Conserved axoneme symmetry altered by a component beta-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 beta-tubulin isoforms [4-10]: basal bodies contain only beta1-tubulin, and only beta2-tubulin is used for assembly of sperm axonemes. A single alpha-tubulin functions throughout spermatogenesis [11,12]. Thus, differences in organelle assembly reside in beta-tubulin. We tested the ability of beta1 to function in axonemes and found that beta1 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 beta1 and beta2, beta1 was co-incorporated at equimolar ratio into functional sperm axonemes. When beta1 exceeded beta2, 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 beta-tubulin requirements for basal bodies and axonemes are distinct, and that key information for axoneme architecture resides in the axonemal beta-tubulin.