The ability to sense Earth's gravitational pull is essential for orientation, navigation, and/or proprioception in many organisms, including plants, animals, fungi, and even protists. We have found that C. elegans dauers, and to a lesser extent, adults, negatively gravitax when migrating on agar in 50 cm vertically oriented test chambers. We hypothesize that this behavior is necessary for dauer larvae to migrate away from poor environmental conditions in a natural setting and that negative gravitactic behavior in dauers may be modulated by other sensory inputs to halt upward movement as the animals approach the surface of the ground. Supporting this hypothesis, we observed that upward migration is attenuated when dauers are exposed to constant light and/or background EM radiation present in the lab. To assess possible molecular mechanisms for gravity sensation, we investigated the requirement for mechanosensory components in the behavior. MEC-4 and MEC-10 are components of the DEG/ENaC channel involved in touch sensation and have been shown to be necessary for the stress response induced by hypergravity. Although we found that gravitaxis is only slightly defective in
mec-10 mutants and
mec-4 mutants show no impairment in the assay,
mec-7 and
mec-12 mutants are entirely defective for the behavior. MEC-7 and MEC-12 are tubulin subunits comprising the 15 protofilament microtubule structures unique to touch receptor neurons (TRNs). Significantly, touch-insensitive
mec-12(
e1605) mutants, in which the 15 protofilament structures are not disrupted, are also defective for gravitaxis, emphasizing the importance for the varied roles that TRN microtubules play in transducing mechanical force. Finally, we found that another, previously unrelated mechanoreceptor, TRPA-1, is also essential for negative gravitaxis in worms. Studies in D. melanogaster have shown that
trpa-1 homologs pyx and pain are required for negative gravitaxis in the absence of light. Although TRPA-1 has been identified as a putative mechanoreceptor in C. elegans, it is primarily known for sensing noxious cold in PVD neurons. A homolog of
trpa-1 is expressed in mammalian inner ear hair cells; hence, C. elegans may provide a potential model for vestibular and auditory systems in humans. Our findings provide evidence that two mechanosensory pathways function in parallel to mediate gravity-sensing in worms.