Centriole evolution

Abstract

Centrioles are cylindrical structures found at the core of the mitotic spindle pole, which also act as basal bodies to nucleate the formation of cilia. Centrioles have a complex, ninefold symmetric structure, and reproduce by an intriguing duplication process. The complexity and apparent self-reproduction of centrioles raises the question of how such a structure could have evolved, making them a favorite topic for theological speculation by 'intelligent design' creationists. In fact, centrioles are capable of robust self-assembly and can tolerate dramatic perturbations while still maintaining basic functionality. Far from being irreducibly complex, centrioles appear to be based on a rather minimal underlying core structure requiring only a handful of genes to construct.

Figure 1

Presence of centrioles correlates strictly with presence of cilia throughout eukaryotic phylogeny. (+) and (−) indicate whether centrioles or cilia are present or absent, respectively, in the given phylum. All phyla that currently lack centrioles appear to have descended from ancestors that once contained centrioles.

Figure 2

Proposed evolution of a proto-centriole capable of nucleating a primitive cilium-like structure. (A) Evolution of tektins allows formation of stable microtubule doublet and triplet structures. (B) Acquisition of one or more appendage proteins allows docking of proto-centriole onto cell cortex. (C) Microtubules can extend from the end of the double structure. (D) If microtubules extend from the end of a docked doublet it would produce a primitive cilium-like structure that could be used for sensory or gliding functions.