A systematic approach to breeding programs for game PART 1

Transcription

1 A systematic approach to breeding programs for game PART 1 Japie van der Westhuizen South African Stud Book and Animal Improvement Association japie@studbook.co.za As with many other things in life, the approach followed in breeding game will determine the final destination. One of the current challenges in game breeding is to distinguish myth from useful fact and to use current knowledge on livestock and other breeding, genetics and breeding practices correctly. An open but critical approach will therefore be more likely lead to success. Success is also relative, as it needs to be measured against predetermined benchmarks. The second challenge for a game breeder is therefore to recognize and set these benchmarks. Achieving goals is therefore an on-going process of assessments and adjustments. To be able to reach the highest levels of efficiency animals with the genetic merit for the traits of economic importance should perform under optimum conditions. Figure 1 illustrates the possible different scenarios comparing differences among wildlife in terms of their genetic merit and phenotypic performance. Figure 1. Utilising the genetic merit by creating optimal conditions. From Figure 1 it is clear that not all animals perform according to their genetic merit, simply because the conditions are not optimal. Likewise, some animals with lower merit might outperform the superior animals due to the treatment they receive. Untying nature from nurture (the genetic merit versus the environmental effect on the observation) is therefore fairly complex but very important to establish the value of each individual animal s merit as a potential parent for the next generation. Game breeders should therefore be aware of all the aids available and also endeavour to utilise them optimally.

2 Start where it matters Successful breeding programs start with objectives and criteria. Sometimes the criteria will be linked to biological efficiency that ultimately underpins financial efficiency. In effect it is important to identify those processes that will influence the quantity and quality of the product or products determining the income of the enterprise. On the other hand, those properties or factors that will drive costs should also be considered. Figure 2 depicts the sequence of decisions and actions. Figure 2. Continuing the quest to breed superior animals by following a logical sequence. Figure 2 shows that plans and criteria follows objectives and serve as the building block of ranking animals according to their adherence to these criteria to enable the breeder to cull those not conforming to these norms. Alternatively these rankings enable the breeder to identify top herd sires (and dams) suitable to have large impact where it matters. Any selection and breeding program therefore depends on the setting of objectives, recording of animals and ranking them according to their genetic merit, fitting these objectives, and using them in the breeding program to achieve these objectives optimally. Some basic Genetic principles Differences on the expression of traits are basically due to two major modes of inheritance, that can be described as either qualitative or quantitative gene action. Although there may be some grey areas as to when these gene actions can be classified as one or the other a simple explanation is as follows: Qualitative traits (expression of traits influenced by less complex gene actions). Examples of traits that can be classified as qualitative traits are coat colour, polledness and congenital genetic defects (as caused by simple gene actions). Some of the typical properties of these traits are:

3 Usually simple inheritance one or only few genes will influence the expression of the trait. The expression of the traits can usually be described as discrete classes, eg. colour variants. Therefore very easily recognisable on the phenotype. Sometimes modifying genes might influence or modify the expression of the original gene s influence. Examples will be when the expression of a coat colour is diluted such as a creamy colour rather than, for instance, dark red. The expression of the trait will not be influenced by the environment. Each animal will carry two possible variants (alleles) of the gene influencing the particular trait. The specific variant or the collective effect of the two variants will determine the expression (observation) of the trait. Figure 3 gives some insight into molecular the structure within the nucleus of each body cell. Figure 3. Illustration of die structure of the chromosome and DNA in the nucleus of body cells. From Figure 3 it can be seen that the code determining the genetic differences among animals is the result of the sequence of base pair molecules on the double helix strings wound up to ultimately construct the particular chromosome. Such a sequence will be associated with a specific gene. Chromosomes furthermore exist in pairs in the cell nuclei where each of the paired chromosome will carry one of each gene s form (known as an allele). In the example of Figure 3, these alleles are indicated as either minus (-) or plus (+) indicating the effect of the gene on the expression of a specific trait, for example coat colour. If, for example, the expression of the trait is governed by a single gene (as could be the case for qualitative traits) and one allele dominates the other, the specific expression will be supressed if the animals has a +- (one copy of each allele) combination. If the desired expression, for example a coat colour of high value will only be expressed where both alleles. The inheritance of these traits or properties therefore follow fairly simple patterns as is illustrated in Figure 4.

