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Feeling the weight of the world: Gravity sensation and sensory integration in C. elegans


All life on Earth, from the smallest microbe to the largest blue whale, is subject to Earth’s gravitational pull. This force may be the only environmental variable that has remained constant for all organisms since the origin of life. Because of this, the ability to sense gravity is present in many species and is often critical for survival. Plants use gravity as a cue for directing root growth. Animals, including humans, sense gravity to facilitate movements and to build spatial awareness. Additionally, gravity sensation is one of many modalities that are integrated by cells and nervous systems to make decisions about behavior. Little is known about gravity sensation compared with other sensory systems. Likewise, polymodality and sensory integration are relatively new and understudied areas of research within sensory biology. To investigate gravity sensation, I developed a novel, large-scale assay for observing gravitactic behavior in C. elegans. I found that the worms negatively gravitax — a behavior that has not previously been observed in this species — and that gravitaxis is altered in the presence of light and electromagnetic fields. A screen of known DEG/ENaC mechanosensory components revealed that MEC-7 and MEC-12, which form specialized microtubules required for gentle touch, are required for negative gravitaxis. However, mutations affecting MEC-4 and MEC-10 — the subunits of gentle-touch transducing ion channels — did not impede worms’ ability to gravitax. Instead, I found that negative gravitaxis depends on the polymodal TRPA-1 channel protein. These findings suggest a previously unidentified connection between DEG/ENaC and TRPA-1 in mechanosensation. I then assayed worms that, through genetic ablation, lacked either the gentle-touch sensitive touch receptor neurons (TRNs) or a pair of proprioceptive PVD neurons, which express MEC-7/12 and TRPA-1. While TRN- worms exhibited behavior similar to N2 controls, worms lacking PVD neurons failed to show negative gravitactic preference. This work contribute to an understanding of gravity sensation and sensory integration in C. elegans that can provide insight into vestibular, auditory, and cognitive disorders.

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