Metadata, data, and all R code for replicating the analyses in the paper: Butterworth et al. 2025: The sicker sex is plastic: Sex specific plasticity determines sex biases in pathogen transmission

Sex differences are predicted to play an important role in the spread and evolution of pathogens. However, attempts to generalize the ‘sicker’ sex are often challenged by intraspecific variability of sex-biases across the infection process. Sex specific plasticity provides a framework to resolve this by elucidating how infection is shaped at the sex, pathogen, environment interface. Using the Daphnia magna and Pasteuria ramosa system, we measure infection outcomes for males and females across three temperatures and seven pathogen densities to quantify how sex specific plasticity shapes susceptibility, pathogen loads, and ultimately transmission. We find unique forms of plasticity at each stage of infection – including equivalent, sex-specific, and divergent plasticity. Integrating these into a single estimate of transmission reveals a clear pattern – male-biased transmission at cold temperatures, and female-biased transmission at warm temperatures. Sex specific thermal plasticity thus determines the ‘sicker’ sex, with implications for pathogen spread and evolution in a warming world.

现有研究预测，性别差异在病原体的传播与演化过程中扮演关键角色。然而，试图归纳“更易染病的性别”的相关尝试，常因感染进程中性别偏向的种内变异而面临挑战。性别特异性可塑性（sex-specific plasticity）为解决这一难题提供了研究框架，通过阐释感染过程如何在宿主性别、病原体与环境三者的交互界面中被塑造。本研究以大型溞（Daphnia magna）-拉姆氏巴斯德菌（Pasteuria ramosa）研究体系为实验模型，设置三种温度梯度与七种病原体密度，分别测定雌雄个体的感染结局，以此量化性别特异性可塑性如何影响宿主易感性、病原体载量，最终影响传播效率。研究发现，感染的不同阶段存在独特的可塑性类型——包括无性别差异型、性别特异性型与分化型可塑性。将这些结果整合为统一的传播效率评估指标后，清晰的模式得以显现：低温环境下病原体传播偏向雄性，而高温环境下传播偏向雌性。因此，性别特异性热可塑性决定了“更易染病的性别”，这一发现对于全球变暖背景下的病原体传播与演化研究具有重要启示。