In toto imaging of germ plasm dynamics reveals an essential role for early distribution of germ granules in germline development

A fundamental question in developmental biology is how the fertilized egg gives rise to all the different cell types of an organism. The traditional view is that the different cell types are specified either by intrinsic factors such as cell fate determinants or via intercellular signaling. In some animals a cytoplasmic determinant-like substance called germplasm specifies the germline. In zebrafish eggs, germplasm is dispersed in form of mRNP complexes called germ granules, which are enriched at the animal pole. After fertilization the distribution of germ granules changes dramatically. The germ granules accumulate in the corners of the first two cleavage furrows of the embryo, to form four large masses that are essential for germline development. Although germ granule movement has been linked to the network dynamics of the microtubular and actin cytoskeleton, a clear mechanistic understanding of the process is currently lacking. Fundamental questions about germplasm dynamics, including "What is the main driving force?" have not been answered yet. To address this gap, we performed careful quantitative analysis of germ granule dynamics relative to dynamic cytoskeletal reorganization in early zebrafish embryos by live-imaging. We identified stereotypic signatures of germ granule dynamics across different regions of the early embryo. Interestingly, we find that the timing of large-scale germ granule movements contrasts prevailing models for the mechanism of germ granule aggregation during cleavage divisions, and rather points to cytokinetic apparatus itself. Using zebrafish mutants affecting the RNA-binding protein Ybx1 (Y-box binding-protein 1), we show that the timing and dynamics of germ granule accumulation in the blastodisc is a crucial factor for appropriate later aggregation into cleavage furrows and eventual distribution to PGCs. Germplasm accumulation in the cleavage furrows is reduced and ectopic aggregates form at the blastoderm margin of ybx1 mutant embryos. Our work establishes Ybx1 as a novel factor with crucial functions in germplasm distribution and suggests that additional factors drive normal germplasm dynamics.