INBREEDING VERSUS LINE-BREEDING IN WILDLIFE
Adapted from the Wildlife Stud Services Journal 2015
Important recent developments in the wildlife industry
The local wildlife industry has grown into an industry to be reckoned with. This dramatic growth in the industry has generated immense interest from different stakeholders e.g. academia, research, private and public sector. At the centre of these developments is the wildlife genetic resources that is a finite and thus require careful harnessing to ensure sustainability. As a result, serious discussions are taking place at different levels of the industry to ensure that utilisation of the wildlife genetic resource takes into account both the short-term and long-term needs of society. In this article, our goal is to illuminate the audience about an aspect of science that should inform the wildlife genetic resource utilisation strategy. We specifically address the topic of preservation of genetic diversity when wildlife genetic resource are utilised in economically important activities such as food production and eco-tourism. The topic of inbreeding is discussed at great length.
Is there a difference between inbreeding and line-breeding?
Technically, inbreeding refers to the mating of close relatives. Note the word close since all animals within a population are related to some extent if we trace ancestry deep enough. Line-breeding, on the other hand, is a form of inbreeding where breeding is focused mainly on a specific ancestor such that the ancestor in question dominates the pedigree of an animal of interest. From a practical point of view, the distinction between inbreeding and line-breeding is rather immaterial since the consequences of the two processes are similar. It should be noted that in other species such as dogs a distinction is made between inbreeding and line-breeding.
Given this background the ensuing discussion will make reference to inbreeding and not line-breeding since that distinction is an unnecessary detail. Perhaps, the most important question is: why should the wildlife community bother about inbreeding?
Importance of inbreeding in wildlife
Most wildlife populations have small census population size and are at great risk of extinction. From a population genetic health standpoint the effective or “genetic” population is more important. Thus, knowledge of the census is not sufficient for management of population genetic diversity. If left unmanaged, inbreeding could lead to devastation of wildlife populations. It would be safe to point out that the level of inbreeding could have already reached unacceptable levels in some of the wildlife populations.
The long-term impact of inbreeding in a population is the erosion of genetic diversity which compromises the genetic health of the population. In the short-term, inbreeding is associated with reduced fitness e.g. high mortality. This phenomena is called inbreeding depression. In other words, the effect of inbreeding are at two levels i.e. the individual animal level (inbreeding depression) and population level (loss of genetic diversity or reduced genetic health).
Briefly, inbreeding is a result of deleterious recessive genes whose expression only occurs when related individuals are mated. Because they are recessive in nature these genes are only expressed when they occur in a dose of two copies. In the absence of inbreeding, these “defective” genes occur in a heterozygous state i.e. a combination of a defective and its normal / active counterpart which suppress its expression. Thus, these deleterious recessive occur in a population albeit at a low frequency in the absence of inbreeding.
A good understanding of inbreeding could be gleaned by reference to this example. Assume that a new deleterious recessive mutation occurs and is a carried by a given individual. A given mating between this individual and any other individual in the population will have a 50% chance of inheriting this mutation. That is, this mutation will run in this family. So long as this mutation is limited to this family and intra-family mating does not occur the recessive mutation will not depress fitness in this family. It is only when members of the same family mate together that the unfortunate situation that the two copies of the recessive mutation comes together will occur. This particular example illustrates the phenomenon of inbreeding at least if we assume a simple trait that is controlled by a single gene. However, the same principle holds for complex traits that are affected by many genes.
Practical actions to consider in managing inbreeding
Now that the concept of inbreeding has been explained, it is important to look at practical interventions that could be considered to prevent or manage inbreeding. In a small population, inbreeding might be unavoidable since almost all the animals within a given location would be related. In this particular case, knowledge of the extent of the relationships among individuals would be useful in order to minimise the effect of inbreeding on the resulting offspring. In practical wildlife populations, particularly those in nature reserves, translocations have been used to manage inbreeding. Translocation refers to introduction of new animals to a given population that are believed to be non-related to the population under consideration. Without knowledge about the pedigrees of the immigrant population and the existing population, discerning the level of relatedness is difficult.
Molecular genetic markers could be used to understand relatedness amongst different populations. However, databases of extensive pedigrees and genetic marker information are rudimentary or at best non-existent for wildlife populations. There is an immediate need for implementation of pedigree recording and collection of molecular marker data. It is encouraging that service providers for wildlife recording services are entering this space. The availability of pedigree and genetic marker data would, however, not be sufficient to address all the ills of inbreeding. Data on the performance of the animals is essential if the effect of inbreeding on performance or fitness is to be evaluated. There is thus a need for establishment of systems for collection of performance data at a specie level so that useful research could be conducted to detect if inbreeding levels have reached thresholds for inbreeding depression.
A recent study by Trinkel et al. (2011) investigated the relationship between inbreeding and susceptibility to bovine tuberculosis in lions in a small enclosed reserve in South Africa. They compared susceptibility of an inbred local lion population and that of an outbred translocated lion population in the same reserve. They found that 30% of the local inbred population died from TB while the mortality was only marginal (3%) in the translocated out-bred population.
There exist opportunities for the wildlife industry to diversify into other economic avenues such as the use of wildlife and game for the production of food. In this scenario the issue of inbreeding will become even more important. If the livestock industry is anything to go by, indications are that development of appropriate systems for collection and processing of animal data would become more important in the wildlife industry. An important development in the long term would be a scenario where sale transactions of wildlife require figures about relatedness of an individual to the general population.
Summary
Inbreeding is a reality in wildlife populations hence the need for interventions that would ensure that the negative consequences of inbreeding are managed or mitigated. Even though the evidence might be limited it could be possible that inbreeding might have already reached critical levels in some wildlife population as has been demonstrated by the study of Trinkel et al. (2011). Inbreeding and its implication for sustainability of the wildlife industry should receive a high priority in both research and business circles
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Literature Cited
Trinkel, M., Cooper, D., Packer, C. and Slotow, R., 2011. Inbreeding depression increases susceptibility to bovine tuberculosis in lions: an experimental test using an inbred-outbred contrast through translocation. Journal of Wildlife Diseases, 47(3), 494–500.
About the Author
Prof Maiwashe is currently employed by the Agricultural Research Council as a Research Team Manager: Animal Breeding and Genetics. He is also acting as a General Manager for the Animal Production Institute. He is a proud graduate of the University of Venda where he obtained his BSc (Agric) degree. He holds a PhD in Animal Breeding and Genetics from Colorado State University in the USA. Prof Maiwashe is rated by the National Research Foundation as an established research scientist. His research focuses on development of selection tools applicable in breeding of efficient and profitable livestock. Most of the selection tools developed from his research have been adopted by the livestock industry and are widely used by farmers in their breeding programmes.
012 672 9028 norman@arc.agric.za www.arc.agric.za
This Article thanks to Wildlife Stud Services (WS2) www.ws2.co.za
