Abstract
BackgroundGravity plays an important role in most life forms on Earth. Yet, a complete molecular understanding of sensing and responding to gravity is lacking. While there are anatomical differences among animals, there is a remarkable conservation across phylogeny at the molecular level. Caenorhabditis elegans is suitable for gene discovery approaches that may help identify molecular mechanisms of gravity sensing. It is unknown whether C. elegans can sense the direction of gravity.ResultsIn aqueous solutions, motile C. elegans nematodes align their swimming direction with the gravity vector direction while immobile worms do not. The worms orient downward regardless of whether they are suspended in a solution less dense (downward sedimentation) or denser (upward sedimentation) than themselves. Gravitaxis is minimally affected by the animals’ gait but requires sensory cilia and dopamine neurotransmission, as well as motility; it does not require genes that function in the body touch response.ConclusionsGravitaxis is not mediated by passive forces such as non-uniform mass distribution or hydrodynamic effects. Rather, it is mediated by active neural processes that involve sensory cilia and dopamine. C. elegans provides a genetically tractable system to study molecular and neural mechanisms of gravity sensing.
Highlights
Gravity plays an important role in most life forms on Earth
Well-fed wild-type C. elegans young adults align their swimming direction with the direction of gravity As time went by, the wild-type worms varied their swimming direction to align with the direction of the gravity vector
To increase the likelihood that this phenotype is due to the mutation at the cat-2 locus, we tested an independent cat-2 allele, n4547, which contains a 1007-bp deletion and is presumably a null mutant [13]. n4547 mutants showed defective orientation with the gravity vector (Fig. S20), supporting the conclusion that the cat-2 gene is required for gravitaxis. To test in another fashion whether dopaminergic neurotransmission is required for gravitaxis, we examined the settling behavior of the mutants for dop-1, dop2, and dop-3, which encode dopamine receptors
Summary
Gravity plays an important role in most life forms on Earth. Caenorhabditis elegans is suitable for gene discovery approaches that may help identify molecular mechanisms of gravity sensing. It is unknown whether C. elegans can sense the direction of gravity. Gravity plays an important role in most life forms on Earth, ranging from single cells to plants [1] and animals. In the first frame (n = 0) of each video, we manually defined an imaging region. The most frequently occurring binary value (mode) within our imaging region, excluding the worm, was subtracted from all subsequent frames in the video. We defined in frame (n + 1) an extended bounding rectangle with 10 pixels added to the width and length of the bounding rectangle of frame (n) to form a search-region for the worm following its displacement in the time span between frame n and frame (n + 1)
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