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，但针对自然种群中，天敌在个体经历性别分配(sex allocation)与双亲抚育(parental care)后，改变或进一步偏倚操作性别比(operational sex ratio)的能力，目前仍鲜有研究。冷杉新松叶蜂(Neodiprion abietis，膜翅目: 松叶蜂科Diprionidae)的雄虫茧始终小于雌虫茧，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个连续世代进行了研究，以探明捕食者、病原物以及寄生蜂(parasitoids)对茧的攻击是否存在与茧体积相关的偏差，并探究天敌的联合作用如何塑造实际操作性别比。冷杉新松叶蜂在茧期的死亡率存在性别偏差，雄虫死亡率是雌虫的1.6倍。雄虫的净死亡率更高，原因是金小蜂科寄生蜂(pteromalid parasitic wasp)与杆状病毒(baculovirus)导致的雄虫偏向性死亡率，高于姬蜂科寄生蜂(ichneumonid parasitic wasps)引发的雌虫偏向性死亡率，且前者的偏差程度更强。雌雄两性对各类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分析显示，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(fitness)产生不同影响。由于此前研究表明，寄主植物叶片质量下降会引发该物种的雌虫偏向性死亡率，因此作用于种群的上行效应(bottom-up factors)与下行效应(top-down factors)，可通过相似或相反的方式影响操作性别比。因此，生态条件的改变有可能改变子代适合度，并产生极端的性比偏差，即便不存在失衡的性别分配。这将限制雌性个体在进行性别分配时，预判操作性别比以及可靠预测不同性别繁殖成功率的能力。