SFs are composed of a family of uniquely localized SFA components. (A) Simplified phylogenetic tree depicting the evolutionary relationships between SFA homologues of selected algae and protists. SFA homologues fall into three evolutionarily distinct groups. Tt: T. thermophila; Cr: C. reinhardtii; Gi: G. intestinalis; Tg: T. gondii. (B) Domain comparison of previously characterized green algae SFA protein (C. reinhardtii; Lechtreck and Silflow, 1997) and unique sequence features identified in SFA orthologous groups. (C) SIM imaging shows the unique localization of T. thermophila Group 2 SF proteins. BB, blue; SF, green; Group 2 SF proteins, white and red. Scale bar, 500 nm. (D) Left: Averaged fluorescence localization of T. thermophila Group 2 SF proteins. Number of SFs analyzed: Cro1p, 101; DisAp, 107; Kdf3p, 105; Kdd6p, 93; Bbc39p, 107; Kdf4p, 157; Bbc29p, 115; and Kdf1p, 109. SFs were gathered from ≥60 cells. Scale bar, 500 nm. Right: Schematic illustrating the relative localization of T. thermophila Group 2 SF proteins along the SF. BB, red; SF, green; Group 2A, orange; Group 2B, cyan. Green dashed lines mark the BB centroid and SF distal end. Arrowheads mark the start and end positions of the Group 2 SF proteins as defined by 30% from peak fluorescence intensity. SF start position is unmarked due to overlapping fluorescence signals from posterior SFs. Error bars indicate SD. Scale bar, 125 nm.