Ultrastructural dynamics of basal bodies during microgamete formation and fertilisation in Plasmodium

Abstract

Cilia and flagella are microtubule-based organelles found in a wide range of eukaryotic organisms that form cilia and flagella are assembled from basal bodies. In the malaria parasite, eight flagellated microgametes are assembled from 8 basal bodies that form de novo as a single group (not next to a parent basal body) in the cytoplasm of microgametocytes extremely rapidly, in as little 8 minutes but is not synchronised. The flagellated microgametes exit from a microgametocyte, each consisting of an axoneme and a haploid nucleus. Fertilisation occurs via a HAP2-mediated fusion with the macrogamete. Despite the essential role of microgametes in malaria transmission and being the only flagellated stage, little is known about the process of basal body formation, biogenesis at the ultrastructure level and their role in fertilisation. We used dual axis serial section electron tomography (ssET) to reveal the unusual single microtubule structure of the basal body and discovered an associated electron dense basal body granule. Using whole cell reconstructions of microgametocytes, free microgametes and macrogametes by serial block face scanning electron microscopy (SBF-SEM), we discovered a deuterosome-like structure only present during the initial formation of 8 basal bodies that could be a nucleating platform for de novo basal body formation. Finally, we reveal that entry of the microgamete into the macrogamete occurs via directed event at a single point of entry with the basal body and granule leading entry. These discoveries highlight the essential functions of basal bodies from initial microgamete assembly to male-female gamete fertilisation.