Clock1a Contributes To The Reproductive Cycle By Directly Regulating Polo-like Kinase 1 In Zebrafish

Mammalian CLOCK is essential for circadian regulation and contributes to reproductive function.Roles of Clock in zebrafish timekeeping and reproduction,however,largely unexplored.Here we found that loss of Clockl a delays sexual maturity later and results in much fewer eggs in a cycle of egg-laying with approximately 7 days in laboratory condition compared with wild types.When we lengthened the cycle to ten days,we observed that the clockla mutants restore the amount of egg production similar to wild types.The paraffin section of H&E staining showed only reduced number of mature oocytes in the ovaries of clock1a until the 10th day after spawning.Among differentially expressed genes in clock1a mutant fish revealed by RNA-seq,we found that polo-like kinase 1(plk1)is down-regulated in clockla mutant fish.Polo-like kinase(Plk1)-mediated phosphorylation of Rec8,an essential protein in meiosis progress,affects the exit time of meiosis.We then showed that plkl is directly controlled by the circadian clock via E'-box,and Rec8 is up-regulated in the ovaries of clock1a mutant fish,presumably resulting in its extended reproductive cycle.Further,injection of BI2536,a Plkl inhibitor,into the body cavity of the wild-type females leads to accumulation of Rec8 and reduction of egg production.Together,these results revealed that the important role of Clockla in zebrafish reproduction,that is,Clockla contributes to the reproductive cycle by directly regulating polo-like kinase-1 and affecting phosphorylation and cleavage of Rec8 during meiosis in zebrafish.