Centriole symmetry: a big tale from small organisms

Centrioles are microtubule-based cylindrical organelles with a 9-fold symmetry. They are essential for axoneme formation in cilia and flagella and for centrosome organization. In the basal hexapods Acerentomon microrhinus, we discovered unusually large centrioles composed of 14 doublet microtubules that serve as templates for cilia and flagella and organize mitotic and meiotic spindles. These observations challenge the long-standing view that centriole symmetry is highly conserved among eukaryotes. Strikingly, daughter centrioles contain a transient cartwheel that is lost after maturation. The length of radial spokes is like that found in 9-fold cartwheels, whereas the diameter of the hub varies according to the dimensions of the centriole cylinder. This suggests that the hub may dictate the master plan for centriole geometry. Finally, the finding that 14-doublet centrioles arise from 9-doublet mothers points to an alternative model for centriole assembly.