Involvement of endogenous circadian rhythm in photoperiodic ovarian response of subtropical tree sparrow, Passer montanus

Abstract

Investigations on the mechanism of photoperiodic time measurement in regulation of ovarian growth and function have been done in the subtropical population of tree sparrow (Passer montanus). Photosensitive female tree sparrows were subjected to various intermittent light dark cycles of different durations such as: 2L/ 2D, 3L/ 3D, 4L/ 4D, 6L/ 6D, 8L/ 8D and 12L/ 12D along with two control groups: one under short days (9L/ 15D) and the other under long days (14L/ 10D) for 30 days. Birds, under short days, did not show follicular enlargement while those experiencing long days exhibited ovarian growth confirming their photosensitive at the beginning of the experiment. Further, the birds under all the intermittent light regimes showed gonadal response except the birds under 8L/ 8D. The mean follicular diameter was significantly greater in the birds under 2L/ 2D as compared to the birds under 12L/12D. On the other hand, no significant variation in follicular diameter was observed among the birds maintained under 2L/ 2D, 3L/ 3D, 4L/ 4D and 6L/ 6D light dark cycles. Further, no significant variation was observed between the groups of birds exposed under 2L/ 2D and14L/10D (gonadostimulatory control group). Histomorphometric analysis of the ovary revealed a significant increase in the thickness of the follicular wall and follicular differentiation in the birds under gonadostimulatory light dark cycles. No significant different in body weight was observed in the birds under any of the light regimes. The above results are in agreement with the avian external coincidence model of photoperiodic time measurement and indicate that an endogenous circadian rhythm is involved during the initiation of the gonadal growth in the female tree sparrow. They further suggest that multiple flashes of light are more effective in inducing follicular growth than a broad pulse of light of same duration.