Evolution and persistence of the cilium.

The origin of cilia, a fundamental eukaryotic organelle, not present in prokaryotes, poses many problems, including the origins of motility and sensory function, the origins of nine-fold symmetry, of basal bodies, and of transport and selective mechanisms involved in ciliogenesis. We propose the basis of ciliary origin to be a self-assembly RNA enveloped virus that contains unique tubulin and tektin precursors. The virus becomes the centriole and basal body, which would account for the self-assembly and self-replicative properties of these organelles, in contrast to previous proposals of spirochaete origin or endogenous differentiation, which do not readily account for the centriole or its properties. The viral envelope evolves into a sensory bud. The host cell supplies the transport machinery and molecular motors to construct the axoneme. Polymerization of cytoplasmic microtubules in the 9+0 axoneme completes the 9+2 pattern.