Malo Aurelio F, Martinez-Pastor Felipe, Garcia-Gonzalez Francisco, Garde Julián, Ballou Jonathan D, Lacy Robert C
Department of Zoology, University of Oxford, Oxford OX1 3PS, UK
Chicago Zoological Society, Brookfield, IL, USA.
Proc Biol Sci. 2017 Aug 30;284(1861). doi: 10.1098/rspb.2017.1159.
Sex ratio allocation has important fitness consequences, and theory predicts that parents should adjust offspring sex ratio in cases where the fitness returns of producing male and female offspring vary. The ability of fathers to bias offspring sex ratios has traditionally been dismissed given the expectation of an equal proportion of X- and Y-chromosome-bearing sperm (CBS) in ejaculates due to segregation of sex chromosomes at meiosis. This expectation has been recently refuted. Here we used to demonstrate that sex ratio is explained by an exclusive effect of the father, and suggest a likely mechanism by which male-driven sex-ratio bias is attained. We identified a male sperm morphological marker that is associated with the mechanism leading to sex ratio bias; differences among males in the sperm nucleus area (a proxy for the sex chromosome that the sperm contains) explain 22% variation in litter sex ratio. We further show the role played by the sperm nucleus area as a mediator in the relationship between individual genetic variation and sex-ratio bias. Fathers with high levels of genetic variation had ejaculates with a higher proportion of sperm with small nuclei area. This, in turn, led to siring a higher proportion of sons (25% increase in sons per 0.1 decrease in the inbreeding coefficient). Our results reveal a plausible mechanism underlying unexplored male-driven sex-ratio biases. We also discuss why this pattern of paternal bias can be adaptive. This research puts to rest the idea that father contribution to sex ratio variation should be disregarded in vertebrates, and will stimulate research on evolutionary constraints to sex ratios-for example, whether fathers and mothers have divergent, coinciding, or neutral sex allocation interests. Finally, these results offer a potential explanation for those intriguing cases in which there are sex ratio biases, such as in humans.
性别比例分配具有重要的适应性后果，理论预测，在生产雄性和雌性后代的适应性回报有所不同的情况下，父母应该调整后代的性别比例。传统上，由于减数分裂时性染色体的分离，人们预期射精中携带X和Y染色体的精子（CBS）比例相等，因此父亲影响后代性别比例的能力一直被忽视。这一预期最近被推翻了。在此，我们通过[具体方式未提及]证明性别比例是由父亲的排他性作用所解释的，并提出了一种可能导致雄性驱动的性别比例偏差的机制。我们确定了一种与导致性别比例偏差的机制相关的雄性精子形态学标记；雄性精子核面积（代表精子所含性染色体）的差异解释了窝仔性别比例22%的变化。我们进一步表明了精子核面积作为个体遗传变异与性别比例偏差之间关系的调节因子所起的作用。遗传变异水平高的父亲射出的精子中，核面积小的精子比例更高。这反过来又导致生出更高比例的儿子（近亲繁殖系数每降低0.1，儿子比例增加25%）。我们的结果揭示了一种潜在的机制，解释了此前未被探索的雄性驱动的性别比例偏差。我们还讨论了为什么这种父系偏差模式可能具有适应性。这项研究平息了认为在脊椎动物中应忽视父亲对性别比例变化的贡献的观点，并将激发对性别比例进化限制的研究——例如，父亲和母亲的性别分配利益是不同、一致还是中立。最后，这些结果为那些存在性别比例偏差的有趣案例（如人类）提供了一种可能的解释。
