Male-limited secondary sexual trait interacts with environment in determining female fitness

Selection for secondary sexual traits (SSTs) elaboration may increase intralocus sexual conflict over the optimal values of traits expressed from shared genomes. This conflict can reduce female fitness, and the resulting gender load can be exacerbated by environmental stress, with consequences for a population’s ability to adapt to novel environments. However, how the evolution of SSTs interacts with environment in determining female fitness is not well understood. Here, we investigated this question using replicate lines of bulb mites selected for increased or decreased prevalence of a male SST—thickened legs used as weapons. The fitness of females from these lines was measured at a temperature to which the mites were adapted (24°C), as well as at two novel temperatures: 18°C and 28°C. We found the prevalence of the SST interacted with temperature in determining female fecundity. At 28°C, females from populations with high SST prevalence were less fecund than females from populations in which the SST was rare, but the reverse was true at 18°C. Thus, a novel environment does not universally depress female fitness more in populations with a high degree of sexually selected dimorphism. We discuss possible consequences of the interaction we detected for adaptation to novel environments.

针对第二性征（secondary sexual traits, SSTs）强化的选择，可能会加剧基于共享基因组表达的性状最优值的基因座内性冲突。这类性冲突会降低雌性适合度，而由此产生的性别负荷可因环境胁迫进一步加剧，进而对种群适应新环境的能力产生影响。然而，目前学界对第二性征演化与环境如何共同作用以决定雌性适合度的机制尚不甚明晰。本研究以球螨（bulb mites）的重复选育系为实验材料，针对雄性第二性征——用作武器的粗壮足肢——的发生率进行高低选育，以此探究上述问题。我们分别在螨类适应的温度（24℃）以及两个非适配温度（18℃与28℃）下，测定了这些选育系雌性个体的适合度。研究发现，第二性征发生率与温度在决定雌性繁殖力方面存在交互作用：在28℃条件下，第二性征发生率较高的种群的雌性个体，其繁殖力低于第二性征发生率较低的种群；而在18℃条件下则恰好相反。由此可见，在具有高度性选择二态性的种群中，新环境并非总是会对雌性适合度造成更显著的抑制作用。最后，我们就本次研究发现的交互作用对种群适应新环境的潜在影响展开了讨论。