Comparison of doublet 1 and the common IDA architecture. (a) Overall view and 90° view. (b–d) Enlarged images of doublet 1 (boxed regions in a) shown at different viewing angles, with the common architecture (average doublets 2–8) used as a control on the right. Blue arrowheads, folded tail of dynein e; black arrowheads, positions where the dynein c tail should emerge (Bui et al., 2008) but is missing on doublet 1; orange arrowheads, additional density on doublet 1 behind dynein e. (e) Sections of the average of doublet 1 (d1; left) and doublets 2–8 (dc; right) of mutant ida4, showing that the lack of dynein c does not change the appearance of the bulb density on doublet 1. The green arrow shows the bulb density on doublet 1. Bar, 20 nm.