Results & discussion
Exposure of wild-type flies to different amounts of the contact insecticide DDT (Gao et al. 2022b) in an incubation vial caused paralysis and death with an efficiency that depended on the insecticide amounts (Fig. 1A). After removal of the dead flies, exposure of a second cohort of flies in the same incubation vial did not compromise survival even at the highest DDT amounts (Fig. 1B). We speculated that the first cohort of flies actively modified and detoxified DDT raising the chance of the second cohort to survive. Alternatively, the first cohort flies might have passively improved survival of the second cohort by adsorption of DDT to their surface. Following this argument, removal of the corpses of the first cohort may cause a depletion of DDT amounts that are not lethal to the second cohort flies. To test this possibility, we added silica beads to vials containing a high DDT amount prior to the incubation with flies (Fig. 1C, D). These flies died. Moreover, flies incubated with these beads removed from the DDT-vial and deposited in a clean vial died as well. Thus, physical contact with DDT depletes the effective amounts of DDT, which, however, remains toxic to the second cohort. This observation indicates that DDT had not decayed due to prolonged usage when the second cohort was exposed to it. Candidate molecules that may interfere with DDT toxicity are cuticular hydrocarbons at the fly body surface. Addition of fly surface wash solutions or vegetable oil (mimicking surface lipids) did not detoxify DDT exposed to the first cohort flies (Fig. 1E). Thus, lipids are probably not involved in DDT detoxification. An alternative mode of DDT detoxification is the contact of the substrate with the proboscis. To study this possibility, we removed the proboscis of the first-cohort flies prior to incubation with DDT. After successful wound-healing, flies without proboscis died upon contact with DDT (Fig. 1F). The second cohort, however, by the majority survived the assay. This indicates that oral DDT mitigation is irrelevant. Next, we sought to reduce the residual toxicity of DDT after incubation with the first cohort. In a simple scenario, we reckoned that cuticular chitin my adsorb DDT and thereby reduce its adverse effects (Fig. 1G). Second-cohort flies were, therefore, added to the vial supplemented with chitin. Against our hypothesis, addition of chitin to the vial after removal of the first-cohort flies reduced survival of the second-cohort flies. Possibly, this effect is due to remobilization of DDT by chitin. Although the mode of function of chitin on DDT is enigmatic, we can draw an important conclusion from this experiment as it demonstrates that in the initial trials without chitin DDT is present but chemically or physically masked or detoxified when the second cohort flies are incubated in the vial after the first cohort. In other words, the first cohort flies do actively, but reversibly, modify the substratum.
According to the kin selection theory, the beneficial effects of a behaviour are more pronounced when the actor and recipient are related. To test whether this applies to our system, we incubated a different wild-type population as a second cohort (Fig. 1H). The survival rate of the second cohort was lower when the wild-type populations differed in the two vials than when the same population was incubated in the consecutive vials.
Next, we addressed the possibility that other insect species thanD. melanogaster might have an identical effect on DDT toxicity. For this purpose, we incubated a honeybee (Apis mellifera ) worker in a vial containing different amounts of DDT (Fig 1I). This incubation was lethal to the honeybee. After removal of the dead honeybee, a cohort of wild-type D. melanogaster was incubated in the same vial. These flies survived this treatment. We conclude that insects, along with their internal detoxification responses, may possess a detoxification mechanism that acts outside their body.
We wondered if this extra-corporeal detoxification response may modify the efficiency of other, unrelated xenobiotics, we repeated the two-cohort experiments with the insecticides Chlorpyrifos and Chlorantraniliprole (Fig. 1J,K). While Chlorpyrifos was detoxified in these assays, Chlorantraniliprole retrained its toxicity. Thus, whereas some chemically different xenobiotics are detoxified by the extra-corporeal detoxification response, some others are not targeted by this process. In conclusion, along with the internal detoxification response, insects have developed an extra-corporeal detoxification mechanism that, in contrast to the former, does not only protect the individual that launches it but the population of insects in the niche (Fig. 2). The altruistic notion comes into play considering that in the field, D. melanogaster flies tend to cluster in their micro-habitat (Soto-Yeber et al. 2018).
We reckon that this altruistic process involves the tarsa. Consistent with recently published findings (4), the insect tarsa appear to be molecularly and genetically autonomous organs involved in xenobiotic defence. One may even speculate that bacteria that colonise especially the tarsa might participate to this detoxification program (Honget al. 2022). The genetic, molecular and cellular mechanisms of the underlying program await identification and characterization. Indeed, the model insect D. melanogaster is a perfect system to advance in ecological genetics in this direction as understanding this problem will have a considerable impact on insect ecology and pest science.