| Summary: | 碩士 === 國立中興大學 === 動物科學系所 === 102 === Evolution of modern chickens can be dated back to 8,000 years ago. House chicken was thought to be originated from Red Jungle fowls in Southeast Asia. The divergence of chicken breeds for egg laying and meat production was dated to about 100 years ago. During the 20th century, selection through population genetics produced the modern commercial strains of meat- and egg-type chickens. One of the most striking differences found between meat- and egg-type chickens was thyroid stimulating hormone receptor (TSHR) signaling, which has a pivotal role in metabolic regulation and photoperiod control of reproduction in birds. Thyroid hormone is the main hormone to regulate energy metabolism, molting, and mating in response to light cue. The aim of this study was to investigate the expression of circadian genes in the hypothalamus of meat- and egg-type hens in response to exogenous thyroid hormone. Hens at 32 weeks of age were injected with saline (1 mL/kg) or 3,5,3 ''-triiodothyronine (T3, 5 μg/kg) through wing vein at 17:00 and sacrificed at 16:00, 22:00, 04:00, 10:00, and 16:00. Results showed that plasma TSH levels were higher in meat-type hens than that of egg-type hens before T3 treatment (P < 0.05). In contrast to meat-type hens, T3 treatment in egg layers resulted in a more dramatic increase of plasma TSH level peaked at 05:00. Expression of energy-related genes, liver kinase B1 (LKB1), uncoupling protein (UCP), and peroxisome proliferator-activated receptor-γ coactivator (PGC-1α) in T3-treated hens exhibited a rhythmic pattern in both types of hens and reached the peak at 22:00 (P < 0.05), suggesting that T3 stimulation mimics the light cue, which in turn drives the hypothalamus to reply an early ignition of gene expressions. Expression of GnRH and Dio2 (deiodinase 2) in T3-treated meat-type hens exhibited an early peak at 22:00, but the control hens showed a dramatic increase of GnRH transcription at 16:00 in the next day (P < 0.05). In egg-type hens, GnRH and Dio2 (deiodinase 2) expression oscillated at a relatively stable pattern in response to T3 treatment. T3 treatment induced a more fluctuating pattern of CLOCK and Baml1 expression during the day in egg-type hens, whereas meat-type hens showed a relative stable pulsatility in response to T3 induction. In meat-type hens, T3 treatment induced a more rhythmic RORα expression to reach the peak at 22:00 (P < 0.05), whereas layers exhibited persistently suppressed RORα expression. Cry1 and Cry2 expression climbed up at 04:00 and peaked at 16:00 in T3-treated meat-type hens, but egg-type hens showed a stable pattern. Per2 pulsatility remained stable by T3 injection in both types of hens. T3 treatment induced a more rhythmic TTF1 (thyroid transcription factor 1) expression to the peak at 22:00 (P < 0.05) in the layers, whereas meat type hens displayed suppressed TTF1 expression down to the wave trough at 10:00 and up to the plateau at 16:00 (P < 0.05). In contrast to layers, therefore, a more dramatic oscillation of GnRH expression in response to T3 treatment in meat-type hens is attributed to the late diurnal fluctuation of Cry1 and TTF1. Based on the results, thyroid hormone regulation on TSH secretion was more dramatic in egg-type hens, but the following effect of GnRH oscillation was not observed, suggesting that egg-type hens may operate through early circadian gene rhythmicity including CLOCK and Baml1 at night to withstand the effect of variable factors on the oscillation of GnRH expression, while meat-type hens exhibited a late diurnal fluctuation of circadian Cry1 and TTF1 expression in response to T3 induction, and thereby indicating an more oscillating GnRH secretion in response to external cues. In conclusion, the circadian gene expression patterns of meat-type hens apparently are more fluctuating in response to stimuli, and resume to the basal pattern in a sluggish way. Thus, the egg-type hens can manage a homeostatic circadian rhythmicity through compensatory mechanisms to withstand the variable cues, while the meat-type hens are less sensitive in maintaining circadian rhythmicity.
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