Tdrd6a recruits Ziwi-bound piRNAs and coordinates deposition of germ plasm mRNAs into primordial germ cells [small RNA]

Germ plasm, the Balbiani body and nuage are evolutionary conserved structures essential for germ cell specification and maintenance. We describe Tdrd6a as a component of these structures with two distinct molecular functions. First, Tdrd6a facilitates the accumulation of the typical antisense-bias of piRNAs, without having effects on piRNA biogenesis signatures. Second, we show that Tdrd6a is required for Balbiani body and germ plasm integrity, and associates with RNA-binding proteins and germ plasm mRNAs. On the cell-biological level, maternally contributed Tdrd6a strongly impacts germ cell formation, but is dispensable for fertility. Using single-cell RNA-sequencing we demonstrate that Tdrd6a promotes early germ cell development and regulates the stoichiometry of germ plasm mRNAs. We propose that Tdrd6a functions as a scaffold to recruit correct ratios of germ plasm transcripts and to accumulate antisense piRNA complexes in order to ensure both specification and maintenance of germ cells. Zebrafish were grown under standard conditions. Total ovary or size selected Oocytes were used to extact RNA from het of tdrd6a mutant fish.