Data from: Opposing effects of mortality factors on progeny operational sex ratio may thwart adaptive manipulation of primary sex ratio

Despite extensive research on mechanisms generating biases in sex ratios, the capacity of natural enemies to shift or further skew operational sex ratios following sex allocation and parental care remains largely unstudied in natural populations. Male cocoons of the sawfly Neodiprion abietis (Hymenoptera: Diprionidae) are consistently smaller than those of females, with very little overlap, and thus, we were able to use cocoon size to sex cocoons. We studied three consecutive cohorts of N. abietis in six forest stands to detect cocoon volume-associated biases in the attack of predators, pathogens, and parasitoids and examine how the combined effect of natural enemies shapes the realized operational sex ratio. Neodiprion abietis mortality during the cocoon stage was sex-biased, being 1.6 times greater for males than females. Greater net mortality in males occurred because male-biased mortality caused by a pteromalid parasitic wasp and a baculovirus was greater and more skewed than female-biased mortality caused by ichneumonid parasitic wasps. Variation in the susceptibility of each sex to each family of parasitoids was associated with differences in size and life histories of male and female hosts. A simulation based on the data indicated that shifts in the nature of differential mortality have different effects on the sex ratio and fitness of survivors. Because previous work has indicated that reduced host plant foliage quality induces female-biased mortality in this species, bottom-up and top-down factors acting on populations can affect operational sex ratios in similar or opposite ways. Shifts in ecological conditions therefore have the potential to alter progeny fitness and produce extreme sex ratio skews, even in the absence of unbalanced sex allocation. This would limit the capacity of females to anticipate the operational sex ratio and reliably predict the reproductive success of each gender at sex allocation.尽管学界已对性比偏差的产生机制开展了大量研究，但在自然种群中，天敌在性分配与育幼行为发生后，能够改变或进一步倾斜操作型性比（operational sex ratio）的能力，目前仍未得到充分研究。雄性冷杉新松叶蜂（Neodiprion abietis，膜翅目：松叶蜂科）的茧始终小于雌性，且两者尺寸几乎无重叠，因此我们可通过茧的大小对茧进行性别鉴定。我们在6个林分中对冷杉新松叶蜂的3个连续同生群开展研究，以检测捕食者、病原菌与寄生性天敌的攻击是否存在与茧体积相关的偏差，并探究天敌的综合作用如何塑造实际操作型性比。冷杉新松叶蜂在茧期的死亡率存在性别偏差：雄性死亡率是雌性的1.6倍。雄性个体的净死亡率更高，原因是金小蜂科（Pteromalidae）寄生蜂与杆状病毒（baculovirus）引发的雄性偏向性死亡率，高于姬蜂科（Ichneumonidae）寄生蜂引发的雌性偏向性死亡率，且偏差程度更强。雌雄个体对各寄生蜂科的易感性差异，与雌雄寄主的体型及生活史差异相关。基于本研究数据开展的模拟实验表明，差异化死亡率的性质发生改变时，会对存活个体的性比与适合度产生不同影响。鉴于此前研究表明，寄主植物叶片质量下降会引发该物种的雌性偏向性死亡率，因此作用于种群的上行（bottom-up）与下行（top-down）调控因子，可通过相似或相反的途径影响操作型性比。因此，生态条件的变化即便在性分配未失衡的情况下，仍有可能改变子代适合度，并引发极端的性比偏差。这将限制雌性个体在进行性分配时，预判操作型性比并可靠预测不同性别繁殖成功率的能力。