Fig. 3. Central-pair-presence and -orientation in normal and mutant cell lines. (A) Cross-sections of flagella from (TbPF16)RNAi and (TbPF20)RNAi cells induced for 3 days. The central pair can be positioned at very different angles relative to the PFR axis. In several sections, one or both microtubules of the central pair can be absent. (B) Loss of some outer dynein arms in (TbPF20)RNAi- and (TbPF16)RNA mutants, and of most of them (eight) in (TbDNAI1)RNAi mutants. Yellow arrows indicate the position of the missing arm. (C) Longitudinal section of a (TbPF16)RNAi mutant where the central pair is visible in a bent region, in this case it appears parallel to the bend plane. (D) Central-pair orientation relative to the PFR proximal axis. A negative angle indicates that lines drawn from the axis of the central pair and of the PFR would cross in direction of doublet 4 of the axoneme (right side of images in B). Sections were classified as belonging to any 15° interval. Owing to the difficulty in unambiguously distinguishing the C1 from the C2 microtubule, angles were limited to 180° but could be more important. As no significant differences could be seen between sections of whole cells or de-membranated samples, data from both sets were pooled. From top to bottom: wild-type (n=82), (TbPF20)RNAi (n=65), (TbPF16)RNAi (n=116) and (TbDNAI1)RNAi (n=44). Whereas a determined orientation is found in wild-type cells, it appeared highly variable in axoneme mutants.