Breeding for disease resistance in farm animals

Abstract

Breeding for Disease Resistance in its Evolutionary ContextPresent-day species of farm livestock have inherited a complex genome from their wild progenitors.Despite the bewildering proliferation of phenotypic variation in breeds within species, genetic structure studies indicate that genetic variation is much less than this would indicate and that genetic difference between extant breeds and their wild relatives is rather small (Rogdakis et al., 1995).This conserved genome from pre-domestication evolution owes much to the co-evolution of livestock host and its parasitic pathogens.This strategy (Khibnik and Kondrashov, 1997) is a continual battle to achieve an ecological symbiotic equilibrium, optimal for the joint survival of the two species.Breeding for disease resistance, the subject of this book, has much to do with trying to lessen the impact of ecological perturbations involved in modern pharmaceutical intervention.It is relevant to note that the fossil record suggests that environmental perturbations of various magnitude, such as the major event that may have resulted in the demise of the dinosaurs 65 million years ago, had significant consequences for the evolution of species.Many species have become extinct and others have been newly created by branching speciation.Sometimes explosive multiplication of hitherto rare species occurred, as in the Cambrian explosion (Morris, 1998).At domestication, another major perturbation in a species' evolution, many animal progenitors would be expected to be near an optimum equilibrium with their pathogens under the prevailing environmental conditions (including stocking rate).In spite of many changes in the environment and in selection objectives following domestication, the wild animal can still be regarded as an informative template for modern livestock, particularly in the fitness and behaviour traits, including disease resistance.