[The evolutionary fates of the flagellum from the unicellular ancestral eukaryote]

The microtubule-based flagellum, or basal body/axoneme, is ancestral: it is a so-called "derived character" defining the extant eukaryote branch, present at the apparition of the early eukaryotic cell. It is a genuine cell compartment. Its structural conservation throughout evolution is remarkable, as well as the molecular conservation of the intraflagellar transport genes. From the start, it links cell sensation and cell locomotion, two properties which require integrated wiring at the cell level. The cell division machinery has been also tightly coupled with this organelle throughout evolution, possibly to ensure the continuity of this cellular polarized organization through division. The association of three basic cell modules, locomotion, sensation and division, all expressing a facet of cell polarity, within the same apparatus, could have given optimal survival value to primitive eukaryotic cells and could explain the evolutionary conservation and success of the basal body/axoneme. In vertebrates, primary cilia are critical sensory organelles, including during early mammalian development. Today the eukaryotic flagellum is not only an important issue for basic cell biology and developmental biology but also for human pathology. Many apparently unrelated severe developmental pathologies turned out to be all connected to the assembly or the function of the primary or beating cilia. The current efforts on human ciliopathies benefit directly from long lasting basic research on the flagellar assembly in experimental models like Chlamydomonas reinhardtii.