Conserved axoneme symmetry altered by a component β-tubulin

Abstract 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 β 1-tubulin, and only β 2-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 β 1 to function in axonemes and found that β 1 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 β 1 and β 2, β 1 was co-incorporated at equimolar ratio into functional sperm axonemes. When β 1 exceeded β 2, 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.