Oki Marine Biological Station, Education and Research Center for Biological Resources, Shimane University, 194 Kamo, Okinoshima-cho, Oki, Shimane, 685-0024, Japan.
Atmosphere and Ocean Research Institute, University of Tokyo, Kashiwa, Japan.
Sci Rep. 2017 Oct 11;7(1):12938. doi: 10.1038/s41598-017-13406-z.
Animal spermatozoa navigate by sensing ambient chemicals to reach the site of fertilization. Generally, such chemicals derive from the female reproductive organs or cells. Exceptionally, squid spermatozoa mutually release and perceive carbon dioxide to form clusters after ejaculation. We previously identified the pH-taxis by which each spermatozoon can execute a sharp turn, but how flagellar dynamics enable this movement remains unknown. Here, we show that initiation of the turn motion requires a swim down a steep proton gradient (a theoretical estimation of ≥0.025 pH/s), crossing a threshold pH value of ~5.5. Time-resolved kinematic analysis revealed that the turn sequence results from the rhythmic exercise of two flagellar motions: a stereotypical flagellar 'bent-cane' shape followed by asymmetric wave propagation, which enables a sharp turn in the realm of low Reynolds numbers. This turning episode is terminated by an 'overshoot' trajectory that differs from either straight-line motility or turning. As with bidirectional pH-taxes in some bacteria, squid spermatozoa also showed repulsion from strong acid conditions with similar flagellar kinematics as in positive pH-taxis. These findings indicate that squid spermatozoa might have a unique reorientation mechanism, which could be dissimilar to that of classical egg-guided sperm chemotaxis in other marine invertebrates.
动物精子通过感知环境中的化学物质来到达受精部位。通常情况下，这些化学物质来自雌性生殖器官或细胞。但也有例外，鱿鱼精子在射出后会相互释放并感知二氧化碳，从而形成聚集体。我们之前已经确定了精子可以进行急转弯的 pH 趋性，但鞭毛动力学如何使这种运动成为可能尚不清楚。在这里，我们表明，转弯运动的开始需要沿着陡峭的质子梯度（理论估计≥0.025 pH/s）下降，跨越约 5.5 的阈值 pH 值。时变运动学分析表明，转弯序列是由两种鞭毛运动的有节奏运动产生的：一种是典型的鞭毛“弯曲手杖”形状，随后是不对称波传播，这使得在低雷诺数范围内能够进行急转弯。这个转弯过程会被一个“过冲”轨迹所终止，该轨迹与直线运动或转弯不同。与某些细菌中的双向 pH 趋性一样，鱿鱼精子也会因类似的鞭毛运动而排斥强酸环境，这与正向 pH 趋性相似。这些发现表明，鱿鱼精子可能具有独特的重新定向机制，这可能与其他海洋无脊椎动物中经典的卵引导的精子化学感应有所不同。
