Transition zone assembly. (A and B) 3D reconstruction model (A) and molecular assembly hierarchies (B) for the C. elegans transition zone based on Schouteden et al. (2015) (A) and Huang et al. (2011), Williams et al. (2011), Jensen et al. (2015), Roberson et al. (2015), Schouteden et al. (2015), and Li et al. (2016) (B). CCEP-290 is an essential component of the central cylinder, whereas MKS and NPHP module components function in assembly of Y-links. MKSR-2 and NPHP-4 are upstream components in the MKS and NPHP assembly pathways, respectively. (C) Recruitment of transition zone components to the site of cilia assembly marked by HYLS-1. Panels show 1.5-fold–, twofold-, and threefold-stage embryos and L1-stage larvae coexpressing GFP:CCEP-290, MKSR-2, NPHP-4, and mCherry:HYLS-1. Initial recruitment of all three components occurs at the twofold stage, with signal reaching maximal levels at the threefold stage. (D) Tomographic slices and 3D reconstruction model of the ciliary base of twofold-stage amphid cilium. A pair of centrioles can be seen, one of which is decorated by a single array of Y-links and a discontinuous central cylinder and enveloped by the ciliary membrane. Note marked widening of the centriole toward the base. See also Video 5. Bars: (C) 10 µm; (C, insets) 1 µm; (D) 100 nm.