Genetic Improvement and Crossbreeding in Meat Goats Lessons in Animal Breeding for Goats Bred and Raised for Meat Will R. Getz Fort Valley State University Appendix J. Genetic Implications of Recent Biotechnologies Appendix Contents Introduction Reproductive technologies Artificial insemination (AI) Embryo transfer (ET) In-vitro fertilization Sex control Cloning Same-sex mating Conservation genetics Molecular technologies DNA fingerprinting for animal identification Marker assisted selection for polygenic traits Gene transfer and transgenic goats Chimeraism Summary Introduction Biotechnology can be broadly defined as the application of biological knowledge to practical needs (Bourdon, 2000). From an animal breeding perspective biotechnologies fall into two general categories, more or less. 1. Reproductive technologies such as artificial insemination and sex control. 2. Molecular technologies which can be used to locate, identify, compare and manipulate genes. Reproductive technologies Animal breeding and reproduction are two distinct disciplines. Animal breeding is a branch of genetics, and reproduction is an aspect of physiology. Yet, the two fields are closely connected. Although the underlying genetic principles of animal breeding are mostly independent of the physiology of reproduction, the practice of animal breeding is not separated from reproductive physiology. Selection and mating decisions are made in the context of available reproductive technologies, artificial insemination. 1
Artificial insemination (AI) Techniques for AI in goats are well developed and the technology is well accepted for use in many circumstances. Dairy goat breeders have successfully used AI for decades. The same technologies can be applied to meat goats. Artificial insemination carries three distinct advantages: It allows a breeder to use a buck that could not otherwise be afforded. It allows the use of a buck that is deceased. It helps control venereal diseases. Despite its availability and general success in goats, artificial insemination is not easy nor is it always practical. Does must be detected in heat, moved to a breeding area, properly inseminated, and at the right time. Estrous synchronization can make AI easier. This can be done artificially through a hormone protocol, or naturally by moving a buck nearby. This reduces the number of days the breeder must be on call for insemination. Average producers can learn AI techniques and be successful. Semen handling all the way back to the source, is a key element to success. Embryo transfer (ET) Embryo transfer is to does (at least to some degree) what artificial insemination is to bucks. This technology involves the collection of embryos from donor (genetically superior) does and the transfer of those embryos to recipient does. In most cases the donor is superovulated through a hormone protocol causing the ovary to develop and release more eggs than normal. The doe is then inseminated and after an interval during which fertilization occurs and the embryonic cells begins to multiply, the embryos are collected and transferred immediately to recipients or frozen for transfer later. As with A.I. embryo transfer allows individual superior parents to have many more offspring than normal and provides goat breeders with access to special animals. Embryo transfer is relatively more difficult and more costly than artificial insemination. Success rates are highly variable because of the many factors that can have an impact. Successful ET requires a skilled technician, and the average goat producer will not have the necessary skills to implement embryo transfer. In-vitro fertilization A developing technology that may make embryo transfer more flexible and cost-effective is invitro fertilization. Fertilization takes place in a test tube. Eggs are collected from donor females, then matured and fertilized in a laboratory. The resulting embryos can either be transferred immediately to recipients or frozen. There are two advantages of this technology: An increase in the number of pregnancies possible. Its potential for decreasing generation interval. The technology also requires substantially fewer sperm which would be an advantage if sperm were in short supply. 2
Sex control Generally speaking sex control is not a big issue for meat goat breeders. That is because both sexes have significant value. For dairy goat breeders, sex control means something because their preference is to have more does born to replace the milking stock in the herd. It is now possible to determine the sex of an embryo by physically removing a few cells and examining the chromosomes. It is possible to sort sperm carrying a male sex chromosome from sperm carrying a female sex chromosome. Sorting rates have been relatively slow but will increase as new technology is refined. The chief reason for sex control is an economic one, but the ability to know the sex of offspring ahead of time will affect selection and mating decisions. Cloning Cloning has significant potential for changing the way you breed meat goats. Herds could become clonal lines, that is, populations of highly select individual breeding goats with the genetic equivalent of identical twins. There are two ways to go about cloning, as described below: Embryo splitting is a relatively simple mechanical technique for cutting an embryo in half to produce twin embryos which can then be transferred to recipient does. It is a relatively slow process because only two identical offspring are produced. It does however increase the efficiency of embryo transfer. Recently developed techniques can create up to about 8 new embryos to be produced from one original. Nuclear transplantation is an entirely different technology. Eggs are matured in vitro, and their nuclei are removed surgically. Then individual cells from a multicell embryo are inserted in each egg, producing a number of identical embryos. The embryos can then be transferred immediately or frozen for use later. In meat goats you might want maternaltype clones for dams and a different type of clone for market animals. If hybrid vigor is substantial, clones could be crossbred all F 1 s. o o Originally, the evidence suggested that goats could be cloned only from undifferentiated embryonic tissue. That would be tissue that had not yet been modified to perform the functions of specific cell types, e.g. muscle cells, liver cells, bone cells, etc. It is now known, based on Dolly evidence, that clones can be created from adult tissue. This is a key breakthrough because it means you can clone goats that are already proven in production. The cloning cycle will be much shorter. Further discussions and detail go beyond the scope of this training module, but many good technical references are available. 3