4 Figure 4. Example of the expression of a trait, eg. Coat colour, influenced by a single gene in the case of dominance. Figure 4 also illustrates the concept of Split (+ - ) animals, namely those carrying the desired allele of the gene but where the specific expression is supressed due to dominance by the corresponding allele of the gene. Currently carriers of desirable genes that but where the expression of the trait is supressed due to dominance (by the other allele) can not be identified by means of a genetic test as these genes have not yet been discovered or tests developed for this purpose. One way of dealing with this problem is to keep proper pedigrees and to accurately record the occurrence of these (and other) traits and properties. Where the mode of inheritance is known, it is then possible to mate carriers ( splits ) that will increase the probability of animals that will express the desired properties. Mating two carriers will result in 25% of the progeny expressing the property and half of the progeny will be carriers. Mating a homozygous animal (one showing the desired trait) with a carrier will result in a 50% probability of the progeny expressing the trait and 25% carriers (the other 25% wil be homozygous for the other, less desirable allele). More complex systems also exist where different levels of gene interactions might influence the expression of certain traits. Such examples are one or more of the following: Modifying genes. Sometimes the expression of certain traits might be modified by genes that are not influencing the trait in the first place. Examples are where colour might be diluted in the presence of certain genes. Allelic series. Some traits are influenced by a possible sieries of alleles that can be present on a specific locus (place) on the chromosome. Each form (allele) will have a position on the dominance chain (eg. A 1 dominant over A that is dominant over a that is dominant over a 1, etc.).

5 Regulatory genes of on-off switches There is evidence that some genes may be regulated by other genes in such a way that the expression they are pre-destined to control may be switched on or off by a different gene (or sets of genes) based on some other (for example environmental) influence. The description of traits influenced by quantitative gene action will follow in PART 2 in next month s edition.

6 A systematic approach to breeding programs for game PART 2 Japie van der Westhuizen South African Stud Book and Animal Improvement Association japie@studbook.co.za As mentioned in PART 1, differences on the expression of traits are mainly due to two major modes of inheritance namely qualitative or quantitative gene action. Following the description of qualitative gene action, the detailed explanation of quantitative gene action follows: Qualitative traits (expression of traits are influenced my a multitude of genes) The expression or measurement of these traits are influenced by many genes, each with a small but additive effect. The performance of the animal (the recording) is furthermore influenced by environmental constraints. The expression of these traits are therefore generally described by the following equation: P = G + E Where: P = The expression (measurement) of the trait G = The sum total of all the many genes influencing (positive or negative) this expression E = Environmental influence on the expression of the trait Most traits of economic importance are inherited in this manner. They include traits such as reproduction (age at first calving or lambing, calving or lambing interval, semen quantity and quality), growth rate (weight at a specific age), body characteristics (horn growth rate, body size) and others. Due to the nature of this type of inheritance, the expression or measurement of these traits follow a continuous pattern and is therefore not expressed in distinct categories. Figure 5 illustrates the type of patterns that can be expected for these traits.

7 Figure 5. Example of the expression of a trait, eg. horn growth rate, influenced by many additive genes and also influenced by the environment. Figure 5 illustrates the continuous scale of measurement of quantitative traits. Also important is to note that the measurement will also be influenced by the environment. Environmental influences include one or more of the following: physical constraints (eg. temperature, humidity, elevation), nutrition (quality and quantity of feed and grazing, minerals), dam influences (age of dam, genetic merit of dam as a mother, protection), herd or flock influences (ability of herd/flock to move or change position, protection measures, hierarchy), seasonal effects (rain patterns, relative birth position in the season), etc. The expression (measurement) of these traits also follow a typical normal distribution as illustrated in Figure 6. Typically recordings on most of the animals will be close to the average with less animals with bigger or smaller measurements further away from the average. Figure 6. The expected normal distribution of traits influenced by many additive genes. It can be seën from the Figure 6 illustration that most of the animals will record measurements closer to the average of the group while the top performers will be fewer (the same is true for the bottom group).

