Unicellular organisms achieve motility through the beating of multiple cilia or a flagellum. In both cases, the mode of beating dictates the movement, and a switch in the type of beat can change movement from forward swimming to rotation. In the coordinated asymmetrical beats of multi-ciliated organisms, the direction of the power stroke is polarised, which prompts the question whether the single flagellum of Leishmania also has a fixed polarisation of its asymmetric beat. In their study, Richard Wheeler and colleagues (Wang et al., 2020) approached this question by developing high frame-rate, dual-colour fluorescence microscopy to visualise flagellar-associated structures in swimming cells. Their analysis revealed that the Leishmania beat has a fixed polarisation; this means that the microtubule doublets 4 and 5 of the axoneme, and the asymmetrically positioned extra-axonemal structure the paraflagellar rod (PFR) always lie on the inside of the tightly curved power stroke. Somewhat surprisingly, loss of PFR or the Leishmania-specific asymmetrical flagellum attachment zone (FAZ) did not affect the polarisation of the asymmetric flagellar beats. Based on these findings, the authors suggest that the fixed polarisation could be attributed to differences between the outer doublets of the axoneme, and, as Leishmania is a very early diverging eukaryote, might be universal among eukaryotes.
