Circadian rhythmicity in photoperiodic regulation of reproductive responses in the Yellow-breasted bunting

Abstract

The role of circadian mechanism in the photoperiodic induction of gonadal growth and function was investigated using photosensitive male Yellow-breasted bunting (Emberiza aureola) by employing ahemeral/resonance lighting cycles, Nanda–Hamner protocols and skeleton photoperiods. In the first experiment, groups of buntings were kept for four weeks under six resonance cycles consisting of a basic photophase of 6 h L combined with dark phases D of varying duration so that the period of LD cycles lengthened systematically by 12 h increments and resulting to LD schedules ranging from LD 6:6 to LD 6:66 for four weeks. Significant testicular growth occurred only in the groups exposed to LD 6:6, LD 6:30 and LD 6:54 with a corresponding increase in serum testosterone levels (TLs). On the other hand, groups experiencing LD 6:18, LD 6:42 and LD 6:66 maintained quiescent condition of their gonads and low serum TLs. In the second experiment, photosensitive buntings were exposed to seven different night-interruption light dark cycles for four weeks that consisted of a basic photophase of 6 h and 1 h photointerruption of the dark phase in the 24 h cycle at different points along with a control group under 7L/17D in order to investigate the differential sensitivity of the photoinducible phase (фi) to light. The night interruption by 1 h light pulse between 12 h and 18 h after the onset of the basic photoperiod led to testicular growth although the rate and magnitude of photoperiodic induction were different for different hours of interruption. Light pulse of 1 h as a continuation of the basic photophase in control group (7L/17D) or as an interruption of dark phase at 20 h and 22 h after the onset of the basic photophase was ineffective in inducing gonadal response. The above results clearly indicate that the buntings possess a time-measuring device that involves an endogenous circadian rhythm in photosensitivity having photoinducible phase which when illuminated leads to a photoperiodic response. The photoinducible phase appears to lie between 13 h and 19 h following the onset of basic photophase in buntings. These results clearly suggest a strong circadian component in photoperiodic time measurement during reproductive responses in buntings and are consistent with the Bunning hypothesis according to which coincidence of environmental photoperiod with an endogenous circadian rhythm in photosensitivity forms the physiological basis of photoperiodism regulating reproductive cycles.