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The breeding season in quail ends when they become relatively refractory to long photoperiods. The processes underlying the development of this refractoriness are dependent upon the thyroid gland.
Thyroidectomized male Japanese quail transferred from short (8 h light: 16 h darkness; 8L: 16D) to long (20L: 4D) daily photoperiods mature their gonads and develop the androgen-dependent cloacal gland at rates only marginally slower than controls. However, when the birds are retransferred to short days thyroidectomized individuals do not show the rapid testicular regression and moult which occurs in controls under such conditions. The testes remain large for a long period, eventually decreasing in size slowly and irregularly. Once such thyroidectomized birds are transferred back to short days the administration of thyroxine has little effect upon the slow rate of gonadal regression but if the hormone is administered during the preceding period of long days normal regression does occur under short days, even though treatment has ceased. This suggests that a process dependent upon thyroid hormones takes place under long daylengths that ensures the termination of reproduction when they are exposed either to short daylengths or to decreasing daylengths after the summer solstice. This view was substantiated by further experiments in which thyroidectomy was performed after the birds had been exposed to long daylengths. When transferred to 8L: 16D normal testicular regression and moult occurred rapidly. Presumably the thyroiddependent process had proceeded to an adequate extent in these quail before the thyroid glands were destroyed. Further support came from treating intact quail chronically with thyroxine. Its administration to quail on 20L: 4D had no observable effect; the testes grew rapidly to maximal size and remained large. However, if the birds were given thyroxine under 12L: 12D the gonads first developed to maximal size and then, after about 12 weeks of treatment, regressed spontaneously whether or not thyroxine was still being administered. It seems that thyroxine had made the quail so relatively refractory that 12L: 12D was no longer sufficiently stimulatory to overcome the developing inhibition on the photoperiodic system. The results in quail are very reminiscent of the finding that the absolute photorefractoriness and spontaneous testicular regression which develops in starlings under long daylengths is thyroid dependent. Quail do not become absolutely refractory on long daylengths and regress their gonads spontaneously but they do develop a 'relative' refractoriness so that only a small decrease in photoperiod is necessary to switch off the reproductive system. We suggest that quail-type refractoriness is homologous with that in starlings and that both involve progressive changes induced by exposure to long daylengths which profoundly alter the photoperiodic response: these changes will not occur without a functional thyroid gland.
J. Endocr. (1985) 107, 211–221
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