The contemporary chicken was most probably developed from its main wild ancestor, the red jungle fowl (Gallus gallus), after its domestication in Southeast Asia in 3,200 BC. Over the years, chicken evolved from the wild form to the multiple contemporary layers, broilers, bantams, game and fancy breeds, as well as the indigenous village chicken available today.
At first sight the diversity within domestic chicken is extensive, which should provide an excellent base for breeding animals that are well adapted to a variety of local environmental conditions. However, the industrialization and globalization of chicken production in the 20th century adversely affected the distribution of chicken genetic resources worldwide, practically limiting the breed composition to commercial stocks of broilers and egg-type, laying hens. Consequently, many chicken breeds have already become extinct or are seriously endangered with extinction.
Currently, the world poultry market is facing recurrent outbreaks of contagious diseases e.g. Avian Influenza or Salmonella which impose serious health and economic constraints due to several interrelated risk factors. First, commercially bred birds which constitute most of the genetic resources nowadays are likely to lose the genetic resistance to various diseases through the long-term process of one-sided selection for production traits. Additionally, extensive vaccination programmes can elicit incomplete immunity. Coupled with large flock sizes on commercial farms amounting to thousands of birds, this can result in very rapid transmission of pathogens. In case of the emergence of the highly contagious diseases like Highly Pathogenic Avian Influenza (HPAI), the whole flock in a given area can be endangered with infection requiring culling of the birds which would seriously threaten the livelihood of livestock farmers, jeopardize commercial poultry production, and seriously impede regional and international trade. Outbreaks of epidemic diseases in commercial flocks also pose a serious hazard to human health, especially in many developing countries where food hygiene needs to be drastically improved.
Immuno-competence can however be enhanced by using the already existing rich sources of genetic variation in chicken, i.e. those gene variants (alleles) that are associated with resistance to disease. However, the main obstacle preventing the direct application of modern molecular techniques in practical chicken breeding programmes is the lack of knowledge on which genes determine the fitness and robustness of the individual. This task is not easy given the complexity of the immune system, the multiple animal-pathogen interactions as well as the not fully understood genomic determination of physiological processes. However, the evolutionary mechanisms that followed the advent of chicken domestication and its distribution to all corners of the world have contributed to the adaptation of the birds to different environments. Since the process of adaptation of the indigenous chicken to often harsh and extreme environmental conditions demanded positive selection towards enhanced immune resistance; these birds are now more likely carrying fixed alleles determining their immuno-competence. However, this link has not been fully elucidated, but there are a number of papers that show existence of genotype × environment impact on the fitness traits including immuno-competence and thus on natural selection. Of course we need breed history and more data to draw any conclusions.
