Caenorhabditis elegansExhibits Positive Gravitaxis

Abstract

10Whether or not the micro swimmer Caenorhabditis elegans senses and respond to gravity is 11 unknown. We find that C. elegans aligns its swimming direction with that of the gravity vector 12 (positive gravitaxis). When placed in an aqueous solution that is denser than the animals, they 13 still orient downwards, indicating that non-uniform mass distribution and/or hydrodynamic 14 effects are not responsible for animal's downward orientation. Paralyzed worms and worms 15 with globally disrupted sensory cilia do not change orientation as they settle in solution, 16indicating that gravitaxis is an active behavior that requires gravisensation. Other types of 17 sensory driven orientation behaviors cannot explain our observed downward orientation. Like 18 other neural behaviors, the ability to respond to gravity declines with age. Our study establishes 19 gravitaxis in the micro swimmer C. elegans and suggests that C. elegans can be used as a 20genetically tractable system to study molecular and neural mechanisms of gravity sensing and 21 orientation. 22 23 24 2 Significance Statement 25Understanding how animals respond to gravity is not only of fundamental scientific interest, 26 but has clinical relevance, given the prevalence of postural instability in aged individuals. 27Determining whether C. elegans responds to gravity is important for mechanistic studies of 28 gravity sensing in an experimentally tractable animal, for a better understanding of nematode 29 ecology and evolution, and for studying biological effects of microgravity. Our experiments, 30which indicate that C. elegans senses and responds to gravity, set the stage for mechanistic 31 studies on molecular mechanisms of gravity sensing. 32When ≥ 1 and ≥ 3, over 73% and 95% of the animals are oriented, respectively, at a polar 131 angle > 90 o . We compute the concentration parameter  for our data by fitting the cumulative 132 distribution function (cdf) associated with equation 2 (SI-section S4) to the experimental one. 133When the animal is at depth d = 4 mm beneath the surface ~ 0.2 (nearly uniform distribution). 134As the animal's depth increases (the animal has more time to align with the direction of gravity), 135 the skewness of the KDE and the magnitude of increase as well For the well-fed WT animals 136  increases at the approximate rate of 0.07 per mm of depth until it asymptotes to ~ 4.3 at ~60 137 mm, and approximately retains this value at depths exceeding 60 mm. KDEs at depths 120 mm 138 < d < 200 mm nearly overlap (SI -Section S5). The inset in Fig. 3 depicts the concentration 139 parameter  as a function of the animal's depth (d, mm) beneath the liquid surface. The data is funded by the Ministry of Education of Taiwan, Global Networking Talent 3.0 Plan (GNT3.0),

Mechanosensory Systems and Sensory Integration Mediate C. elegans Negative Gravitaxis

“…Our findings that C. elegans dauer larvae exhibit pronounced negative gravitaxis contrasts with prior studies which found no gravitactic behavior in dauers, or that reported positive reorientation toward the gravitational vector in adults 16,17 . These differences are likely attributable to several parameters of assay design.…”

“…In previous studies, C. elegans dauer larvae showed no gravitactic preference, in contrast with C. japonica , which negatively gravitax on vertically oriented Petri plates 16 . Other studies reported that C. elegans adults show a tendency to orient downwards when swimming in liquid, suggesting potential positive gravitaxis 17 . However, as in Drosophila and the ascidian Ciona , the ability of nematodes to undergo gravitaxis is likely context-dependent 18,19 .…”

Antagonistic Inhibitory Circuits Integrate Visual and Gravitactic Behaviors

“…At present, the only comprehensive synaptic connectivity maps (i.e., connectomes) are from two invertebrates: the adult nematode Caenorhabditis elegans [20] and a larva of the tunicate Ciona intestinalis [21]. Although there are conflicting reports on whether C. elegans has a gravity response [18,22], Ciona larvae clearly do. The most commonly described gravitactic behavior in Ciona larvae is negative gravitaxis [23][24][25].…”

C. elegans: a sensible model for sensory biology

“…Despite its simplicity, recent studies reveal that C. elegans can detect more of this universe than previously thought. It is even possible that worms can detect gravitational forces (Chen, Ko, Chuang, Bau, & Raizen, 2019). We invite the reader to ponder what else this amazing creature can do!…”