4.1. Multi-generational influences of DEHTP on fitness
The fitness evaluation was performed in reproduction, lifespan and behavior. In the aspect of reproduction, the oscillation between stimulation and inhibition over generations was also reported in the multi-generational effects of ionic liquids and antibiotics (Zhang and Feng, 2022; Zheng et al., 2022). On one hand, the alteration from inhibition in earlier generation (e.g., in F2) to stimulation in later one (F3) showed generation-dependent hormesis, which was also observed in the reproductive effects of antibiotics (Zheng et al., 2022), ionic liquids (Shi et al., 2021; Zhang and Feng, 2022) on C. elegans and those of graphene oxide on Acheta domesticus (Dziewięcka et al., 2020). Hormesis over generations (also referred as trans-hormesis) were found in various organisms (Agathokleous et al., 2022; Costantini, 2022). On the other hand, the alteration in effects across generations indicated that the organisms actively adjusted fetal response to the maternal challenges, which is essential to ensure the fitness or adaption to sustain the population survival (Yue et al., 2021). Notably, the responses in the offspring did not always match the stresses that the offspring actually experience. From an ecological perspective, pre-conditioning hormesis can improve resilience to subsequent exposure (Costantini et al., 2010; Costantini et al., 2014). From a health perspective, it may even strengthen therapeutic benefit (Dhawan et al., 2020). However, pre-conditioning hormesis is costly under the mismatch between the actual environment in offspring and the predicted environment by parents. Therefore, the long-term effects of DEHTP deserve ecological concerns.
The trade-off effects between lifespan and reproduction by DEHTP were also observed in the toxicities of other pollutants, e.g., ionic liquids (Shi et al., 2021; Zhang and Feng, 2022) and antibiotics (Yu et al., 2017). The trade-off relationship showed a common strategy to balance energy investment in adapting environments for better fitness. On one hand, more reproduction at the expense of lifespan is essential to the population’s survival and continuation (Yu et al., 2017). On the other hand, lower reproduction with an extended lifespan aids individual organisms to escape from environmental stress. Both aspects were observed in the effects of DEHTP but in different generations. Again, the trade-off effects over generations also supported the active adjustment of organisms to environmental stresses. The results collectively indicated that phthalate might potentially disturb adaptation and evolutionary changes.
In the aspect of behavior, the results demonstrated that DEHTP caused avoidant behavior in nematodes, which allows the individuals to escape from environmental stresses. Notably, the influences of DEHTP on reproduction, lifespan, growth and behavior were all observable at 0.8 mg/L and even lower concentrations that are environmental realistic (e.g., 0.07-6.1 mg/kg as the arithmetic mean values in dust) (Nagorka et al., 2011). The results also brought up an assumption that DEHTP resulted in caloric restriction by less foraging behavior and less efficient food uptake. The caloric restriction decreased the overall available energy, which subsequently resulted in energy allocation among life traits according to the energy budget theory (Matyja et al., 2020; Yu et al., 2018). The energy allocation also influenced the trade-off relationships over generations (Wang et al., 2022a). Moreover, the caloric restriction is important in the evolutionary responses with more resilient to other stressors, and therefore partially explained the pre-conditioning hormesis (Speakman, 2020). Therefore, further studies on the caloric restriction would explain the aforementioned trade-off relationships between reproduction and lifespan, and also the generational effects.