Transcriptome sequencing reveals genetic mechanisms underlying the transition between the laying and brooding phases and gene expression changes associated with divergent reproductive phenotypes in chickens.

Transition from laying to incubation behavior in chicken is an interesting topic in reproductive biology. The decline of incubation behavior in chicken population has led to considerable phenotypic differences in reproductive traits between breeds. However, the exact genetic mechanism of the reproductive phase transition still largely unknown and little is known about the gene expression changes that contribute to the phenotypic differences. We performed mRNA sequencing to investigate the molecular mechanism underlying the transition from laying to brooding and to detect difference in gene regulation underlying the phenotypic diversification using two chicken breeds. The majority of gene expression changes during phase transition were steroidogenesis and hormone-releasing genes. Brooding chickens shared a conservative pattern of greatly inhibited steroidogenic enzyme genes in the pituitary gland, therefore, low levels of steroidogenic enzymes might result in reproductive defects such as ovary regression and brooding onset. The conserved network responsible for brooding behavior was maintained by steroid biosynthesis and hormonal interactions. Interestingly, three transcription factors, SREBF2, NR5A1 and PGR, act as central signal modulators of steroid biosynthesis and hormonal interactions during the transition from laying to brooding modes at the molecular level. Furthermore, Genes correlated with protein synthesis and accumulation showed expression variation between breeds, which might result in different concentrations of and sensitivities to reproduction-related hormones. This study provided a new insight in neuroendocrine system at the molecular level, and helps to understand the genetic and hormonal responses that ultimately translate into behavior in chicken.