The Annual Ovarian Cycle of the Reeves' Pond Turtle Chinemys reevesii (Reptilia: Geoemydidae) Based on Seasonal Variations in the Serum Vitellogenin Level and Follicular Growth

摘要

To demonstrate the annual ovarian cycle of a multiclutched turtle Chinemys reevesii, we quantified vitellogenin (VTG, a yolk-precursor protein) in the serum collected monthly from turtles kept in an outdoor enclosure. We also sacrificed wild adult females (one or two individuals per month) captured from a river site in Kyoto, Japan, and observed oviductal eggs and follicles assigned to five size classes: C_1 to C_5, in ascending order. The seasonal variation in the serum VTG level showed a sharp peak in late spring and a broad peak during autumn, indicating that vitellogenesis accelerated rapidly in spring, decreased in summer, increased slowly but steadily in autumn, and ceased in winter. From May to July, ovulations occurred in succession preceded by the C_4-to-C_5 growth of follicles, but without substantial growth of C_1–C_3 follicles. The vernal peak of vitellogenesis would therefore contribute largely to the sequential growth of the follicles prepared for the second and third clutches of the breeding season. In August, when the successive ovulations had been completed, no remarkable growth of C_1–C_3 follicles was observed anymore, reflecting the ovarian quies-cence. Newly-formed C_1 follicles appeared in September when C_2 and C_3 follicles markedly increased in number but C_4 and C_5 follicles were still absent. In October and November, C_4 and C_5 follicles were observed again, suggesting that the follicles for the first clutch of the next breeding season reached preovulatory size before hibernation. The production and remarkable growth of follicles occurring from September to November would account for the broad peak of vitellogenesis in autumn. Thus the observed seasonal variations in the serum VTG level and follicular growth were concordant with each other.