Table 1.

Core genes of circadian rhythm and their functions in livestock.

Genes	Pigs	Sheep	Cattle
ARNTL (BMAL)	Crude protein intake of three meals regulates the protein expression [11].	Decreasing brown adipose tissue in neonates [45].
	Promoting the secretion of progesterone and estradiol and inhibiting the apoptosis of granulosa cells [27].	Sensitized to long-term light or without melatonin [45].	Increasing the transcript levels of NR1D1 and Prostaglandin G/H synthetase 2 (PTGS2) in uterine stromal (USCs) [71]; Increasing the production of prostaglandin F2α (PGF2α) [71].
	Controlling the expression of ovarian genes in porcine granulosa cells [27].	Different expression peaks in long-day or short-day conditions [42].
	Suppression of mRNA levels of circadian genes [27].	Rapidly alternating photoperiods causes 2.5-h earlier expression peak [41].
CRY1	Reducing the expression of pro-inflammatory cytokines Interleukin-6 (IL-6) and Tumor-necrosis-factor alpha (TNF-α) [22].	High expression in night or along with high melatonin concentration [36,37,40].
	Stabilizing the circadian oscillation and keeping the body temperature constant [22].
PER2	Crude protein intake of three meals regulates the protein expression [11].	Decreasing brown adipose tissue in neonates [45];	Increasing the proliferation of granulosa cells treated with FSH [50];
	Calcium supplementation at different times inhibits mRNA expression in placenta [26].	Synchronizing with the plasma insulin and glucose concentrations [41,42].	Decreasing the progesterone production and expression of STAR mRNA in FSH-treated granulosa cells [50];
		long-term light or decreased melatonin reduces mRNA expression [37,45].	Enhancing αs–casein protein synthesis of bovine mammary epithelial cells (BMEC) and an overall increase in milk protein content [57].
		Different expression peaks in long-day or short-day conditions [42].
PER1	Crude protein intake of three meals regulates the protein expression [11].	Increasing the prolactin secretion [36].
	Calcium supplementation indicator [26].	High expression in night [37,40].
	A transmitter of light signal. The expression in the PA and DH was elevated in the animals treated with CO [28].	In SCN, PER1 peaks early in the day [38].
NR1D1		Decreasing the plasma insulin and glucose concentrations [41].	Repressing PTGS2 gene expression [71]
		In PT, melatonin inhibits mRNA expression [40].
		Rapidly alternating photoperiods causes 2.5 h earlier expression peak [41].
CLOCK	Calcium supplementation indicator [26].	Decreasing the plasma insulin and glucose concentrations [41].	Increasing estradiol production, and luteinizing hormone receptor (LHr) and P450 aromatase (P450arom) mRNAs in FSH-treated granulosa cells [50];