8 Typically selection of the more desirable animals as parents for the next generation should therefore take place within an environment where each animal has an equal chance of performing. Differences will then be mostly effected by genetic differences among the group of selection candidates. Figure 7 illustrates the expected differences in performance of groups of animals in different environments (or that have been treated differently). Figure 7. Illustration of the expected normal distribution in the expression of the same trait in two different environments. The mere fact that one environment puts a bigger constraint on the expression of a trait does not mean that those animals are of inferior genetic merit. The inferior performance could simply be due to these environmental constraints. This creates a unique challenge for game breeders to overcome. In the livestock industry, systems and methodologies have been developed to deal with this. It is based on the fact that families share the same genes. The, so called Mixed Models result in the prediction of each animal s genetic merit free from known environmental influence (such as location, year of birth, age of dam, season, sex, etc.). The prerequisite is, however, that the traits considered should be recorded on as many animals as possible. This, in itself poses some careful planning for game breeders and tot make optimum use of darting events to record as many as possible important traits. Steps to establish a similar system for game breeders will be in the first place to record the parentage of individuals (even if DNA markers are used to confirm dams and sires) and recording of the traits, as mentioned.

9 SA Stud Book renders such a service to the livestock industry and has been instrumental in the development and application of sophisticated mathematical mixed models for the prediction of genetic merit for farm animals (Beef Cattle, Dairy Cattle, Sheep, Goats, Pigs and lately, Boxer Dogs). These developments are also based on a reliable data set designed and managed in accordance with international standards set by the International Committee for Animal Recording (ICAR). Stud Book s system and genetic evaluation conform to ICAR s Certificate of Quality. The initial uptake of game data has taken place on Stud Book s logix Game system. Once enough data has been collected the research team will engage in determining the genetic component in the expression of these traits (also called the heritability) followed by the first genetic predictions. Notes on Inbreeding and outcrossing. Inbreeding is defined as Mating animals that are more related to each other than the average of the population. The main result of inbreeding is that the homozygosity (proportion of identical alleles of the gene on the chromosomes) is increased, therefore reducing genetic variation (a larger portion of the animals in the population have identical gene-pairs). It therefore also increases the chance that some gene will be fixated, meaning that the lack of diversity could cause a situation where all animals will carry exactly the same alleles, including deleterious genes. In many cases this in turn leads to lower fitness (fertility and survivability) and no breeding escape routes in terms of natural selection ( survival of the fittest ) due to the sameness of all animals. Sometimes a milder form of inbreeding is the result of continuous selection of superior animals and concentrating on one or only a few traits as breeding goals. Genetically superior animals tend to produce offspring that are also genetically superior when compared to others. Basically these offspring also tend to have higher inbreeding but are particularly more related to the superior common ancestors. This is known as linebreeding as these ancestors form family lines. The, so called, eco types in the same species are prime examples of lines that occur in nature, primarily due to mating behaviour in certain species or due to the isolation of sub populations for various reasons. A golden rule of thumb is to avoid inbreeding at all cost, if possible or to at least keep the rate of increase low. Numerous research projects and scientific reports show that nothing really good result from inbreeding, especially when natural selection is unable to counter the harmful effects due to a rapid increase in inbreeding over generations. A quick way to fix this is the, so called, outcrossing. These are mating plans where animals from different unrelated lines are mated. The result is progeny where the inbreeding coefficient is drastically reduced or even nullified within one generation. Good breeding practices will also ensure regular changes of breeding (hopefully with unrelated) males before sires will be mated to own offspring.

10 Important considerations for sensible game breeding programs. All successful breeding programs rely on accurate information and very clear objectives. This is only possible when proper reliable records are kept. If possible, the first step will be to keep accurate pedigrees. Observations and recordings of economically important traits or properties should also be based on objective measurements or at least be repeatable placing the potential breeding animals in the same ranking with successive evaluations. Due to the nature of game farming and breeding, the maximum number of observations and measurements should take place when darting or when animals are contained. This includes taking biological samples for the necessary DNA (mostly for parentage verification or proof of identification) or other tests. Evaluation of suitable candidates as breeding stock should be done within contemporary (treatment or environmental influence) groups where ranking reflects the genetic merit for the desirable trait. Like all good business practices, any breeding program should also be assessed regularly and objectively by comparing outcomes to predetermined goals. Breeding goals should at least be based on longer term sustainability of the enterprise. Although relatively high prices may influence profitability in the short term, this should not be the longer term deterrent to take into consideration. Generally goals should include cost of production relative to total output. Game, like other farm animals, have requirements related to maintenance (mainly feed resources) and show certain limitations regarding reproduction rate and growth parameters. Inclusion of these properties, together with others of economic importance, should always be considered in when setting up breeding objectives and mating plans.