Centrioles are not present at the base of mature C. elegans cilia. (A) Immunofluorescence micrograph of amphid cilia in L4-stage larva expressing GFP:CCEP-290 (transition zone) and stained for HYLS-1 (ciliary base) and glutamylated tubulin (axoneme). 10 cilia form a bundle that is in contact with the external environment via a channel formed by the sheath and socket glia cells. Note that cell bodies are located ∼100 µm from the ciliary base and, therefore, outside the field of view. (B) Transmission electron micrographs of L4-stage amphid cilium at the level of the proximal segment of the axoneme, transition zone, and ciliary base with explanatory schematics. Morphologically recognizable centriole/basal body structures are missing from the ciliary base. MTs, microtubules. (C) Schematic of centriole pair in C. elegans early embryo. SAS-6 is a component of the cartwheel/central tube, whereas SAS-4 is associated with the outer microtubule wall. (D) SAS-6 and SAS-4 are not present at the base of mature cilia. Panels show centrioles in two-cell and gastrula-stage embryos and ciliary base in L4-stage larvae of a strain coexpressing GFP:SAS-4/SAS-6:GFP and mCherry:HYLS-1 (mCh:HYLS-1). (E) Tomographic slices and 3D reconstruction model of the ciliary base of L1-stage amphid cilium. Axonemal microtubules continue proximally as cilium widens at the base. Note additional electron densities between doublet microtubules and cell membrane (arrowheads). Transition zone elements including Y-links (arrows) can be seen in more distal planes. See also Videos 1 and 2. Bars: (A) 5 µm; (B) 200 nm; (D, insets) 1 µm; (D) 10 µm; (E) 100 nm.