Organization of the cytoplasm into dynamic assemblages, such as RNA/protein (RNP) granules, reflects physiological control of gene expression programs. However, the pathways of RNP granule formation and their function during development remain largely unclear. Moreover, in animals with preformed germ cell specification, such as C. elegans, maternal germplasm components asymmetrically condense into RNP (germ) granules that partition exclusively to the germline precursor (P) lineage, lending to the hypothesis that germ granule condensation is a physiochemical property of germ cells that is linked to the embryonic germ line establishment. However, recent findings challenged this view as failed asymmetric segregation of the canonical markers of P granules, PGL and GLH proteins, have no obvious role in germline fate specification [Gallo et al. 2010]. We set out to clarify this enigma by carefully studying the localization behavior and function of other known P granule-associated components. In particular, we focused on maternal RNA regulators that have–unlike PGL/GLH proteins–also an obvious non-granular phase. This includes GLD-2 and GLD-3, which form together a cytoplasmic poly(A) polymerase (cytoPAP). Using a suit of new antibodies, we found that both cytoPAP components are asymmetrically organized in dividing germline precursors. Their cytosolic phase is present at the “somatic” and “germline” poles; however, at the “germline” pole they condense into granules that partition exclusively to germline precursors. Strikingly, these germplasm granules segregate independently of phosphatase PPTR-1, which regulates asymmetric condensation of PGL/GLH-positive P granules. Instead, cytoPAP components follow a MEX-5-dependent dissolution pathway that acts in a later stage of mitosis. In contrast to PGL/GLH, condensation of cytoPAP-containing germ granules strongly correlates with nuclear accumulation of PIE-1, an essential germline fate determinant. Moreover, we uncovered an important functional interdependency between the non-granular form of both cytoPAP components and cortical polarity establishment, suggesting that RNA modifiers act in germline precursors to break cellular symmetry. Taken together, we identified two types of germ granules that depend on different molecular pathways for their asymmetric condensation and distribution. These differing mechanisms of intracellular phase separation appear to be essential for soma-germ line distinction. Thus, our work provides a paradigm for the intricate relation between position-dependent cytoplasm organization and cell fate establishment.
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