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.