Objective To explore the effect of ionizing radiation on the fertility of male mice and its offspring and the intergenerational and transgenerational genetic effect mechanism of ionizing radiation through sperm DNA methylation sequencing.Methods Eight-week-old(8 w)C57BL/6 male mice were irradiated with 60Co radiation source at a dose of 3 Gy(3 Gy-F0 group,n=60)and non-irradiated mice of the same age were used as controls(0 Gy-F0 group,n=60).Afetr 5-,6-,7-,8-,9-,10-,11-,and 12-week radiation,the mice began to breed with healthy females,and the first generation(F1 generation)male mice then breed with healthy female mice to obtain the offspring(F2 generation)male mice.The structure of testis was detected by hematoxylin-eosin staining;serum follicle-stimulating hormone(FSH),testosterone(T)and luteinizing hormone(LH)levels were determined by enzyme-linked immunosorbent assay.Automatic sperm analysis system was used to detect sperm concentration and activity.The DNA of F0 generation sperm was extracted and analyzed by genome-wide DNA methylation sequencing.MassARRAY methylation sites were detected in sperm DNA of F1 generation mice and verified by quantitative polymerase chain reaction(qPCR).Results Compared with the 0 Gy-F0 group,male mice in the 3 Gy-F0 group gradually regained their reproductive ability 7 weeks after ionizing radiation.There was no significant difference in the number of surviving offspring between the 3 Gy-F0 group and 0 Gy-F0 group 10-11 weeks after radiation(P＞0.05).There were no significant differences in body weight,testicular morphology,or sperm concentration of F1 generation mice between the 3 Gy-F1 group and the 0 Gy-F1 group(all P＞0.05).However,compared with the 0 Gy-F1 group,the contents of LH,FSH and T in the 3 Gy-F1 group were all decreased(all P＜0.05),the testicle volume,total sperm motility rate,forward motility rate and the fertility were considerably decreased(all P＜0.05).DNA methylation sequencing showed that more differentially methylated genes were enriched in the pathway regulating microtubule formation.MassARRAY methylation sites analysis showed that the methylation level of Mid1 was significantly increased(P＜0.05 or P＜0.01).Mid1 was verified down-regulated in Fl and F0 sperm by qPCR(P＜0.05 or P＜0.01).However,there were no significant differences in volume of testes,testicular index,sperm concentration,sperm motility,hormone levels or Mid1 expression level between 0 Gy-F2 and 3 Gy-F2 mice in F2 generation male mice(all P＞0.05).Conclusion Sperm damage in mice caused by ionizing radiation at a dose of 3 Gy can recover by itself.However,it may decrease sperm activity by regulating Mid1 methylation level of sperm in F1 mice,thus affect the fertility of F1 mice,but has no effect on the fertility of male F2 mice.