Tubulin tails

The kaleidoscopic structures in the electron micrographs above are the bundles of microtubules (axonemes) found in moth and fly sperm tail flagella, shown in cross-section. The axonemes above show the pattern typical of cilia and flagella in insects and other arthropods: nine doublet microtubules surround a central pair of singlet microtubules, and each doublet is associated with a singlet 'accessory' microtubule. In moths (top left and right) the accessory microtubules have an unusual 16-protofilament structure, and are visibly larger than those in Drosophila melanogaster (bottom left). In a transgenic Drosophila male — in which β-tubulin from the moth Heliothis virescens was co-expressed with the Drosophila tubulins — aberrant axonemes were formed in which both larger and smaller accessory microtubules are visible (bottom right). In other words, the moth tubulin was clearly able to 'force' some of the accessory microtubules assembled in the fly male germ cells to take on the moth-specific structure, even though moth tubulin made up only a small part (about 10%) of the tubulin in the spermatids. This shows that the architecture of the microtubule cytoskeleton can be directed by a component β-tubulin.