Genetic dissection of the predisposition to feather pecking of laying hens

Feather pecking is a complex trait which may be caused by endogenous factors (i.e. genetic factors or, physiological control mechanisms) or environmental factors (feeding, density, housing conditions). The objective of this study was to identify and characterise candidate genes responsible for feather pecking in laying hens. In this study, in order to facilitate the determination of a possible genetic predisposition to that complex trait, feather pecking was considered to be a redirected exploratory behaviour occurring when there is no opportunity for normal exploration, which may consequently trigger off severe feather pecking. Differences in intrinsic exploratory behaviour can be best measured in freshly hatched chicks in a well-defined and easily controlled environment. The exploratory behaviour of two chicken lines differing in the tendency to feather pecking (High versus Low Feather Pecking) was investigated. Observations of the freshly hatched chicks in a well-defined environment showed differences in the locomotory activity taking the form of exploratory episodes of the chicks from the two investigated lines. The number of exploratory episodes was significantly higher in the chicks from the High Feather Pecking line (P<0.001). Blood and brain samples were collected and used for further molecular analyses. Seven candidate genes (HOXB8, TPH1, TPH2, ACADS, CRHR1, PRKG1, PRKG2) were identified from literature. The coding regions of five candidate genes (HOXB8, TPH1, TPH2, ACADS, CRHR1) were screened for single nucleotide polymorphisms (SNPs). All found SNPs were silent. RNAs isolated from chicken brains of Low and High Feather Pecking lines were subjected to two-colour microarray experiments using the Chicken Neuroendocrine array from ARK-Genomics. A loop design was applied. A statistical analysis was carried out using a clone-by-clone mixed model approach, applied to each technical replicate. P-values for each clone were calculated as an average of P-values across technical replicates. Eight clones met the significance criteria and, on the basis of their sequences BLASTed against the chicken genome sequence, four candidate genes (HSC70, MBP, APOA1, TLR7) were selected for further detailed analysis. To increase a statistical power the joint analysis of all three technical replicates for each clone was applied. The clones and genes obtained after this alternative statistical analysis were not further analysed. Realtime quantitative RT-PCR carried out for all eleven candidate genes revealed a different expression for five of them (P<0.05): TPH2, PRKG1, HSC70, MBP, APOA1. The two main conclusions that can be drawn in this study are: · There is a significant difference in exploratory behaviour between the High Feather Pecking and Low Feather Pecking line (in line with the hypothesis of feather pecking being redirected exploratory behaviour) · There is a significant difference in gene expression of five candidate genes. A general tendency for lower expression of almost all analysed candidate genes in the High Feather Pecking line was observed during realtime quantitative RT-PCR This study was conducted as a comprehensive effort to elucidate the genetic foundations of the complex trait feather pecking. The results of the study have revealed some very interesting differences in the behaviour and gene expression in the two observed lines (High and Low Feather Pecking) which should be further analysed in the future.