Figure 3(A) Top view of cartwheel reconstruction from P. tetraurelia. Scale bar, 50 nm. Dark orange arrow marks the D2-rod. Purple arrow marks part of the pinhead. Dotted yellow box with arrows denotes the central hub-focused reslice shown in panel B; dashed white box with arrows denotes the spoke-focused reslice shown in panel D. (B) Reslice of central hub-containing region with spokes in P. tetraurelia. Scale bar, 50 nm. Dashed light yellow line denotes the zoomed view shown in panels C, black line with arrows indicates the ~25 nm repeat distance between D1 densities formed by merged spokes. Dark orange arrow marks the D2-rod. (C) Zoomed view displaying periodic repeats of the central hub (CH) and several cartwheel inner densities (CIDs) in P. tetraurelia. CIDs, red arrowheads and red plot profile; CH, blue arrowheads and blue plot profile. Overlay between CID and CH peaks is plotted on the right in red and blue. Mean distance between CID peaks, red 8.6 +/- .8 (n= 5). Distances between CH peaks split into two distinct populations: smaller (within a ring-pair), green 3.1 +/- 1.3 (n= 6); larger (between ring-pairs), blue 5.5 +/- .7 (n= 5); average periodicity, black 4.2 +/- 1.3 (n= 11). (D) Serial z-projections of ~4 nm thickness from one cartwheel repeat unit of P. tetraurelia. Left-most z-projections display the central hub, right-most projection shows the microtubule wall. Blue circles delineate one ring-pair. White arrows mark individual emanating spokes. White arrowheads mark fused spokes. White dashed line follows the tilt of the spokes. Black arrow indicates the final merged spoke (D1 density). Dark orange arrow marks the D2-rod. Scale bar, 50 nm. (E) Top view of cartwheel reconstruction from C. reinhardtii. Scale bar, 50 nm. Dark orange arrow marks the D2-rod. Purple arrow marks part of the pinhead. Dotted yellow box with arrows denotes the central hub-focused reslice shown in panel F; dashed white box with arrows denotes the spoke-focused reslice shown in panel H. (F) Reslice of central hub-containing region with spokes in C. reinhardtii. Scale bar, 50 nm. Dashed light yellow line denotes the zoomed view shown in panel G, black line with arrows indicates the ~25 nm repeat distance between D1 densities formed by merged spokes. Dark orange arrow marks the D2-rod. (G) Zoomed view displaying periodic repeats of the central hub (CH) and several cartwheel inner densities (CIDs) in C. reinhardtii. CIDs, red arrowheads and red plot profile; CH, blue arrowheads and blue plot profile. Overlay between CID and CH peaks is plotted on the right in red and blue. Mean distance between CID peaks, red 8.8 +/- 1.0 (n= 5). Distances between CH peaks split into two distinct populations: smaller (within a ring-pair), green 3.5 +/- 1.3 (n= 6); larger (between ring-pairs), blue 5.1 +/- .4 (n= 5); average periodicity, black 4.2 +/- 1.3 (n= 11). (H) Serial z-projections of ~4 nm thickness from one cartwheel repeat unit of C. reinhardtii. Left-most z-projections display the central hub, right-most projection shows the microtubule wall. Blue circles delineate one ring-pair. White arrows mark individual emanating spokes. White arrowheads mark fused spokes. White dashed line follows the tilt of the spokes. Black arrow indicates the final merged spoke (D1 density). Dark orange arrow marks the D2-rod. Scale bar, 50 nm. (I) Three-dimensional rendering of the cartwheel reconstruction from P. tetraurelia. Left side, cartwheel oriented along the correct proximal-distal axis; right side, inverted proximal-distal axis, showing the asymmetry of spoke inclination. Dark orange arrows indicate the D2-rod. Purple arrows indicate part of the pinhead. Dashed yellow box, inset of one spoke unit, with the major and minor tilt angles of the spokes relative to the central hub. White asterisks denote subunits of ring-pairs. (J) Model of P. tetraurelia (left side) and C. reinhardtii (right side) cartwheel structures. Dashed grey box denotes one repeat unit of the cartwheel, dashed black lines and boxes display cross sections of spokes.