The large variability in melatonin blood levels in ewes is under strong genetic influence.

The present study was conducted to assess the degree of genetic determination of the variability in the mean nocturnal plasma concentration of melatonin in sheep. Three hundred twelve ewes born from 18 males and with known genealogy were sampled at the summer and the winter solstices. The nocturnal plasma melatonin concentration was defined as the mean of four plasma samples taken at hourly intervals in the middle of the night (2200-0200). Identity of the father ( P < 0.001) and the solstice ( P < 0.05) were significant. Melatonin concentrations varied considerably among individuals [338.4 ± 197.5 (SD) pg/ml; range 26.6-981.3 pg/ml] and between rams regarding the melatonin concentrations of their daughters (range from 202.9 to 456.3 pg/ml). Inheritance was analyzed by a statistical model that allows discrimination of genetic effects from nongenetic effects and that estimates repeatability and heritability coefficients. Both the repeatability coefficient between solstices (0.60) and heritability coefficient [0.45 ± 0.07 (SE)] were high. These results demonstrate that the variability in plasma melatonin concentration in ewes is under strong genetic control.