Table. 1. Sequences of the internal variable region in β-tubulins tested for axoneme assembly in Drosophila melanogaster spermatogenesis.

IVRs shaded: amino acids 55–57 in Drosophila β2, β1, and derivatives; 55–63 in β3 and β3β2C. The IVR is flanked by conserved sequences, as shown. B2t⁶ makes axonemes, but they are nonfunctional [26, 27]. ODAs are substantially deficient (Table 2, Fig. 1, online supplemental Table S1). Neither Drosophila β3 nor β3β2C (a chimera with β3 sequences joined to the β2 C-terminus [20]; Table S1) can alone support axoneme assembly, and rarely make doublets. In the few examples, ODAs are absent [20, 38]. Both of these proteins inhibit ODA addition when co-expressed with wild type β2 (Table 3, Fig. 2, Table S1). All other β-tubulins tested for axoneme assembly in Drosophila spermatogenesis consistently have ODAs, even when axonemes are otherwise defective (examples shown in online Fig. S1).