The single flagellum of Leishmania has a fixed polarisation of its asymmetric beat

Eukaryotic flagella undergo different beat types necessary for their function. The single flagellum on Leishmania parasites, for example, undergoes a symmetric tip-to-base beat for forward swimming and an asymmetric base-to-tip beat to rotate the cell. Asymmetric beats are most commonly associated with multi-ciliated tissues or organisms where the asymmetry has a constant polarisation. We asked whether this also holds for the single Leishmania flagellum. To do so, we used high frame rate dual colour fluorescence microscopy to visualise intracellular and intraflagellar structure in live swimming cells. This showed that the asymmetric Leishmania beat has a fixed polarisation. As in Chlamydomonas, this asymmetry arose from an asymmetric static curvature combined with a symmetric dynamic curvature. Some axoneme protein deletion mutants give flagella which retain static curvature, but lack dynamic curvature. We saw that these retain a fixed polarisation. Similarly, deletion mutants which disrupt vital asymmetric extra-axonemal and rootlet-like flagellum-associated structures also retain a fixed polarisation. This indicated that beat asymmetry does not originate from rootlet-like and extra-axonemal structures and is likely intrinsic to either the nine-fold rotational symmetry of the axoneme structure or due to differences between the outer doublet decorations.