Same-sex mating This represents another reproductive technology with implications for meat goat breeding and improvement. Using nuclear transplantation, embryo transfer, and nuclear fusion it is at least theoretically possible to create individual offspring whose parents are both male, or both female. It may never become possible, but the genetic rationale for it is to increase the rate of genetic change by increasing accuracy or selection and selection intensity. Conservation genetics Without genetic variation there is no possibility of sustained genetic improvement. Genetic diversity is critical in populations are to adapt to changing economic or production environments. The current increasing emphasis on uniformity in large populations in controlled production environments tend to result in a loss of genetic diversity. Alleles that may be important in a changed production environment may become extremely rare of even lost if steps are not taken to preserve the breeds and types of animals that are their reservoir. Conservation genetics is the branch of genetics dedicated to preserving germplasm. Evidence from laboratory animals and plants suggests that breeds or lines that are not currently considered competitive in the existing production environment may still harbor potentially useful alleles that are not found elsewhere. It is expensive to maintain live populations of less profitable animals and we can not depend on wealthy or dedicated benefactors to take up the cause in every case. One way to conserve these alleles is to insert them, via crossbreeding, into the more profitable types of stock and thus holding them there. Another option is to identify and take proactive steps to store the alleles in the form of sperm, eggs, embryos and live animals as is being done under the National Animal Germplasm Program at the USDA/ARS/National Center for Genetic Resources Preservation on the campus of Colorado State University. Several goat types and breeds are part of that ongoing program. Molecular technologies While reproductive technologies focus on the sperm, egg or embryo, molecular technologies go a step deeper into the body system and operate at the level of individual genes segments of strands of deoxyribonucleic acid (DNA). Here are just a few molecular technologies that are likely to influence meat goat breeding now and in the future. DNA fingerprinting for animal identification Even on well-managed and well-organized meat goat farms there can be mistakes made in parentage. DNA fingerprints appear graphically in a form that remind us of the bar codes for pricing merchandise. The pattern of bands represents the presence of specific alleles at a number of loci. DNA fingerprinting has implication for the management of pedigree populations in that paternity can be settled once and for all. From a genetic standpoint, the major benefit of DNA fingerprinting would be an increase in accuracy of selection. Sires are often misidentified and the faulty pedigrees cause genetic predictions to be less reliable, especially for young sires with few progeny data. For meat goat breeders who must use multiple-sire pastures could identify sires 4
through fingerprinting, the amount of genetically useful information would increase dramatically. This technology can be used to decrease the time required for a population to achieve purebred status in grading-up programs or in programs of repeated backcrossing to import an allele. Why is this? Because of segregation of alleles. It causes the actual breed composition of any particular individual in a new population, to vary away from the average. At the cellular level some individuals may have inherited up to 90% of their alleles may derive from one parent rather than the other. Fingerprinting can identify those individual that have the more desirable gene composition. It has potential for predicting hybrid vigor for a particular cross. Hybrid vigor is greater when the parent breeds are more distantly related. Marker assisted selection for polygenic traits Marker assisted selection uses genetic markers which are a detectable gene or DNA fragment to identify alleles at a linked locus, i.e. alleles on the same chromosome. It is likely to be more difficult with polygenic traits however, than for simply-inherited traits. There are several problems, but one major challenge in selection for polygenic traits is that differences in performance caused by any single gene may be too small to detect. No single one will have overriding effects on the trait. Still, some geneticists believe that some quantitative traits may be controlled through quantitative trait loci (QTLs) which can be detected. An example is the Boorolla gene for fecundity in sheep; a major gene controlling a quantitative trait. Gene transfer and transgenic goats Much of the interest in gene transfer comes from its potential for creation of products to be used in human or animal disease therapies; especially through a product such as milk, which can be harvested on a daily basis. In the area of animal breeding, gene transfer could be a tool for increasing genetic variation within populations. Gene transfer is a long way from commercial application. Success rates are poor and the procedures are very expensive. Transferring genes is made complex because the structural parts of genes depend on regulatory parts to function correctly. Sometimes they depend on entirely different genes for proper regulation. Several different parts, and in some cases different genes must be transferred. Gene transfer and many other molecular techniques have been more successful in plants than in animals in recent years. That may be because many important traits in plants are simply-inherited and simply-regulated. Their biological mechanisms are less complex. Nonetheless, a recent example includes the insertion of a gene in the goat genome which results in spidersilk being produced in goat milk. Chimeraism Occasionally one may be asked about the possibility of creating a goat x sheep chimera, sometimes known as a geep or a shoat (not to be confused with a young pig). A chimera is an animal which has at least two different populations of cells, that are genetically distinct, and which originated in different fertilized eggs. The name comes from the mythological creature Chimera which was said to be made out of three different creatures: lion, goat and serpent.. Chimeras are artificially produced by mixing cells from two different organisms. About 20 years 5
ago there was the report of such a sheep x goat mix that was produced by combining embryos for each species. Pictures were taken for verification, but it was not viable, and remains something of laboratory lore. Summary In a world surrounded by biotechnology and futuristic concepts, the beauty of traditional selection is that it allows only fully functional animals, animals with working genes and gene combinations, to survive and reproduce. Information contained in this document is part of a web-based training and certification program for meat goat producers (http://www2.luresext.edu/goats/training/qa.html) that was developed with funding received by Langston University from USDA/FSIS/OPHS project #FSIS-C-10-2004 entitled "Development of a Web-based Training and Certification Program for Meat Goat Producers." Collaborating institutions/organizations include Alcorn State University, American Boer Goat Association, American Kiko Goat Association, American Meat Goat Association, Florida A&M University, Fort Valley State University, Kentucky State University, Langston University, Prairie View A&M University, Southern University, Tennessee Goat Producers Association, Tennessee State University, Tuskegee University, United States Boer Goat Association, University of Arkansas Pine Bluff, and Virginia State University. 6