1. The partitioning of resources between male and female offspring is a fundamental aspect of the reproductive biology of animals. Sex allocation can be adjusted by controlling offspring sex ratio or by sex‐specific resource allocation towards individual eggs. The latter is well‐known in birds and reptiles, but less studied in other organisms, especially haplodiploids with females that can adjust offspring sex via fertilization control.
2. Here, we investigated whether egg size can affect sex allocation in a haplodiploid thrips, Pezothrips kellyanus. First, we established that larger eggs are more likely to be fertilized and become females than smaller eggs, which remain unfertilized and become males. This size‐dependent fertilization may be regulated by anatomical constraints that allow sperm access only to larger eggs. Furthermore, across both sexes, larger eggs developed into offspring with higher fitness, suggesting that egg size is a reliable proxy for resource allocation in this species.
3. During the early phase of reproduction, mating increased investment by females towards egg size thereby facilitating fertilization success and increased female offspring survival. However, this came with a reduced lifetime fecundity, reflecting that mating manipulates reproductive strategies in favour of increased daughter production. This may be in conflict with the optimal offspring sex ratio of females which is theoretically less female‐biased than for males.
4. We detected both egg size‐mediated sex allocation and a mating effect on resource allocation under both laboratory and field conditions, suggesting that they are maintained by natural selection.
5. We provide evidence for a finely tuned egg size‐dependent fertilization mechanism that mediates sex allocation strategies and is moderated by mating. Furthermore, our findings establish a new perspective about potential effects of sexual conflicts on egg size and sex allocation.

中文翻译：

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单倍体蓟马种类的卵子大小介导的性别分配和交配调控的生殖投资

1. 雄性和雌性后代之间的资源分配是动物生殖生物学的基本方面。可以通过控制后代性别比例或通过针对单个卵的性别特定资源分配来调整性别分配。后者在鸟类和爬行动物中是众所周知的，但在其他生物中研究较少，特别是具有雌性的单倍体可通过受精控制来调节后代性别。
2. 在这里，我们调查了卵的大小是否会影响单倍体蓟马（Pezothrips kellyanus）的性别分配。首先，我们确定较大的卵比较小的卵更容易受精并成为雌性，较小的卵仍未受精而成为雄性。这种与大小有关的受精可能受到解剖学上的限制，使精子只能进入较大的卵子。此外，在两性中，较大的卵发育成具有较高适应性的后代，这表明卵大小是该物种资源分配的可靠代表。
3. 在繁殖的早期阶段，雌性交配增加了对卵大小的投资，从而促进了受精成功并增加了雌性后代的存活率。但是，这会降低一生的繁殖力，这表明交配会控制生殖策略，从而增加子代的产量。这可能与女性的最佳后代性别比相抵触，后者在理论上比男性偏见要少。
4. 我们在实验室和田间条件下都检测到了蛋大小介导的性别分配以及对资源分配的交配效应，表明它们是通过自然选择维持的。
5. 我们提供了微调的卵子大小受精机制的证据，该机制介导性别分配策略并通过交配来调节。此外，我们的发现建立了关于性冲突对卵子大小和性别分配的潜在影响的新观点。