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Atypical centrioles

Typical centrioles are made of 9 triplets of microtubules organized with radial symmetry. Centrioles can vary the number of microtubules and can be made of 9 doublets of microtubules (as in Drosophila melanogaster) or 9 singlets of microtubules as in C. elegans. Atypical centrioles are centrioles that do not have microtubules, such as the Proximal Centriole-Like found in D. melanogaster sperm, or that have microtubules with no radial symmetry, such as in the distal centriole of human spermatozoon. Atypical centrioles may have evolved at least eight times independently during vertebrate evolution and may evolve in the sperm after internal fertilization evolves.

It wasn't clear why centriole become atypical until recently. The atypical distal centriole forms a dynamic basal complex (DBC) that, together with other structures in the sperm neck, facilitates a cascade of internal sliding, coupling tail beating with head kinking. The atypical distal centriole's properties suggest that it evolved into a transmission system that couples the sperm tail motors to the whole sperm, thereby enhancing sperm function.