In brief: Dietary phytoestrogens disrupt a specific stage of ram spermatogenesis, causing subtle decreases in sperm quality by affecting the expression of pathways involved in the structural integrity of the spermatozoa. This paper demonstrates for the first time that ram reproduction is compromised by oestrogenic pasture, whilst also providing a longitudinal model for the impact of phytoestrogens on male fertility.
Abstract: Compounds with oestrogen-like actions are now common in both the Western diet. The long-term impacts and underlying mechanisms by which oestrogenic compounds alter male reproduction, however, are unclear. To investigate this, we used a longitudinal sheep model examining the impact of oestrogenic pasture consumption on semen quality and production, testicular size, sexual behaviour and the seminal plasma proteome of Merino rams (n = 20), over a full spermatogenic cycle and in the subsequent breeding season. Throughout the study period, sexual behaviour, sperm production and motility were similar between the exposed and non-exposed rams (P > 0.05). However, between 5 and 8 weeks of exposure to dietary phytoestrogens, rams produced a higher percentage of spermatozoa with a specific malformation of the sperm midpiece and reduced DNA integrity, compared to non-exposed rams (P < 0.001). Investigation into the seminal plasma proteome revealed 93 differentially expressed proteins between phytoestrogen-exposed and control rams (P < 0.05). Exposure to phytoestrogens increased the expression of proteins involved in cellular structure development, actin cytoskeleton reorganisation, regulation of cell function and decreased expression in those related to catabolic processes. The greatest fold changes were in proteins involved in the assembly of the sperm flagella, removal of cytoplasm, spermatid development and maintenance of DNA integrity. After returning to non-oestrogenic pasture, no differences in any measure were observed between treatment groups during the subsequent breeding season. We conclude that dietary phytoestrogens can transiently disrupt specific stages of ram spermatogenesis, causing subtle decreases in sperm quality by affecting the expression of pathways involved in the structural integrity of the spermatozoa.