aléatoire unique

Stystème I. L'effectif de la descendance par sujet reproducteur correspond à la distribution de Poisson avec la moyenne = 1 chez les deux sexes.

Système II. Chaque acoouplement reproduit un mâle et une femelle pour la génération suivante.

Expérience 2. Une population d'effectif Nt a été subdivisée en Np sous-ligné'es, coroposées ohacune de Nsuh sujets

Système A. Subdivisions répétées de la population totale et mélange des sous-lignles à chaque génération t_m.

Système B. to a combine 1'accouplement par permutation circulaire à l'intérieur du groupe propose par Kimura et Grow (1963) et les systèmes I et II mentionnés pour 1'expérience 1. Dans les deux systèmes A et B, on a pratique 1'accouplement aléatoire au sein des sous-lignees.

Les résultats sont récapitulés ci-après:

1. Quand une population Start subdivise'e, le risque d'extinction diminuait à mesure que le nombre des sous-lignées augmentait. Cela s'explique par le fait que la possibilité de fixation tend à s'accroltre à mesure que l'effectif de la sous-lignée augmente, tandis que son orientation est aléatoire dans chaque lignée, de sorte que la subdivision permet de maintenir le gène plus efficacement dans 1'ensemble. Dana le cas extrême où de nombreusee lignées consanguines seraient maintenues par accouplement entre frèree et soeurs, la proba­bilité d'extinction du gène serait négligeable.

2. (a) Chez une population d'effectif déterminé à accouplement aléatoire par paire, ei pour chaque couple de parents un mâle et une femelle contribuaient à la génération suivante, l'effectif réel de la population pourrait doubler, par rapport au cas ou la sélection de la descendance est aléatoire. (b) la consanguinité progresserait plus rapideirent si la population était subdivisée, par rapport à une population d'effectif Bemblable se reproduisant par accouplement aléatoire. En revanche, si l'on pratiquait 1' accouplement par permutation circulaire à 1'intérieur du groupe, le taux d'accroissement était très faible par rapport à celui obtenu avec une population à accouplement aléatoire.

3. Si ces sous-lignées étaient constamment mélangées à certains intervalles, 1'aug­ mentation de la consanguinité pourrait être considérablement réiduite, á savoir: (a) plus l'intervalle est court dans le mélange des sous-lignées, plus l'augmentation temporaire de la consanguinité est faible; (b) plus l'effectif des sous-lignées est important, plus l'accroissement de la consanguinity est faible; (c) 1' inconvénient indiqué en (b) était moindre dans le système II que dans le système I.

On peut done conclure qu'il faut assurer la conservation d'une population a faible effect if en la subdivisant en plusieurs lignèes (de 3 à 5) et en pratiquant 1'accouplement par permutation circulaire à l'intérieur du groupe parmi les sous-lignées toutes les 5 à 10 générations, plutÔt qu'en conservant une population entière par accouplement aléatoire. Afin de réauire le risque d'extinction d'un gène souhaitable et aussi de réduire le taux de consanguinité au sein d'une population, il convient de maintenir ausei constuit que possible le nombre des descendants issus de chaque géniteur.

Resumen

Se ha avanzado muy poco en la formulacióin de principios de conservación y manejo de recursos genéticos animales. De manera que no existe un medio seguro de mantener una población en un estado genético determinado (Hickman, 1979). En cualquier raza o población en peligro de extinción, el aumento del tamano de la población tiene una importancia decisive para el mantenimientode la variabilidad genética indispensable. Salvo que se disponga de nuevas tecnologias tales como el empleo de semen congelado y de óvulos fertilizados con-geladoe, los esfuerzos en materia de conservaci6n de recursos genéticos deben concentrarse en el descubrimiento de técnicas de selecci6n genética apropiadas que impidan la pérdida de genes deseables presentes en la poblacion.

