Model for how WDR-60 contributes to efficient dynein-2–mediated retrograde IFT, and the crossing of the TZ to recycle the IFT machinery. (A) In wild-type C. elegans cilia, kinesin motors carry dynein-2 as a cargo on anterograde IFT trains across the TZ to enter the cilium compartment and reach the ciliary tip. After rearrangement, dynein-2 transports trains in the retrograde direction, crossing the TZ barrier to return them to the base of the cilium, so they can be recycled. (B) In the absence of WDR-60, less dynein-2 is recruited and loaded onto anterograde IFT trains to be incorporated into cilia. Consequently, fewer dynein-2 motors are available at the ciliary tip to power the newly rearranged retrograde trains. These underpowered dynein-2–driven trains move at slower velocities and tend to accumulate at the distal side of the TZ, unable to generate enough force to cross this barrier. (C) Disrupting the NPHP module reduces the resistance offered by the TZ to the passage of IFT trains, facilitating the exit of retrograde IFT to the cilium base and compensating for the less efficient WDR-60–deficient dynein-2 trains.