Electroreception: Worms leap to insects for dispersal

Summary

Electroreception is employed by some fishes to locate prey or predators. However, why the nematode C. elegans senses electric fields is unclear. A new study shows that electroreception helps these microscopic worms to attach themselves to insects for transportation.

C. elegans is widely used to study how an animal responds to different environmental cues at the behavioral, neural, and genetic levels. Despite having a compact nervous system with only 302 neurons, C. elegans features a remarkable ability to sense conventional and non-conventional sensory cues. Previous studies have shown that C. elegans, like humans, is capable of sensing touch, tastants, odors, sound, and light^{1, 2}. C. elegans also senses other sensory cues that humans cannot, such as electric fields³.

Electroreception refers to the ability of an organism to sense and react to environmental electric stimuli. Fishes are the most renowned species that sense electric fields, but electroreception is also found in some terrestrial animals^{4, 5}. Electroreception can be roughly divided into two sub-types, passive and active⁵. For passive electroreception, animals (e.g., sharks, rays, flies, and bees) respond to electric fields that are presented by other objects, including abiotic sources and other organisms. Conversely, active electroreception occurs when animals (e.g., electrical eels) respond to the distortion of their self-generated electric fields. It is well known that animals use electroreception to detect prey, avoid predators, communicate, and facilitate dispersal.

C. elegans and some other nematode species have been reported to manifest a behavior called electrotaxis (also known as galvanotaxis) under electric fields⁵. Electroreception in C. elegans shows unique and shared features compared to those found in fish. Ampullary electroreceptors are exclusively used by aquatic organisms for passive electroreception. These are low-frequency-tuned electrosensory systems that typically function from < 0.1 Hz to up to 10–50 Hz sinusoidal (alternating current, AC) electrostatic fields⁴. C. elegans moves towards a cathode during AC cycles only when the frequency is lower than 1 Hz⁶. Morphologically, the primary cilia of electrosensory cells found in C. elegans amphids and fish ampullary electroreceptors are exposed to a cutaneous pore, which is open to the external environment^{3, 5}. However, ampullary electroreceptors are typically much more sensitive, with a minimal detectable electric field strength of 10⁻⁹ to 10⁻⁶ V/cm⁷. In sharp contrast, the threshold for C. elegans electrotaxis behavior is about 3 V/cm³. Because of this extremely high threshold, it is arguable whether electroreception plays an ecologically relevant role in C. elegans. A new study by Takuya Chiba and colleagues in this issue of Current Biology now provides strong evidence that electroreception can help C. elegans disperse in the wild by attaching to insects, like bumblebees, through electrostatic interactions⁸.

C. elegans is most commonly found as dauer (a German word meaning "enduring or persisting") larvae in the wild⁹. Dauer is a dispersal phase of C. elegans, similar to the infective juvenile stage of parasitic nematodes. Nictation, a behavior in which dauer worms stand on their tail and wave their body, greatly enhances the possibility for dauers to attach to passing animals (e.g., snails, slugs, and isopods)¹⁰ (Figure 1A). In the laboratory, dauers can commonly be found on the lid of a starved culturing dish used to grow C. elegans. It is widely believed that this phenomenon is most likely due to the dauers climbing along the wall of the dish to the lid. However, Chiba et al. decided to take a closer look at this phenomenon in a more natural setting. C. elegans is routinely maintained in the laboratory using a nematode growth medium (NGM), which restricts the movement of worms on a 2D smooth surface. Clearly, this is very different from its natural habitat. To mimic the natural habitat and facilitate nictation, the authors used dog food agar (DFA) mixed with Kimwipes to create a 3D rough surface. This new experiment led to a surprising finding that a dauer suddenly appeared on the lid, and simultaneously, a dauer disappeared from the DFA surface. The authors defined this novel behavior, in which dauers directly transfer across a gap, as "leap". Subsequently, they found that this leaping behavior is a passive movement.

Figure 1. C. elegans dauer larvae employ electroreception to attach to bumblebees for dispersal.

