Positioning of IFT trains in the trypanosome flagellum and models for IFT trafficking. (A) Cross section of the trypanosome flagellum observed by conventional TEM. The arrowhead indicates an IFT particle positioned at the level of doublet 4. The cartoon shows the main structural components of the axoneme, with the numbering of microtubule doublets (Branche et al., 2006) superposed on the original image. Doublet numbering follows the conventional rules: a line perpendicular to the middle axis of the central pair microtubules is drawn and makes contact with the A tubule of only one doublet that is defined as number 1. The numbering follows the clockwise orientation defined by the dynein arms. Dynein arms are shown in orange, radial spoke in violet, central pair projections in yellow, and the PFR in blue. (B) The restricted presence of IFT on doublets 3–4 and 7–8 can be explained by two models: either some doublets are used specifically for anterograde transport and the other ones support retrograde IFT (Model 1) or bidirectional trafficking takes place on all doublets (Model 2). Only one doublet was highlighted for the sake of clarity.