Before the experiment, we surmised the PET-MPs to increase mortality. But the experimental results, PET-MPs did not cause high mortality, which is different from our envision. One explanation of the result could be that the chemical properties of PET is highly inert and it has been designed for food packaging purposes (Zimmermann et al., 2019). The previous reports highlighted PET-MPs do not negatively affect the survival, metabolism and feeding activity of Gammarus pulex (Weber et al., 2018). There was no significant effect on the head capsule lengths、weight 、HSP70 level in Chironomus riparius exposed to PET microfibers (Setyorini et al., 2021). Another potential explanation is related to peritrophic matrix (PM). PM, mainly composed of chitin and protein (Lehane, 1997), is a four-layered membrane secreted by Drosophila cardia (King, 1988). PM acts as physical barrier between innergut epithelial cell and food, and can protect innergut epithelial cells against mechanical damage caused by granular food (Hegedus et al., 2009). It has been reported that PET-MPs are densely packaged in the digestive tract of Gammarus pulex, indicating they may be covered in the PM and subsequently processed (Weber et al., 2018). According to this hypothesis, we speculated that the PM protects the flies from the damage of PET-MPs to the digestive system. Thus, no negatively effect was observed on flies exposed to PET-MPs. The possible explanation for the result could be due to the purification effect of microplastics. Drosophila exposed to 1µm of Polystyrene fluorescent microplastics, cleared them in 24 h (Matthews et al., 2021). The diameter of flies’ gut is 160µm (Buchon et al., 2013), and the larger particles are easy to form intestinal obstructions. The size of the microplastics used in our experiment is 2µm. We surmised that the microplastics may be excreted smoothly from flies’ intestines.
An interesting phenomenon was found in our experiment. The exposure to 1 g/L PET-MPs increased the lifespan of the flies. The possible reason for this phenomenon is hormetic effects. Slight stress would disturb the homeostasis of the organism, forcing the animal to make adaptive response. Slight stress can improve functional capacity of the organism and even prolong longevity: this phenomenon is called hormesis (Minois and Rattan, 2003). Exposure to mild stress, such as hypergravity (Le Bourg and Minois, 1999), heat shock (Sørensen et al., 2007), low dose radiation (TG, 2003), cold shocks (Le Bourg, 2007), can slightly extend the life of flies. We speculated microplastics can also be seen as a mild stress. Exposure to PET-MPs induces faster growth in the giant snail Achatina reticulata (De Felice et al., 2021). Low concentration of MPs can stimulate the activity of SOD, and activate the antioxidant system, while SOD activity and the destruction of oxidant system at high concentrations (Trestrail et al., 2020). In our previous study, PET-MPs enhanced the spontaneous activity of flies. We speculated that microplastics can induce the adaptive responses of flies and enhance biological performance to extend the lifespan of flies.
Interestingly, the phenomenon of prolonging life only appeared in male flies. Sex specific hormetic in Drosophila is very common. The beneficial effects of mild stress often occur in male flies. For example, hypergravity (Le Bourg et al., 2000), irradiation (TG, 2003)and heat shock (Sørensen et al., 2007), induced hormesis for life span restricted to male. The reason could be that male flies and female flies respond differently to environmental intervention. The same genes in different sexes act in various environments. For example: mei-41 is a gene encoding the protein of DNA repair (Sekelsky et al., 2000). Furthermore, mei-41 gene is necessary for hormesis, and the lack of hormesis is shown in mutants with inactivated mei-41 (Moskalev et al., 2011). After radiation exposure 72 hours, mei-41 is overexpressed in male flies. Whereas there are no expression changes of mei-41 in female flies (Zhikrevetskaya et al., 2015). Another reason may be related to the dissimilar metabolic levels on flies. Recently reported intestinal sex differences in metabolic gene expression (Hudry et al., 2016). sex-biased intestinal metabolism might contribute to sex differences in whole-body. In previous experiment, our results found that microplastics have a significant effect on male lipid and glucose content, but no obvious effect was observed on female flies. There is no doubt that many differences in development and physiology exist between male and female flies. Hence, in future experiments, the issue of sex specific deserves further investigation.
Despite no negative effects of PET-MPs on the flies’ lifespan in this experiment, we still cannot ignore microplastics potential risks. Plastic products contain additives such as antimicro-bial agents, heat stabilizers, plasticizers, flameretardants, flame retardants, UV stabilizers, pigments, fillers, and to provide specific properties (Lithner, 2011), while pure plastic powder was chosen in our experiment. Studies have shown that at the highest leachate of recyclable plastics, the survival of barnacle Amphibalanus amphitrite larvae was significantly lowered (Li et al., 2016). In addition, due to higher surface area, microplastics are easier to accumulate persistent organic pollutants (POPs) such as fluorobenzenes(CBS), perfluorochemicals (PFCS), polyfluorobiphenyls (PCB), heavy metals, viruses, bacteria, and so on (Mato et al., 2001). A recent study indicated that MPs can aggravate the toxicity of Cadmium and induce an enhancement of gene silencing in somatic tissues of Drosophila (Zhang et al., 2020). Therefore, evaluating the synergistic effects of MPs interaction with additives and environmental pollutants is critical in future studies.