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Context: Several polymorphisms in the melatonin receptor 1A gene (MTNR1A) have been related to reproductive performance in ovine. Aims: To investigate the effect of the Rsa I and Mnl I polymorphisms on ram seminal quality. Methods: Eighteen Rasa Aragonesa rams were genotyped for the Rsa I (C/C, C/T, T/T) and Mnl I (G/G, G/A, A/A) allelic variants of the MTNR1A gene. Individual ejaculates were analysed once a month throughout the whole year. Sperm motility, morphology, membrane integrity, levels of reactive oxygen species (ROS), phosphatidylserine (PS) inversion, DNA fragmentation and capacitation status were assessed. The effect of the season and polymorphisms on seminal quality was evaluated by mixed ANOVA. Key results: Both polymorphisms had an effect on membrane integrity and viable spermatozoa with low levels of ROS and without PS translocation, and Rsa I also on motile and DNA-intact spermatozoa. An interaction between both polymorphisms was found, pointing to a negative effect on seminal quality of carrying the T or A allele in homozygosity. Differences were higher in the reproductive than in the non-reproductive season. Conclusions: Mutations substituting C by T and G by A at Rsa I and Mnl I polymorphic sites, respectively, in the MTNR1A gene in rams could decrease the seminal quality. Implications: Genotyping of rams based on melatonin receptor 1A could be a powerful tool in sire selection. Some variants of the melatonin receptor gene affect seasonal reproduction in sheep, but it is not clear how they affect males and their spermatozoa. This study explores the impact of two genetic variants of the melatonin receptor gene on ram seminal quality throughout the year. Results revealed that there is a detrimental effect on seminal quality, more pronounced during the reproductive season, depending on the variant carried, which highlights the potential of genotyping for optimal sire selection in breeding programs. Image by Victoria Peña-Delgado. This article belongs to the Collection Dedication to Jim Cummins. [ABSTRACT FROM AUTHOR]