Objective To investigate the effect and mechanism of polyunsaturated fatty acids(PUFAs)on the structure and function of testis in transgenic mice with Δ15 fatty acid desaturase(Δ15 Des).Methods C57BL/6 wild-type(WT)male mice and Δ15 Des transgenic male mice were chosen as control group(n=5,WT group)and experimental group(n=5,TG group).They were all fed with 6%arachidonic acid(ARA)feed for 8 weeks,and were executed under anesthesia.Testicular and epididymal tissues were separated,and the composition and fatty acid content of testicular tissues were detected by gas chromatography(GC).Sperm quality analyzer(CASA)was used to detect sperm morphology and motility[total sperm motility,linear sperm motility,mean sperm path velocity(VAP),curve sperm velocity(VCL),and linear sperm velocity(VSL)]in epididymis;hematoxylin-eosin(HE)staining was used to observe morphological characteristics of testicular tissue.The expression of free fatty acid receptor 1(FFAR1),FFAR2,FFAR3,FFAR4,and peroxisome proliferator-activated receptor α(PPARα),PPARβ,and PPARγ messenger RNA(mRNA)in testis of both groups were detected by real-time fluorescence quantitative PCR(RT-qPCR).The levels of FFAR4 and PPARγ protein in testis of both groups were detected by Western blot.Results Compared with WT group,ARA and docosatetraenoic acid(DTA)levels in testis of TG group decreased(P<0.05 or P<0.01),while linoleic acid(LA),eicosapentaenoic acid(EPA),and docosahexaenoic acid(DHA)levels increased(P<0.05 or P<0.01).There was no significant difference in sperm morphology and quantity between both groups.Linear sperm motility and VSL of TG group were higher than those of WT group(P<0.05),but there was no difference in total sperm motility,VAP,and VCL(P>0.05).Compared with WT group,there were fewer vacuoles and more mature sperm in seminiferous tubules in TG group.Compared with WT group,the expressions of FFAR4 and PPARαmRNA in the testis of TG group up-regulated(P<0.01 or P<0.05),PPARβ mRNA down-regulated(P<0.05),and FFAR4 protein level increased(P<0.05).Conclusion n-3 PUFAs can improve the structure and function of testicular tissue in Δ15 Des transgenic mice,and its mechanism might be related to the activation of downstream signal molecules by binding FFAR4.