Uno de los problemas más importantes que plantea la conservación de una población pequeña consiste en evitar todas las clasea de azar causadas por la consanguinidad que inevitablemente se produce cuando se limita el número de animales reproductores. Desde este punto de vista, se llevó a cabo un estudio de simulación con computadora para in— vestigar como afectan los eistemas de apareamiento el grado de consanguinidad y la con-Biguiente influencia sobre las probabilidades de extinción de un alelo dsseable en una poblacióin deterrainada.

Las suposiciones de este estudio son aquellas que normalmente emplean los in-vestigadores, a saber: (1) no superposición de generaciones; (2) tamaño constante de la población; (3) acoplamiento en parejas; (4) igual propcrción de sexos, y (5) ninguna relación entre los miembros de la primera generación.

Métodos: (1) se calcularon las relaciones genéticae entre todos los miembros de la poblaclón y se obtuvieron para cada generación loe coeficientes de consanguinidad de la progenia producida por los apareamientos designados. (2) En una poblaci6n de talla N, apareada al azar, basada en la fórmula: f_n = 1/2N + (1-1/N)f_n-1 + (l/2N)f_n_₂> los co­eficientes de consanguinidad esperados en la n generación se ajustaron al resultado de la simulación, y ae estimó la talla efectiva Ne de la población.

Experimento 1. Una población simple apareada al azar

Sistema I. El número de la progene por cruzamientos individuales sigue la dis-tribucion de Poisson, con una media = 1 an ambos sexos.

Sistema II. Gada pareja acoplada reproduce un macho y una hembra para la siguiente generación.

Etperimento 2. Una población de talla NT se subdividió en Np aublineas compuesta cada una de Nsuh individuoa

Sistema A. Subdividir repetldaa veces toda la población y mezclar las sublineas en cada t_m generación.

Sistema B. Se combinó el apareamiento ciroular en grupo propuesto por Kimura y Grow (1963) y tambien fueron combinados los aietemas I y II mencionados en el Experimento 1. En los dos Siatemas, Ay B, se practicó el apareamiento al azar dentro de las aublineas.

Los resultados se reaumen de la manera siguiente:

1. Cuando ae subdividia una poblacion, disminuya la probabilidad de extinción al aumentar el número de aublineas. Esto es debido a que la probabilidad de fijaoióh tiende a aumentar a medida que aumenta la talla de la sublinea, mientras que su orientación es aleatoria en cada linea y por lo tanto la subdivisión permite mantener la totalidad del gene de manera más eficaz. En el caso extremo en que muohaa lineas conaanguineaa se mantuvieran por apareamientos entre hermanos y hermanas, la probabilidad de extinción del gene aeria minima.

2. (a) En una población de talla fija apareada al azar, si un macho y una hembra contribuyen a la prócima generación, el tamaño efectivo de la población podrfa aumentar al doble, comparación con el caso en que el número de la progenia es aleatorio. (b) sub-dividiendo la población la consanguinidad podria avanzar más rápidamente. comparada con una gran poblacián del mismo tamaño apareada al azar. Eh cambio, practicando el apareamiento circular en grupo, el indice de aumento seria muy pequeño en comparación con el de la población apareada al azar.

3. Si las aublineas indicadas se mezclaran repetidamente a ciertos intervaloe, podria suprimirse considerablemente el aumento de la consanguinidad, es decirt (a) cuanto mas corto sea el intervalo en las mezolas de aubllneas, manor será el aumento temporal de la consanguinidad: (b) cuanto mayor sea la talla de las aubiineas, manor será el aumento de la conaanguinidad, y (c) el inconveniente señalado en (b) fué manor en el Siatema II que en el Siatem I.

Se llega, pues, a la conclusión de que la conaervación de una población pequeña debe efectuarse subdividiendo la población en varias (3 - 5) lineas y praoticando el apareamiento circular en grupo entre aublineas cada 5 - 10 generaciones, en lugar de conservar toda una población por apareamientos al azar. Para disminuir la probabilidad de extinción de un gene deseable sai como para disminuir la tasa de conaanguinidad en una población determindeda deberá mantenerse lo más conatante poaible el número de deacendientes por cria individual.