(A) General steps of how a dauer larva leaps onto a bumblebee that carries electrostatic charges. Step 1, a crawling dauer on a 2D surface touches a protruded 3D substance, like a pillar structure. Step 2, the dauer then initiates nictation behavior in which it stands on its tail and waves its head. Step 3, the nictation dauer senses the electric field generated by a nearby bumblebee; opposite charges are induced within the dauer; the dauer also moves further up to decrease the surface tension to prepare for taking-off. Step 4, the dauer passively leaps to the bumblebee due to electrostatic force. Step 5, the dauer attaches to the bumblebee for dispersal. (B) Multiple dauer worms can leap at the same time. A single nictating dauer carries as many as 80 dauers as a “dauer tower” that attaches to an electrically charged bumblebee. Worm larvae and bumblebees were not drawn on scale.

By using a high-speed camera, the authors captured the process of this leaping behavior. Interestingly, only nictating dauers can leap. Moreover, they do not bend their body before the leap. Instead, these dauers maintain a straight body posture before and after taking off. What’s even more surprising is that the flying phase appears to be a process of acceleration. These observations have led the authors to hypothesize that the driving force of this leaping behavior is not generated by dauers themselves, but instead requires external forces, such as the electric potential difference that was found between the lid and the agar.

To further confirm that leaping behavior is driven by electric fields and to obtain quantitative results, the authors built a new setup. First, DFA was replaced with a microdirt substrate (MDS) containing a defined array of micro-posts on its agar surface to facilitate nictation. Second, the agar surface was completely separated from the upper glass electrode lid, resulting in no physical contact between the two. This physical gap ensures that any dauer that appears on the lid is purely there as a result of leaping. With this setup, the authors confirmed that leaping behavior only takes place under electric fields, with a strength threshold of 200 kV/m. Both positive and negative electric fields induce similar leaping behavior. The authors also found that dauers routinely move up before taking off to decrease the contact area between the tail and agar, thus overcoming the surface tension that counteracts electrostatic forces (Figure 1A). By adding detergent to decrease the surface tension, the take-off speed increased. This finding is consistent with leaping as a passive behavior. More importantly, electroreception is required for this leaping behavior. tax-6(p678) mutant worms, which lose the ability to sense electric fields³, are defective in leaping, suggesting that certain stages before the take-off require neural processing. Thus, leaping behavior is also an active process at least partially, especially considering that tax-6(p678) mutant worms exhibit normal nictation behavior.

The electric field strength (200 kV/m) required to induce leaping behavior in C. elegans far exceeds the upper limit of those seen in aquatic animals. It is also worth noting that air is a good electrical insulator compared to the aquatic environment, which makes it possible for terrestrial animals to carry significantly more electrostatic charges. Thus, it is highly possible that dauers can electrostatically interact with other animals in nature. To directly test this theory, the authors used bumblebees that are known to be highly electrostatically charged in the wild^{11, 12}. These bumblebees were artificially charged by rubbing them against a Canadian goldenrod flower. Further experiments confirmed that the charge of bumblebees obtained in the lab was comparable to those observed in the wild. When the charged bumblebees were put close to the nictating dauers, leaping behavior was detected (Figure 1A). The electric field strength calculated was about 724 kV/m, exceeding the 200 kV/m leaping threshold. Strikingly, as many as 80 dauers were able to leap at the same time (Figure 1B). The leaping distance between dauers and bumblebees was about five times the dauer's body length, which is also biologically meaningful.

Previous studies have indicated that C. elegans can attach to flying insects, such as moth flies and fruit flies^{10, 13}. This leaping behavior has also been found in other nematode species besides C. elegans⁸. These observations suggest that it is highly possible that microscopic worms can attach to charged insects for transportation to new habitats or resources in the wild. This new study provides a crucial clue to further understand the global distribution and evolution of nematodes. These exciting findings also raise many interesting questions. For example, considering that leaping behavior is a rather passive phenomenon, how electroreception actively contributes to this behavior is unclear. Several sensory neurons, such as ASJ, ASH, and AWC^{3, 14}, are essential for electroreception in adult C. elegans. Whether dauers sense electric fields using the same neural mechanism is unknown. The activities of electrosensory cells in sharks and skates are tuned by voltage-activated calcium channels and some potassium channels^{15, 16}. Whether similar channels function in C. elegans electroreception has not been tested. A genetic locus containing a Piwi-interacting small RNA (piRNA) cluster, nict-1, is associated with natural variation of dauer nictation¹⁷. Whether C. elegans wild isolates also show variation in leaping behavior remains unclear; if so, what would be the genetic basis of such variation? It will be interesting to investigate these questions in future studies.