The Role of Membrane-Bound Extracellular Vesicles During Co-Stimulation and Conjugation in the Ciliate Tetrahymena thermophila

During sexual reproduction, the freshwater ciliate Tetrahymena thermophila sheds membrane-bound vesicles into the extracellular environment (cEMVs: ciliary extracellular micro-vesicles). We provide evidence that 100 nm vesicles shed from the cilia of starved cells promote mating between cells of complementary mating types. A proteomic analysis revealed that these EMVs are decorated with mating-type proteins expressed from the MAT locus, proteins that define a cell’s sex (one of seven). Once the mating junction is established between cells, smaller 60 nm vesicles (junction vesicles) appear within the extracellular gap that separates mating partners. Junction vesicles (jEMVs) may play a role in remodeling the mating junction through which gametic pronuclei are exchanged. Evidence is presented demonstrating that cells must be able to internalize extracellular signals via some form of endocytosis in order to trigger conjugation. Finally, an evolutionarily conserved fusogen (Hap2) implicated in pore formation also appears necessary for jEMV processing. This system offers an excellent opportunity for studies on ectosome shedding, intercellular signaling and shed vesicle uptake by macro-pinocytosis, as they relate to sexual reproduction in the ciliate Tetrahymena thermophila.