The primary cilium as a gravitational force transducer and a regulator of transcriptional noise.

Circumstantial evidence has suggested that the primary cilium might function as a gravity sensor. Direct evidence of its gravity-sensing function has recently been provided by studies of rohon beard neurons. These neurons showed changes in the variability of gene expression levels that are linked to the cyclic changes in the Earth's gravitational field due to the Sun and Moon. These cyclic changes also cause the tides. Rohon beard neurons, after the primary cilia have been selectively destroyed, no longer show changes in gene expression variability linked to the cyclic changes in Earth's gravitational field. After the neurons regrow their primary cilia, the link between variability in gene expression levels and the Earth's changing gravitational field returns. This suggests two new functions for the primary cilia, detecting the cyclical changes in the Earth's gravitational field and transducing those changes into changes in the variability (stochastic nature) of gene expression.