The primary cilium is a gravity sensor

Life on Earth evolved in a gravitational field that shows very small cyclic changes in magnitude due to the moon, the same changes that contribute to tides. While there is evidence that animal cells can detect and respond to dramatic changes in gravitational force, such as microgravity, there is little if any evidence that animal cells can directly detect and use normal variation in gravitational force. Since zebrafish embryos in simulated microgravity show a marked increase in gene variance, we wondered whether the Earth's sinusoidal gravitational field could be used to regulate the stochastic nature of gene expression. In neurogenin‐3.1:gfp transgenic zebrafish embryos, GFP fluorescence intensity levels in the rohon beard neurons at high tide were more variable than at low tide in the same embryos. When the primary cilia were selectively destroyed, gene expression levels were uniformly high across the tides. When the primary cilia recovered, two different noise states were once again observed at high tide and low tide. This suggests that the primary cilium detects the cyclic change in gravitational force and uses it to regulate the stochastic nature of gene expression. The ubiquitous nature of the primary cilium suggests that using the change in Earth's gravitational force due to the moon to regulate the stochastic nature of gene expression might be a general feature of all vertebrate cells.