Department of Bioscience, Section for Genetics, Ecology and Evolution, Aarhus University, Ny Munkegade 114, Building 1540, 8000 Aarhus C, Denmark.
Biology Department, Evolution and Optics of Nanostructures Group, Ghent University, K.L. Ledeganckstraat 35, 9000 Gent, Belgium.
Proc Biol Sci. 2018 Mar 28;285(1875). doi: 10.1098/rspb.2017.2887.
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 characterized 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.
性分配理论预测，当儿子和女儿具有不同的生殖价值时，父母应该调整后代的性别比例，使其偏向具有更高适应度回报的性别。单倍二倍体物种直接控制后代的性别比例，但具有染色体性别决定（CSD）的物种被认为受到孟德尔分离的限制。现在越来越多的证据表明，CSD 物种可以战略性地调整性别比例，但潜在的机制尚不清楚。有一种假设认为，通过配子产生的偏向，适应性控制更有可能在异配子性别中进化。我们在两种具有适应性雌性偏置性别比的社会性蜘蛛物种的雄性中调查了这一假设，在一种具有平等性别比的亚社会性物种中也进行了调查。我们使用流式细胞术量化了产生雄性（0）和雌性（X）决定精子的能力，结果表明，社会性物种的雄性产生的携带 X 染色体的精子明显多于 0 染色体的精子，平均为 70%。这与产生更多女儿的情况一致。亚社会性物种的雄性产生的携带 X 染色体的精子偏向性明显较低，为 54%。我们还研究了宿主与其内共生菌之间的基因组间冲突是否可以解释雌性偏向性。下一代测序表明，影响性别比例的五种常见细菌内共生菌属大部分不存在，排除了内共生细菌在社会性蜘蛛中偏向性别比例的可能性。我们的研究为通过偏向配子产生来控制性分配提供了证据，这是 CSD 物种通过异配子性别适应性调整后代性别比例的一种机制。
