Molecular genetics and livestock selection: Approaches, opportunities and risks

Full text: There are over 1,200 million cattle worldwide that provide a source of food, motive power and clothing. Cattle were first domesticated about 12,000 years ago with both the archaeological and molecular evidence suggesting that this occurred in the Near East and that domesticated cattle then spread to Africa and Europe. Traditionally breeding was carried out at a local level, often using a limited number of shared bulls. The selection of individuals with particular characteristics suited to local environments, needs and preferences led to the emergence of distinct breeds with characteristic phenotypes. In 1993 there were 783 cattle breeds worldwide, although the definition of a breed is often vague. With the introduction of artificial insemination (AI) in the more developed countries during 1950s particular bulls with desirable characteristics were more widely used in preference to local bulls. The use of AI, coupled with improvements in management in Europe and North America, allowed rapid progress to be made in the improvement of simple production traits. Breed improvement has been further enhanced by the development of statistical methods to maximize genetic gain achieved by selection on traits that can be readily measured. Consequently, where the economic environment supports high input agriculture, there has been a dramatic increase in milk yield and meat produced from the improved stock. The unfortunate consequence of intensive selection in these areas has been the reduction of genetic diversity, both within the selected breeds, as the superior individuals within these breeds have been used as breeding stock, and also through the replacement of traditional breeds. While the use of improved breeds in areas advantaged by good environmental conditions and a favourable economic climate has allowed the increase in production, all-be-it with the penalty of lost diversity and damage to the environment occasioned by intensive farming practices, in less developed and environmentally less favoured areas the use of these breeds presents a greater cause for concern. Local breeds are usually adapted to survive in their local environments eg with increased tolerance of extremes in temperature or in the face of particular disease or parasite challenge. Attempts at the inappropriate and/or unmanaged introduction of improved dairy breed into some areas has met with disastrous consequences. In 1993, 112 of the 783 cattle breeds worldwide were at risk of extinction. The greatest risk is the replacement of local stock that are adapted for survival in the face of disease challenge with disease sensitive stock in areas where standards and resources to provide extensive veterinary care are not available. Much work has been carried out over the past 10 or so years to produce genetic and physical maps of the bovine genome. In the first instance these maps were composed predominantly of anonymous markers, but more recently genes, and expressed sequence tags (ESTs) have been added to the genome maps of cattle. Use of genetic maps together with other molecular genetic approaches, like micro-array technology to examine gene expression, production-associated traits are under the control of several genes, which have varying levels of effect on the trait, and are generally referred to as Quantitative Trait Loci. To date considerable success has been reported in localising QTL for a wide range of traits, however two notable successes have identified the major genes involved in increased muscling and milk production. Knowledge of the loci controlling individual traits will allow the direct selection for favourable alleles at these loci. In the first instance this can be done by marker-assisted selection with markers linked to the gene involved in the trait. However, ultimately, knowledge of the allelic variation within that gene will allow more efficient selection to be carried out. There are several advantages of using markers in selection programmes, rather than relying on phenotype based selection. In using markers it will be possible to introgress favourable alleles for particular traits from one breed into another, taking advantage of specialised characteristics of different breeds, for example to maintain disease resistance while increasing production. By using information on the markers spanning the genome, as well and the genes under selection, it will also be possible to maintain the widest possible genetic diversity within breeds. Thus considered and well-managed use of molecular information will help preserve the genetic diversity of cattle populations. (author)