Conserved axoneme symmetry altered by a component β-tubulin

Ninefold microtubule symmetry of the eukaryotic basal body and motile axoneme has been long established [1–3]. In Drosophila, these organelles contain distinct but similar β-tubulin isoforms [4–10]: basal bodies contain only β1-tubulin, and only β2-tubulin is used for assembly of sperm axonemes. A single α-tubulin functions throughout spermatogenesis [11,12]. Thus, differences in organelle assembly reside in β-tubulin. We tested the ability of β1 to function in axonemes and found that β1 alone could not generate axonemes. Small sequence differences between the two isoforms therefore mediate large differences in assembly capacity, even though these two related organelles have a common evolutionarily ancient architecture. In males with equal β1 and β2, β1 was co-incorporated at equimolar ratio into functional sperm axonemes. When β1 exceeded β2, however, axonemes with 10 doublets were produced, an alteration unprecedented in natural phylogeny. Addition of the tenth doublet occurred by a novel mechanism, bypassing the basal body. It has been assumed that the instructions for axoneme morphogenesis reside primarily in the basal body, which normally serves as the axonemal template. Our data reveal that β-tubulin requirements for basal bodies and axonemes are distinct, and that key information for axoneme architecture resides in the axonemal β-tubulin.