IFT-dependent axoneme extension. (A) Schematic of anterograde IFT machinery in C. elegans. Assembly of the proximal segment depends on the coordinated transport of IFT trains by kinesin-II and OSM-3 together, while assembly of the distal segment depends on OSM-3 alone. (B) Recruitment of fluorescently tagged IFT components (green in merge) to the site of cilia assembly marked by the transition zone components MKS-6 and MKSR-2 (red). Panels show 1.5-fold–, twofold-, and threefold-stage embryos and L1- and L3-stage larvae coexpressing CHE-11:mCherry, KAP-1:GFP, OSM-3:mCherry, or OSM-6:GFP and GFP:MKSR2 or MKS-6:mCherry as indicated. Initial recruitment of KAP-1 and OSM-6 occurs at the twofold stage of CHE-11 and OSM-3 at the threefold stage. IFT trafficking is only observed at L1. (C) Tomographic slice through base of the amphid channel in L1 larva, with magnified view and 3D reconstruction model of a single cilium at the level of the transition zone and proximal axoneme. Note cilium appears to terminate close to the transition zone. See also Video 6. Bars: (B) 10 µm; (B, insets) 1 µm; (C) 200 nm; (C, insets) 100 nm.