Tdrd6a and Tdrd6b together are required for germ plasm formation

Germ cell specification is often driven by germplasm (Gp): phase-separation-based structures in the embryo, formed by maternal RNA and proteins. In the zebrafish, these oocyte-derived factors form a large structure known as the Balbiani body (Bb), which is also required for proper oocyte polarization. How the formation of these entities is regulated, especially in vertebrates, remains unclear. In this study, we show that two multi-Tudor proteins, Tdrd6a and Tdrd6b, are together required for Gp formation in zebrafish. While the Bb in the oocyte is affected in absence of both Tdrd6a, and not Tdrd6b, this does not affect oocyte functionality. In contrast, Gp is largely dispersed at the 4-cell stage, resulting in the absence of primordial germ cells during later development and sterility. Furthermore, we show that the Prion-like domain of Tdrd6b is relevant for Tdrd6b aggregation as well as for its interaction with Buc and that this modulated by Tdrd6b Tudor domains. The implication of Tdrd6a and Tdrd6b in Gp stability is an important step in our understanding of how this phase-separated structure is controlled during development.