Atypical and distinct microtubule radial symmetries in the centriole and the axoneme of Lecudina tuzetae.

Abstract

The centriole is a minute cylindrical organelle present in a wide range of eukaryotic species. Most centrioles have a signature 9-fold radial symmetry of microtubules that is imparted onto the axoneme of the cilia and flagella they template, with 9 centriolar microtubule doublets growing into 9 axonemal microtubule doublets. There are exceptions to the 9-fold symmetrical arrangement of axonemal microtubules, with lower or higher fold symmetries in some species. In the few cases where this has been examined, such alterations in axonemal symmetries are grounded in likewise alterations in centriolar symmetries. Here, we examine the question of microtubule number continuity between centriole and axoneme in flagellated gametes of the gregarine Lecudina tuzetae, which have been reported to exhibit a 6-fold radial symmetry of axonemal microtubules. We used time-lapse differential interference microscopy to identify the stage at which flagellated gametes are present. Thereafter, using electron microscopy and ultrastructure-expansion microscopy coupled to STimulated Emission Depletion (STED) super-resolution imaging, we uncover that a 6- or 5-fold radial symmetry in the axoneme is accompanied by an 8-fold radial symmetry in the centriole. We conclude that there can be plasticity in the transition between centriolar and axonemal microtubules.