Cytoplasmic streaming drifts the polarity cue and specifies the cell polarity in Caenorhabditis elegans zygotes

Cell polarisation is required to define body axes during development. The position of spatial cues for polarisation is critical to direct the body axes. InCaenorhabditis eleganszygotes, the sperm-derived pronucleus/centrosome complex (SPCC) serves as the spatial cue to specify the anterior–posterior axis. Approximately 30 minutes after fertilisation, the contractility of the cell cortex is relaxed near the SPCC, which is the earliest sign of polarisation and called symmetry breaking (SB). It is unclear how the position of SPCC at SB is determined after fertilisation. Here, we show that SPCC drifts dynamically through the cell-wide flow of the cytoplasm, called meiotic cytoplasmic streaming. This flow occasionally brings SPCC to the opposite side of the sperm entry site before SB. Our results demonstrate that cytoplasmic flow determines stochastically the position of the spatial cue of the body axis, even in an organism likeC. elegansfor which development is stereotyped.