Data from: Male spiders control offspring sex ratio through greater production of female-determining sperm

Sex allocation theory predicts that when sons and daughters have different reproductive values, parents should adjust offspring sex ratio towards the sex with the higher fitness return. Haplo-diploid species directly control offspring sex ratio, but species with chromosomal sex determination (CSD) were presumed to be constrained by Mendelian segregation. There is now increasing evidence that CSD species can adjust sex ratio strategically, but the underlying mechanism is not well understood. One hypothesis states that adaptive control is more likely to evolve in the heterogametic sex through a bias in gamete production. We investigated this hypothesis in males as the heterogametic sex in two social spider species that consistently show adaptive female biased sex ratio and in one subsocial species that is characterised by equal sex ratio. We quantified the production of male (0) and female (X) determining sperm cells using flow cytometry, and show that males of social species produce significantly more X-carrying sperm than 0-sperm, on average 70%. This is consistent with the production of more daughters. Males of the subsocial species produced a significantly lower bias of 54% X-carrying sperm. We also investigated whether inter-genomic conflict between hosts and their endosymbionts may explain female bias. Next generation sequencing showed that five common genera of bacterial endosymbionts known to affect sex ratio are largely absent, ruling out that endosymbiont bacteria bias sex ratio in social spiders. Our study provides evidence for paternal control over sex allocation through biased gamete production as a mechanism by which the heterogametic sex in CSD species adaptively adjust offspring sex ratio.性分配理论（Sex allocation theory）预测，当子代雄性与雌性具有不同的繁殖价值时，亲本应调整后代性别比例，偏向于能带来更高适合度收益的性别。单倍二倍体物种（haplo-diploid species）可直接调控后代性别比例，但学界此前认为染色体性别决定（chromosomal sex determination, CSD）物种会受孟德尔分离定律的限制。现有越来越多证据表明，CSD物种可策略性调整性别比例，但其背后的调控机制尚未明确。有假说提出，异型配子性别通过配子产生偏向性，更易演化出适应性性别调控能力。我们以雄性作为异型配子性别，针对两种始终表现出适应性雌性偏向性别比例的群居蜘蛛物种，以及一种以均等性别比例为特征的亚群居蜘蛛物种，对该假说展开了验证。我们通过流式细胞术量化了决定雄性（0）与雌性（X）的精子细胞的产生量，结果发现群居物种的雄性所产生的携带X染色体的精子数量显著多于携带0染色体的精子，平均占比达70%，这与其会产生更多雌性后代的现象相符。而亚群居物种的雄性仅表现出54%的X染色体精子占比，其偏向性显著更低。我们还探究了宿主与其内共生体之间的基因组内冲突是否能够解释雌性偏向的性别比例现象。下一代测序结果显示，五种已知会影响性别比例的常见细菌内共生体属在样本中几乎不存在，由此排除了内共生细菌导致群居蜘蛛性别比例偏向的可能性。本研究证明，在CSD物种中，异型配子性别可通过配子产生偏向性实现父本对性分配的调控，这是其适应性调整后代性别比例的一种机制。