Hormesis and trade-offs: a comment

The lifelong effects of anoxia hormesis in solitary bees

The stimulatory and protective response known as hormesis elicits an often over compensatory response resulting in life-history trait improvements. There are an array of abiotic and biotic agents that have been shown to trigger hormesis; most commonly studied are chemicals, temperature, and low oxygen. Investigations into low-oxygen exposures that activate the hormetic response reveal that insect performance can be dramatically improved by single short low-oxygen events, but the focus of this work has been primarily on short-term, transitory protection afforded by hormesis. Few reports examine whether the effect is longer lasting or lifelong. We previously reported that one hour of anoxia was enough to induce a hormetic response in the alfalfa leafcutting bee, Megachile rotundata (Hymenoptera: Megachilidae). Here, we investigated the long-term effects of this response by looking at starvation resistance, flight, and locomotory activity throughout the life of the adult bees. In addition, we studied the effects of anoxia hormesis on multiple reproductive metrics. Anoxia hormesis had lifelong positive effects for flight in both sexes. We also recorded higher starvation survival in bees that experienced hormesis. This improvement in performance came at a steep reproductive cost (ie reduction in fecundity). However, no costs or benefits were passed to the next generation. We hypothesize that using anoxia hormesis in the context of pollination services by this species should result in bees that are more active in the field, thereby increasing the numbers of visits to flowers throughout their entire life.

Sublethal chemical stimulation of arthropod parasitoids and parasites of agricultural and environmental importance

An increasing number of studies have reported stimulation of various organisms in the presence of environmental contaminants. This has created a need to critically evaluate sublethal stimulation and hormetic responses of arthropod parasitoids and parasites following exposure to pesticides and other contaminants. Examining this phenomenon with a focus on arthropods of agricultural and environmental importance serves as the framework for this literature review. This review shows that several pesticides, with diverse chemical structures and different modes of action, applied individually or in combination at sublethal doses, commonly stimulate an array of arthropod parasitoids and parasites. Exposure at sublethal doses can enhance responses related to physiology (e.g., respiration, total lipid content, and total protein content), behavior (e.g., locomotor activity, antennal drumming frequency, host location, and parasitization), and fitness (longevity, growth, fecundity, population net and gross reproduction). Concordantly, the parasitic potential (e.g., infestation efficacy, parasitization rate, and parasitoid/parasite emergence) can be increased, and as a result host activities inhibited. There is some evidence illustrating hormetic dose-responses, but the relevant literature commonly included a limited number and range of doses, precluding a robust differentiation between sub- and superNOAEL (no-observed-adverse-effect level) stimulation. These results reveal a potentially significant threat to ecological health, through stimulation of harmful parasitic organisms by environmental contaminants, and highlight the need to include sublethal stimulation and hormetic responses in relevant ecological pesticide risk assessments. Curiously, considering a more utilitarian view, hormesis may also assist in optimizing mass rearing of biological control agents for field use, a possibility that also remains neglected.

Neonicotinoid Exposures that Stimulate Predatory Stink Bug, Podisus maculiventris (Hemiptera: Pentatomidae), Reproduction Do Not Inhibit Its Behavior

Exposure to sublethal amounts of pesticide can compromise life-history traits and behavior of natural enemies thereby reducing their effectiveness as predators. However, sublethal exposures to pesticides and other stressors may also stimulate insects, a dose-response phenomenon known as hormesis. We previously reported stimulatory effects on reproduction in the beneficial insect predator Podisus maculiventris (Say) (Hemiptera: Pentatomidae) following exposure to sublethal concentrations of imidacloprid. Here we examined whether these same treatments stimulated behavior and/or predation of P. maculiventris. Stimulation of some behaviors occurred at a reproductively hormetic concentration and two additional sublethal concentrations, depending upon bioassay design and sex. We observed no substantial inhibition of behavior or predation at a reproductively hormetic concentration, demonstrating that reproductive fitness in P. maculiventris may be stimulated without compromising behaviors important in its effectiveness as a natural enemy.

Meta-analytic evidence for the anti-aging effect of hormesis on Caenorhabditis elegans

Mild stress-induced hormesis, as a promising strategy to improve longevity and healthy aging, meets both praise and criticism. To comprehensively assess the applicability of hormesis in aging intervention, this meta-analysis was conducted focusing on the effect of hormesis on Caenorhabditis elegans. Twenty-six papers involving 198 effect size estimates met the inclusion criteria. Meta-analytic results indicated that hormesis could significantly extend the mean lifespan of C. elegans by 16.7% and 25.1% under normal and stress culture conditions (p < 0.05), respectively. The healthspan assays showed that hormesis remarkably enhanced the bending frequency and pumping rate of worms by 28.9% and 7.0% (p < 0.05), respectively, while effectively reduced the lipofuscin level by 15.9% (p < 0.05). The obviously increased expression of dauer formation protein-16 (1.66-fold) and its transcriptional targets, including superoxide dismutase-3 (2.46-fold), catalase-1 (2.32-fold) and small heat shock protein-16.2 (2.88-fold) (p < 0.05), was one of the molecular mechanisms underlying these positive effects of hormesis. This meta-analysis provided strong evidence for the anti-aging role of hormesis, highlighting its lifespan-prolonging, healthspan-enhancing and resistance-increasing effects on C. elegans. Given that dauer formation protein-16 was highly conservative, hormesis offered the theoretical possibility of delaying intrinsic aging through exogenous intervention among humans.

Hormesis and radiation safety norms: Comments for an update

Hormesis can be explained by evolutionary adaptation to the current level of a factor present in the natural environment or to some average from the past. This pertains also to ionizing radiation as the natural background has been decreasing during the time of the life existence. DNA damage and repair are normally in a dynamic balance. The conservative nature of the DNA repair suggests that cells may have retained some capability to repair damage from higher radiation levels than that existing today. According to this concept, the harm caused by radioactive contamination would tend to zero with a dose rate tending to a wide range level of the natural radiation background. Existing evidence in favor of hormesis is substantial, experimental data being partly at variance with results of epidemiological studies. Potential bias, systematic errors, and motives to exaggerate risks from low-dose low-rate ionizing radiation are discussed here. In conclusion, current radiation safety norms are exceedingly restrictive and should be revised on the basis of scientific evidence. Elevation of the limits must be accompanied by measures guaranteeing their observance.

Which Is the Most Significant Cause of Aging?

It becomes clearer and clearer that aging is a result of a significant number of causes and it would seem that counteracting one or several of them should not make a significant difference. Taken at face value, this suggests, for example, that free radicals and reactive oxygen species do not play a significant role in aging and that the lifespan of organisms cannot be significantly extended. In this review, I point to the fact that the causes of aging synergize with each other and discuss the implications involved. One implication is that when two or more synergizing causes increase over time, the result of their action increases dramatically; I discuss a simple model demonstrating this. It is reasonable to conclude that this might explain the acceleration of aging and mortality with age. In this regard, the analysis of results and mortality patterns described in studies involving yeasts and Drosophila provides support for this view. Since the causes of aging are synergizing, it is also concluded that none of them is the major one but many including free radicals, etc. play significant roles. It follows that health/lifespan might be significantly extended if we eliminate or even attenuate the increase of a few or even just one of the causes of aging. While the synergism between the causes of aging is the main topic of this review, several related matters are briefly discussed as well.