Discussion

Leishmania parasites undergo a dramatic shape change from the insect life cycle stages to adapt to their mammalian host, changing from a long thin promastigote to a short, near-spherical, amastigote. The flagellum also changes from having a long motile 9+2 axoneme to a short collapsed 9+0 (9v) axoneme, lacking the central pair, presumably linked with changes to the flagellar pocket structure. These changes are likely vital for pathogenesis. We used electron microscopy, electron tomography and fluorescence microscopy of tagged proteins to determine the changes to the ultrastructure and molecular composition of the flagellum and flagellar pocket. This revealed a 9+2 axoneme can restructure to 9+0, and that the concurrent restructuring of the flagellar pocket involves the reorganisation of the Leishmania flagellum attachment zone (FAZ), a structure in the flagellar pocket neck homologous to the trypanosome FAZ. We are now expanding this analysis to the sub-pellicular microtubules which define cell shape, using serial block face scanning electron microscopy and electron tomography of whole cells. This is revealing how modular self-organising cytoskeletal elements are used by trypanosomes to achieve the restructuring of the flagellum, the flagellar pocket and FAZ, and the sub-pellicular microtubules to allow Leishmania parasites adapt their shape to different hosts.