Introduction
According to W. D. Hamilton’s inclusive fitness theory (kin selection),
a trait or behaviour is altruistic when the fitness cost of the actor is
lower than the fitness benefit of the recipient which is directly
proportional to the genetic relatedness between actor and recipient
(rb>c ; r=relatedness, b=benefit for recipient,
c=cost for actor; (West et al. 2007)). In insects, usually
eusocial species such as ants, bees and termites are considered to show
altruistic behaviour. This extends to the point that “an animal acting
on this principle would sacrifice its life if it could thereby save more
than two brothers, but not for less” (Hamilton 1963). Here, we report
on our observations during exposure of the non-eusocial fruit flies
(Drosophila melanogaster ) to insecticides arguing that first
visitors of a contaminated site are able to detoxify the site to the
benefit of the second visitors while they die.
Xenobiotics including plant secondary metabolites and insecticides
challenge insects in their daily life as they may perturb cell, tissue
and organ physiology at worst causing death. For survival, hence, they
have developed elaborate structural and molecular defence mechanisms to
escape or disarm xenobiotic toxicity (Gao et al. 2022a). First,
the cuticle that covers the body and the endings of the digestive system
serves as a barrier to some extent preventing xenobiotics penetration.
If xenobiotics overcome the cuticle barrier, potent genetic and
molecular programs are elicited for detoxification. The molecular
players of the detoxification response have been studied extensively in
various insect species. They act in concert in different internal
tissues such as the fat body and the midgut. A key entry site of
xenobiotics are the ends of the legs, the tarsa. These body parts are
designed to sense the substratum with gustatory sensilla and need to
have a cuticle with adapted higher permeability (Ling et al.2014; Dinges et al. 2021) and flexibility. A subtype of these
sensilla, in addition, may have pores permitting uptake of small
molecules. Thickening of the tarsal cuticle in response to continuous
exposure to insecticides has been reported in mosquitos (Balabanidouet al. 2019). Thus, the tarsa are dynamic cuticular structures
communicating with the proximal environment. Our finding suggest that an
extra-corporeal detoxification mechanism may exist in insects that
protects insects against their proximal environment. As protection
extends to insects visiting the site of the toxic micro-environment
after the first visit of their relatives, we consider this behaviour as
altruistic.