Chilling out or heating up? Thermal plasticity of seminal fluid proteins in Drosophila melanogaster

Seminal fluid proteins (SFPs) are key mediators of post-mating interactions, shaping fertility, sperm use and female physiology with major consequences for both sexes. Because they often enhance male fitness while imposing costs on females, SFPs are central to sexual selection and sexual conflict. Their composition and allocation strategies are influenced by both environmental conditions and male state. However, how abiotic ecological factors regulate SFP expression, and how this interacts with sperm competition risk, remains poorly understood. Here, we use adult flies from a wild Drosophila melanogaster population to test how exposure to natural temperature variation (20°C, 24°C and 28°C) across two-time scales (48h and 13d), and under either no or high sperm competition risk, interact to shape SFP production and transfer to females. SFP production and transfer profiles were significantly affected by temperature, both consistently reduced after long-term (13d) exposure to low (20°C) and high (28°C) temperatures, compared to 24°C. Proteins from the sex peptide and ovulin networks, which mediate female post-copulatory responses and are key to male fitness, followed this same pattern. Our results show that natural temperature fluctuations, well within the optimal reproductive range for flies in the wild, can have a strong effect on SFP production and transfer, and thus on post-copulatory sexual selection dynamics.