Hormesis and trade-offs: a comment.

Single or multiple exposures to mild stress at younger ages often have positive health beneficial effects throughout life, indicating that rescue systems turned on at a young age can be effective even in old age (e.g. Le Bourg 2011). This seems to be at variance with the notion of the so-called trade-offs that due to a limitation of resources it is impossible for an organism to increase all aspects of fitness (see for example, Stearns 2000). Recently, this view has again been raised in an article by McClure et al. (2014), published in Evolution. These authors have reported that Drosophila melanogaster flies challenged by a pretreatment with dead spores of the fungus Metarhizium robertsii lived longer, survived better under heat stress, and had a higher fecundity. Thus, these flies displayed the well-established phenomenon of physiological hormesis (for hormesis terminology, see for example Calabrese et al. 2007; Wiegant 2014). However, McClure et al. (2014) also showed that the pretreatment by dead fungal spores decreased by 10% the survival time of the flies after infection by live spores of the same fungus, and concluded that “when it occurs, hormesis leads to trade-offs with other fitness traits” and that, if applied to human patients, “the consequences of hormetic treatments for infected patients could be dire”. Thus, the balance of positive and negative effects of hormesis on fitness traits could be definitely on the negative side, precluding relying on hormesis in therapy or health maintenance and improvement. In our opinion, this viewpoint is not completely accurate because several other studies performed with flies and other systems show that a mild stress can have positive, negative or no effects on fitness-related traits. For instance, Lints and Le Bourg (1989) showed that female D. melanogaster flies living continuously in hypergravity had a lower and