A domestication related mutation in the thyroid stimulating hormone receptor gene (TSHR) modulates photoperiodic response and reproduction in chickens.

The thyroid stimulating hormone receptor gene (TSHR) has been suggested to be a "domestication locus" in the chicken. A strong selective sweep over TSHR in domestic breeds together with significant effects of a mutation in the gene on several domestication related traits, indicate that the gene has been important for chicken domestication. TSHR plays a key role in the signal transduction of seasonal reproduction, which is characteristically less strict in domestic animals. We used birds from an advanced intercross line between ancestral Red Junglefowl (RJF) and domesticated White Leghorn (WL) to investigate effects of the mutation on reproductive traits as well as on TSHB, TSHR, DIO2 and DIO3 gene expression during altered day length (photoperiod). We bred chickens homozygous for either the mutation (d/d) or wild type allele (w/w), allowing assessment of the effect of genotype at this locus while also controlling for background variation in the rest of the genome. TSHR gene expression in brain was significantly lower in both d/d females and males and d/d females showed a faster onset of egg laying at sexual maturity than w/w. Furthermore, d/d males showed a reduced testicular size response to decreased day length, and lower levels of TSHB and DIO3 expression. Additionally, purebred White Leghorn females kept under natural short day length in Sweden during December had active ovaries and lower levels of TSHR and DIO3 expression compared to Red Junglefowl females kept under similar conditions. Our study indicates that the TSHR mutation affects photoperiodic response in chicken by reducing dependence of seasonal reproduction, a typical domestication feature, and may therefore have been important for chicken domestication.