Eukaryotic flagella undertake different beat types as necessary for different functions; for example, the Leishmania parasite flagellum undergoes a symmetric tip-to-base beat for forward swimming and an asymmetric base-to-tip beat to rotate the cell. In multi-ciliated tissues or organisms, the asymmetric beats are coordinated, leading to movement of the cell, organism or surrounding fluid. This coordination involves a polarisation of power stroke direction. Here, we asked whether the asymmetric beat of the single Leishmania flagellum also has a fixed polarisation. We developed high frame rate dual-colour fluorescence microscopy to visualise flagellar-associated structures in live swimming cells. This showed that the asymmetric Leishmania beat is polarised, with power strokes only occurring in one direction relative to the asymmetric flagellar machinery. Polarisation of bending was retained in deletion mutants whose flagella cannot beat but have a static bend. Furthermore, deletion mutants for proteins required for asymmetric extra-axonemal and rootlet-like flagellum-associated structures also retained normal polarisation. Leishmania beat polarisation therefore likely arises from either the nine-fold rotational symmetry of the axoneme structure or is due to differences between the outer doublet decorations.

真核生物鞭毛根据不同的功能承担不同的节拍类型；例如，利什曼原虫鞭毛在向前游动时会经历一个对称的尖端到底部的节拍，以及一个不对称的底部到尖端的节拍来旋转细胞。在多纤毛组织或生物体中，不对称的节拍是协调的，导致细胞、生物体或周围液体的运动。这种协调涉及动力冲程方向的极化。在这里，我们询问单个利什曼原虫鞭毛的不对称节拍是否也具有固定的极化。我们开发了高帧率双色荧光显微镜来可视化活游泳细胞中的鞭毛相关结构。这表明不对称的利什曼原虫节拍是两极分化的，动力冲程仅发生在相对于不对称鞭毛机械的一个方向上。在鞭毛不能跳动但具有静态弯曲的缺失突变体中保留了弯曲的极化。此外，不对称轴突外和小根样鞭毛相关结构所需的蛋白质缺失突​​变体也保持正常极化。因此，利什曼原虫节拍极化可能源于轴丝结构的九倍旋转对称性，或者是由于外部双峰装饰之间的差异。