Shared and Distinct Mechanisms of Compartmentalized and Cytosolic Ciliogenesis

Most motile and all non-motile (also known as primary) eukaryotic cilia possess microtubule-based axonemes that are assembled at the cell surface to form hair-like or more elaborate compartments endowed with motility and/or signaling functions. Such compartmentalized ciliogenesis depends on the core intraflagellar transport (IFT) machinery and the associated Bardet-Biedl syndrome complex (BBSome) for dynamic delivery of ciliary components. The transition zone (TZ), an ultrastructurally complex barrier or 'gate' at the base of cilia, also contributes to the formation of compartmentalized cilia. Yet, some ciliated protists do not have IFT components and, like some metazoan spermatozoa, use IFT-independent mechanisms to build axonemes exposed to the cytosol. Moreover, various ciliated protists lack TZ components, whereas Drosophila sperm surprisingly requires the activity of dynamically localized TZ proteins for cytosolic ciliogenesis. Here, we discuss the various ways eukaryotes use IFT and/or TZ proteins to generate the wide assortment of compartmentalized and cytosolic cilia observed in nature. Consideration of the different ciliogenesis pathways allows us to propose how three types of cytosol-exposed cilia (primary, secondary and tertiary), including cilia found in the human sperm proximal segment, are likely generated by evolutionary derivations of compartmentalized ciliogenesis.