Characterisation of centriole biogenesis during multiciliation in planarians

Background Information Dense multicilia in protozoa and metazoa generate a strong force important for locomotion and extracellular fluid flow. During ciliogenesis, multiciliated cells produce hundreds of centrioles to serve as basal bodies through various pathways including deuterosome‐dependent (DD), hyper‐activated mother centriole‐dependent (MCD) and basal bodydependent (BBD) pathways. The centrosome‐free planarian Schmidtea mediterranea is widely used for regeneration studies because its neoblasts are capable of regenerating any body part after injury. However, it is currently unclear how the flatworms generate massive centrioles for multiciliated cells in the pharynx and body epidermis when their cells are initially centriole‐free. Results In this study, we investigate the progress of centriole amplification during the pharynx regeneration. We observe that the planarian pharyngeal epithelial cells generate their centrioles asynchronously through a de novo pathway. Most of the de novo centrioles are formed individually, whereas the remaining ones are assembled in pairs, possibly by sharing a cartwheel, or in small clusters lacking a nucleation center. Further RNAi experiments show that the known key factors of centriole duplication, including Cep152, Plk4 and Sas6, are crucial for the centriole amplification. Conclusions and Significance Our study demonstrates the distinct process of massive centriole biogenesis in S. mediterranea and helps to understand the diversity of centriole biogenesis during evolution. Research article: In this study, we characterise the progress of centriole biogenesis in regenerating pharynx of the centrosome‐free Schmidtea mediterranea. To differentiate into multiciliated epithelia, centriole‐free neoblasts produce massive centrioles mainly de novo. Most of the de novo centrioles are formed individually, whereas the remaining are assembled in pairs possibly by sharing a cartwheel, or in small clusters lacking a nucleation centre. Both centriole biogenesis and ciliogenesis are asynchronous during the differentiation.