Catálogo de publicaciones - libros

The first complete genome sequence of an organism was for yeast, in 1996. Since then, the much larger task of doing a complete human sequence has been completed. Those of major domestic animals are following rapidly. It will always be impossible to foresee the full potential of such an explosion in knowledge, but aspects of gene-based technologies are already beginning to have an impact in the livestock sector. The first and most obvious area of impact concerns feed supply, which constitutes 50–75 percent of total costs in many livestock systems. Production costs for maize and soybean are being reduced by genetic modification of the crop for herbicide and insect resistance. Maize has been modified to reduce phosphorous and nitrogen excretion in swine and poultry, and also to provide a more valuable amino acid balance. Genetic modification of the animal is also possible. Most dramatically, the insertion of a growth hormone in the DNA of fish accelerates growth. However, in this and all other cases, the genetic modification (GM) of animals has produced profound physiological disturbances. At the same time, the administration of GM-produced growth hormone to dairy cows is now routine in the United States of America and several other countries. This is not permitted in Europe, where the attitude to all GM technologies has been much more cautious. Conventional selection programmes continue to deliver steady genetic improvement in all animal populations. New molecular methods offer the prospect of enhancing genetic gains, particularly for traits that are difficult or expensive to measure, or which have low heritability. Gene technologies have much to contribute to the control of disease in animals. As pressure to reduce antibiotic and drug use increases, genetically modified vaccines with proven specificity and distinguishable from natural infections are already in use. DNA typing is helping with rapid and precise diagnosis. In addition, the interaction of some pathogens (e.g. scrapie) with the genotype of the animal calls for the application of DNA technologies. Following the BSE epidemic in Europe, safety of livestock-derived foods is high on research and regulatory agendas. DNA techniques are already in use for tracking of sources of Salmonella enterica and Escherichia coli outbreaks, as well as for traceability of product in the food chain. Finally, gene-based technologies can facilitate the measurement and conservation of genetic diversity in animal populations.

Polymerase Chain Reaction (PCR) technology allows scientist to amplify, copy, identify, characterize and manipulate genes in a relatively simple way. Exploitation of the technology to devise new products and translate these to the commercial sector has been remarkable. Molecular technologies are not difficult to establish and use, and can appear to offer developing countries many opportunities. However, developing countries should look in a different way at the apparent advantages offered. Whilst molecular biological science appears to offer solutions to many problems, there are a number of drawbacks. This desire to adopt the latest technology often overrides any considerations of the use of more conventional technologies to address needs. The conventional, and often more practical, methods already provide many specific tools in the disease control area. Changing the technology can also deflect critical resources into the molecular field in terms of laboratory funding and training. This may cause redundancy of staff, limit further development in conventional techniques, and polarize scientists into the older (less glossy) and newer (molecular) camps. Animal disease diagnosis still primarily utilizes conventional techniques such as Enzyme Linked Immunosorbent Assay (ELISA). This will not change drastically in developing countries, but developments will combine such methods with more discriminatory molecular techniques, and a balanced and parallel development is needed. An understanding of the use and possible advantages of the various technologies is required by both scientists and policy-makers in developing nations. Vaccines based on molecular science could have a real impact in developing countries, but “vaccinology” needs to examine both the animal (immunology of target species) and the disease agent itself. This is a research-based science and, as such, is expensive, with no surety of success. Developing countries should exploit links with developed countries to provide the “field” genetic resource (endemic disease situation) in order to devise and test vaccines developed through molecular studies. Development of technologies cannot be divorced from an understanding of the epidemiology of the diseases found in developing countries. This is frequently not undertaken due to the many competing demands on the scarce resources available. However, increased livestock trade possibilities may provide the focus and catalyst needed to ensure that animal health science is applied appropriately and usefully for the benefit of developing countries.

Genetic improvement by conventional breeding is restricted to those genetic loci present in the parental breeding individuals. Gene addition through transgenic technology offers a route to overcome this restriction. The transgene can be introduced into the germ cells or the fertilized zygote, using viral vectors, by simple co-culture or direct micro-injection. Alternatively, the transgene can be incorporated into a somatic cell, which is then incorporated into a developing embryo. This latter approach allows gene-targeting strategies to be employed. Using pronuclear injection methods, transgenic livestock have been generated with the aim of enhancing breeding traits of agricultural importance, or for biomedical applications. Neither has been taken beyond the development phase. Before they are, in addition to issues of commercial development, basic technological issues addressing inefficiency and complexity of the methodology need to be overcome, and appropriate gene targets identified. At the moment, perhaps the most encouraging development involves the use of viral vectors that offer increased simplicity and efficiency. By combining this new technology with transgenes that evoke the powerful intracellular machinery involved in RNA interference, pioneering applications to generate animals that are less susceptible to infectious disease may be possible.

Genomic DNA of 15 randomly selected unrelated animals and from two sire families (11 animals) of the Sahiwal breed of Zebu cattle were investigated. Four oligonucleotide probes — (GTG)_5, (TCC)_5, (GT)_8 and (GT)_12 — were used on genomic DNA digested with restriction enzymes Alu I, Hinf I, Mbo I, Eco RI and Hae III in different combinations. All four probes produced multiloci fingerprints with differing levels of polymorphisms. Total bands and shared bands in the fingerprints of each individual were in the range of 2.5 to KB. Band number ranged from 9 to 17, with 0.48 average band sharing. Probes (GT)_8, (GT)_12 and (TCC)_5 produced fingerprinting patterns of medium to low polymorphism, whereas probe (GTG)_5 produced highly polymorphic patterns. Probe (GTG)5 in combination with the HaeIII enzyme was highly polymorphic with a heterozygosity level of 0.85, followed by (GT)_8, (TCC)_5 and (GT)_12 with heterozygosity levels of 0.70, 0.65 and 0.30, respectively. Probe GTG_5 or its complementary sequence CAC_5 produced highly polymorphic fingerprints, indicating that the probe can be used for analysing population structure, parentage verification and identifying loci controlling quantitative traits and fertility status.