Exploring the ultrastructure of cell division coordination in Toxoplasma gondii through expansion microscopy

The ability of protozoan parasites to rapidly proliferate within their host is at the core of their mechanisms of pathogenesis. Toxoplasma gondii, resorts to flexible cell division modes, resulting in variable outputs. It can proliferate by means of endodyogeny, endopolygeny and schizogony. These modes of division share mechanistic features such as lack of chromatin condensation, nuclear fission by semi-closed mitosis and de novo daughter cell assembly. However, the underlying mechanisms of this flexibility are only partially understood. The centrosome has long been staged at the center of regulation. However, its biogenesis and homeostasis are poorly understood. Here, we have dissected the contribution of different centrosomal components in T. gondii, using ultrastructure expansion microscopy. We analyze the phenotypes displayed by conditional mutants of centrosomal proteins TgSAS6 and TgCEP250L1 during asexual cell division and delve into pre-sexual division leading up to sexual differentiation. This work uncovers unexpected features of microtubule nucleation and centriole biogenesis. Overall, our work proposes a model for the modular organization of compartmentalized functional domains which may ultimately underly cell division flexibility allowing these parasites to adapt to different niches and proliferate accordingly.