Description

Multiple condensates occupy the perinuclear space of C. elegans germ cells, where they coordinate RNA surveillance to ensure proper gene expression (Lev and Rechavi 2020; Sundby et al. 2021). The most well-studied of these condensates are P granules, phase-separated germ granules required for maintenance of germ cell identity and fertility (Kawasaki et al. 1998; Updike et al. 2014). P granule morphology and localization is well documented in C. elegans development (Strome et al. 1982). Adjacent to P granules are Mutator foci, which are nucleated by MUT-16 and required for the amplification of small interfering RNAs (siRNAs) to create a robust and heritable silencing signal (Phillips et al. 2012). During development, faint Mutator foci are occasionally seen in the P₄ germline blastomere of 30-cell embryos, but are most robust and numerous in the Z2 and Z3 progenitor germ cells (PGCs) of 100-cell embryos (Uebel et al. 2020). A third germline condensate, Z granules, are situated between P granules and Mutator foci and facilitate transgenerational epigenetic inheritance of silencing signals. Z granule components ZNFX-1 and WAGO-4 colocalize with P granules in early embryos, but begin to de-mix from P granules in the Z2/Z3 PGCs to form separate Z granule condensates (Wan et al. 2018). Lastly, recently discovered SIMR foci also intimately localize within this cluster of germline granules. SIMR-1, a key component of SIMR foci, is a Tudor domain protein that mediates production of secondary siRNAs for piwi-interacting RNA (piRNA)-targeted mRNAs (Manage et al. 2020). While P granule, Z granule, and Mutator foci localization through embryonic development has been previously described, the embryonic appearance of SIMR foci is not known.

Here we use endogenously tagged SIMR-1::mCherry to investigate the embryonic onset of SIMR foci. We further visualize embryonic P granules with PGL-1::BFP and Mutator foci with MUT-16::GFP to compare fluorescence and interaction with SIMR foci. Live imaging of embryos reveals diffuse cytoplasmic expression of SIMR-1 in all stages (Figure 1A-E), similar to previous observations of MUT-16 expression (Uebel et al.. 2020). Because SIMR foci are present in the germlines of adult hermaphrodites and localize adjacent to P granules, we focused our analysis on the germline blastomeres and progenitor germ cells of embryos (Figure 1A’-E’). In the 2-cell embryo, P granules segregate to the posterior P₁ germline blastomere, yet no punctate SIMR foci are present (Figure 1A, A’). Similarly, no SIMR foci are found in 8-cell embryos as P granules begin associating with nuclear pores in the P₃ germline blastomere (Figure 1B, B’). The 28-cell embryo yields the first observable SIMR focus adjacent to perinuclear P granules in the P₄ germline blastomere (Figure 1C, C’). While we consistently observe SIMR foci in 28- to 50-cell embryos (n = 3), these foci are few and faint. Around the 100-cell stage, the P₄ cell gives rise to the Z2 and Z3 PGCs, and it is here that we reliably observe bright and numerous SIMR foci (Figure1D, D’). These bright foci also persist in the Z2/Z3 of late-stage embryos of 300 or more cells (Figure1E, E’). Our data reveals the previously unknown embryonic appearance of SIMR foci.

Consistent with their localization in adult germ cells, embryonic SIMR foci appear adjacent to both P granules and Mutator foci. Interestingly, the appearance of fewer, faint SIMR foci in the P₄ cell and more numerous, bright SIMR foci in the Z2/Z3 PGCs is similar to the timing of Mutator foci formation in embryos (Uebel et al.. 2020). Both the de-mixing of Z granules and the appearance of robust Mutator foci and SIMR foci in the PGCs correlates with the onset of embryonic germline transcription (Seydoux and Dunn 1997, Wan et al. 2018, Uebel et al. 2020). Taken together, this observation suggests that the arrival of newly produced mRNAs in the Z2/Z3 PCGs may necessitate or facilitate the coordinated reorganization of germ granule components for efficient RNA surveillance.