Conclusions and perspectives
The HPA axis is a neuroendocrine system of critical importance in the regulation of energy metabolism and stress responses. Its level of activity influences production traits negatively and several robustness traits positively. The recent history of genetic selection for production traits such as growth rate, feed efficiency and leanness (all negatively influenced by GR hormones) has probably contributed to the reduction of HPA axis activity and consequently to a decrease of the robustness of modern, high-productivity animals. In the context of sustainable breeding, the genetic selection objective aims at a better balance between production and robustness traits. HPA axis activity should then be increased to improve robustness, but at the same time, the high production level of modern genotypes should not be compromised. It is very difficult to envisage phenotypic selection on HPA axis activity in farm animal populations, but marker-assisted selection may present a realistic alternative. A high genetic variability is present in the various components of the HPA axis. As with every other trait studied in livestock species, large numbers of genes are probably involved, and (particularly for these traits) they must be expected to interact intensively. Current developments in bioinformatics make it increasingly feasible to quantify the effects of very large numbers of polymorphisms (SNPs) on the phenotypes of interest (genomic prediction) and on their underlying mechanisms (‘quantomics’, www.quantomics.eu). The limiting factor in such approaches is an accurate estimation of the effect of each marker: this requires observations of the trait on large numbers of animals, updated regularly. But once this exercise is completed, the actual selection programme does not require phenotypic records anymore.
Such a selection strategy based on the genetic variation of neuroendocrine stress systems would be complementary to quantitative approaches such as the integration of robustness phenotypes or environmental sensitivity in the selection programmes.