Genetic and Environmental Architecture of Ram Fertility Traits: A Review

Background/Objectives: Ram fertility is essential for sheep production, influenced by genetic, physiological, behavioral, and environmental factors. This narrative review synthesizes findings from over 190 peer-reviewed publications to evaluate the phenotypic indicators, genetic architecture, molecular candidates, and management conditions influencing testicular development, semen quality, and reproductive performance in rams. Methods: A narrative synthesis of peer-reviewed studies was conducted, integrating findings from quantitative genetics, genome-wide association studies, transcriptomics, and controlled environmental and management experiments. Emphasis was placed on studies evaluating fertility-related traits across breeds, ages, and production systems. Results: Recent genomic and transcriptomic studies have identified potential biomarkers (e.g., IGF1, IGFALS, FOXO1) and gene networks linked to ram fertility, including semen quality, scrotal circumference, and endocrine regulation. For instance, genome-wide association studies (GWASs) have identified candidate genes such as SLC2A8 and MAPK3, which are associated with spermatogenesis and semen quality. Additionally, Y-linked SNPs such as ZFY16: g.146 C > T have been linked to testicular development. Genetic potential is heavily modulated by environmental constraints. Heat stress emerges as a disruptor of testicular thermoregulation, with recent evidence highlighting the vulnerability of spermatogenesis even in adapted breeds. Management interventions, specifically nutritional supplementation and hormonal modulation via melatonin, are discussed as effective strategies to mitigate environmental impacts. Conclusion: Improving ram fertility will require an approach that prioritizes phenotypic traits supported by candidate genes identified through transcriptomic analyses and GWASs. Integrating these genetic tools together with cost-effective nutritional and hormonal management strategies can further improve semen quality, libido, and testicular traits, thereby enhancing fertility gains while maintaining sheep breed adaptability across production systems.