Are Caenorhabditis elegans magnetoreceptive?

A diverse array of species on the planet employ the Earth’s magnetic field as a navigational aid. As the majority of these animals are migratory, their utility to interrogate the molecular and cellular basis of the magnetic sense is limited. Vidal-Gadea and colleagues recently argued that the worm C. elegans possesses a magnetic sense that guides their vertical movement in soil. In making this claim they relied on three different behavioural assays that involved magnetic stimuli. Here, we set out to replicate their results employing blinded protocols and double wrapped coils that control for heat generation. We find no evidence for a magnetic sense in C. elegans, and demonstrate that iron-contamination from the laboratory setting can result in false positive results. We further show that the Vidal-Gadea hypothesis is problematic as the adoption of a correction angle relative to the Earth’s magnetic inclination does not necessarily result in vertical movement.


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The ability to sense the Earth's magnetic field is a widespread sensory faculty in the 31 animal kingdom. Magnetic sensation has been shown in migratory birds (Merkel and 32 Wiltschko, 1965;Zapka et al., 2009), mole rats (Nemec et al., 2001), pigeons (Wu 33 and Dickman, 2012), and turtles (Lohmann et al., 2004). While behavioral evidence 34 supporting the existence of a magnetic sense is unequivocal, the underlying sensory 35 mechanisms and neuronal circuitry that transduce and integrate magnetic information 36 are largely unknown. A major impediment to progress in the field is the lack of 37 genetic and molecular tools in magnetosensitive species. One such model system 38 could be the nematode Caenorhabditis elegans, which has proved to be a powerful 39 tool to explore a wide variety of senses. It has been claimed by Vidal-Gadea et al.

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(2015) that C. elegans possess a magnetic sense which can easily be exploited for 41 mechanistic investigation (see also Bainbridge et al., 2016). They argue that C. 42 elegans possess a magnetic sense that is employed for vertical orientation, worms 43 adopting a correction angle relative to the inclination of the Earth's magnetic field.

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This conclusion was based on results from three assays which they developed: (1) a 45 "vertical burrowing assay"; (2) a "horizontal plate assay"; and (3) a "magnetotaxis 46 assay". Here, we set out to replicate the aforementioned behavioral assays, adopting 47 several critical controls that were absent in the original study. attracted to the benzaldehyde. Conversely, if worms are pre-exposed to 100% 4 protocols we found that worms preferred 1% benzaldehyde (n=11, p<0.005,

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Radio frequency contamination within this room is very low, with intensities below 68 0.1nT between 0.5 to 5MHz (see Figure 2A

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In the first magnetic assay described by Vidal-Gadea, starved animals were injected 73 into agar-filled plastic pipettes ( Figure 3A). Worms were allowed to migrate overnight, 74 and the number on each end of the tube were counted. In the absence of an external 75 field the authors reported that animals preferentially migrated downwards, however, 76 when exposed to an inverted Earth strength magnetic field worms migrated upwards.

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This preference was reversed in the case of fed animals. We repeated these 78 experiments, but observed no effect of inverting the magnetic field on the burrowing 79 index when the worms were starved (Mann-Whitney U-test, n 1 =38, n 2 =40, U=681, n.

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In their second behavioural assay, Vidal-Gadea placed ≈50 worms in the center of an 86 agar plate ( Figure 3C). This plate was placed within a single wrapped Merritt coil 87 system which permitted the generation of either null or horizontal magnetic fields of 88 Earth strength intensity (either 32.5T or 65T). They reported that in the absence of 89 magnetic stimuli worms displayed no directional preference, whereas in the presence 90 of a horizontal field worms distributed in a biased direction 60° either side of the 91 imposed vector. We replicated these experiments, treating each plate as an 92 experimental unit. Blind analysis of worm orientation revealed no effect on orientation 93 behavior when applying a 32.5µT stimulus (Rayleigh-test, r=0.20, n=24, n. s) or a 94 65µT stimulus (Rayleigh-test, r=0.25, n=24, n. s., Figure 3D). Nor did we observe any 95 directional preference in our control experiments (32.5µT: Rayleigh-test, r=0.10, 96 n=24, n. s.; 65 µT: Rayleigh-test, r=0.11, n=24).

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In their third behavior assay worms were placed in the center of a horizontal agar 100 plate between two different goal areas ( Figure 3E). An extremely strong neodynium 101 magnet generating a field up to 0.29T (approximately 8,000 times Earth strength), 102 was placed beneath one of the goal areas. Vidal-Gadea reported that in the absence 103 of this magnet worms were distributed evenly between the goal areas, however, if the 104 magnet was present worms migrated towards it. We replicated their set up placing a 105 strong neodynium magnet under one goal area, but added an equally size non-106 magnetic brass control under the opposing goal area. We observed no preference for 107 the goal area associated with the neodynium magnet (n=49 plates, P>0.5, Wilcoxon 108 signed rank test, Figure 3F). As false-positives in magnetoreception have been 109 associated with contamination of biological material with exogenous iron we asked 6 this by growing worms on agar plates spiked with magnetite particles, and repeated 112 the magnetotaxis assay. We found a weak but significant preference for the goal 113 area under which the magnet resided (Wilcoxon signed rank test, n=29, V=670.5,

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Why are our results different from those of Vidal-Gadea? We have gone to great 118 lengths to employ the same protocols. We have used worms from the same source,

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we have employed the same neodymium magnets, we have used the same assay 120 plates, and the same synchronization and starvation protocols. There were, however,

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It is pertinent to note that other attempts to elicit magnetoreceptive behavior in C.
170 elegans have also been unsuccessful (Njus et al., 2015). Collectively, these data 171 indicate that C. elegans is not a suitable model system to understand the molecular

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We used 24cm long tubes filled with 3% chemotaxis agar (see Figure 3A), each end 221 was closed with a plastic stopper. The tubes contained three small holes (3mm in 10 During filling of the tubes great care was taken to avoid air bubbles at the ends of the 224 tubes. Tubes with air bubbles were discarded. 1.5µl of 1M NaN 3 was added to each 225 end-hole of a test tube and 50 were injected into the center-hole ( Figure 3A). The 226 test tube was then covered with aluminum foil and placed upright in a holder. The