Extensive flagellar remodeling during the complex life cycle of , an early-branching trypanosomatid.

is a monoxenous kinetoplastid flagellate that constitutes the most basal branch of the highly diverse parasitic trypanosomatids, which include human pathogens and This makes uniquely informative for the evolution of obligatory parasitism from free-living lifestyle and the evolution of human parasitism in some trypanosomatid lineages. It has typical promastigote morphology but also forms surface-attached haptomonads and amastigotes. Haptomonads form by attachment to a surface via a large bulge at the base of the flagellum, which is then remodeled into a thin attachment pad associated with flagellum shortening. Promastigotes and haptomonads multiply by binary division, and the progeny of a haptomonad can either remain attached or grow a flagellum and resume swimming. Whole genome sequencing and transcriptome profiling, in combination with analysis of the cell ultrastructure, reveal how the cell surface and metabolism are adapted to parasitism and how characteristic cytoskeletal features are conserved. Our data demonstrate that surface attachment by the flagellum and the flagellar pocket, a -like flagellum attachment zone, and a -like cytostome are ancestral features, while evolution of extant trypanosomatids, including the human parasites, is associated with genome streamlining and diversification of membrane proteins.