Germline P Granules Are Liquid Droplets That Localize by Controlled Dissolution/Condensation

TLDR

In sexually reproducing organisms, germ cells are specified during embryogenesis, and in *C. elegans* this involves P granules that localize to the posterior of the one‑cell embryo, a process whose physical nature remains poorly understood.

Abstract

In sexually reproducing organisms, embryos specify germ cells, which ultimately generate sperm and eggs. In Caenorhabditis elegans, the first germ cell is established when RNA and protein-rich P granules localize to the posterior of the one-cell embryo. Localization of P granules and their physical nature remain poorly understood. Here we show that P granules exhibit liquid-like behaviors, including fusion, dripping, and wetting, which we used to estimate their viscosity and surface tension. As with other liquids, P granules rapidly dissolved and condensed. Localization occurred by a biased increase in P granule condensation at the posterior. This process reflects a classic phase transition, in which polarity proteins vary the condensation point across the cell. Such phase transitions may represent a fundamental physicochemical mechanism for structuring the cytoplasm.