竞争环境中精子合作的动力学
The dynamics of sperm cooperation in a competitive environment
摘要：
精子合作已经在各种分类群中进化，通常被认为是对精子竞争的反应，但这种集体运动形式的好处仍然不清楚。在这里，我们使用精细尺度成像和最小的数学模型来研究啮齿动物属$Peromyscus$的精子聚集。我们证明，随着聚集体中精子细胞数量的增加，该组以更持久的线性移动，但没有增加速度;然而，这种好处在较大的聚集体中被抵消，因为该组的几何形状迫使精子彼此游泳。结果是聚集体尺寸和平均速度之间的非单调关系，理论预测和经验观察的最佳值均为6-7个精子/聚集体。为了理解性选择在驱动这些精子群动态中的作用，我们比较了两个具有不同交配系统的姐妹物种，发现在激烈竞争下进化的$P.\,maniculatus$（高度混杂）的精子比缺乏竞争的$P.\,polionotus$（严格一夫一妻制）的精子更容易形成最佳大小的聚集体。我们结合数学和实验研究协调精子运动揭示了几何形状，运动和组大小对精子速度的重要性，并建议这些物理变量如何与进化选择压力相互作用，以调节竞争环境中的合作。
英文摘要：
Sperm cooperation has evolved in a variety of taxa and is often considered a response to sperm competition, yet the benefit of this form of collective movement remains unclear. Here we use fine-scale imaging and a minimal mathematical model to study sperm aggregation in the rodent genus $Peromyscus$. We demonstrate that as the number of sperm cells in an aggregate increase, the group moves with more persistent linearity but without increasing speed; this benefit, however, is offset in larger aggregates as the geometry of the group forces sperm to swim against one another. The result is a non-monotonic relationship between aggregate size and average velocity with both a theoretically predicted and empirically observed optimum of 6-7 sperm/aggregate. To understand the role of sexual selection in driving these sperm group dynamics, we compared two sister-species with divergent mating systems and find that sperm of $P.\,maniculatus$ (highly promiscuous), which have evolved under intense competition, form optimal-sized aggregates more often than sperm of $P.\,polionotus$ (strictly monogamous), which lack competition. Our combined mathematical and experimental study of coordinated sperm movement reveals the importance of geometry, motion and group size on sperm velocity and suggests how these physical variables interact with evolutionary selective pressures to regulate cooperation in competitive environments.
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