Thiamethoxam induces meiotic arrest and reduces the quality of oocytes in cattle

Thiamethoxam (TMX) is a neonicotinoid insecticide, the residues of which have been detected on various crops. In addition to its specific acetylcholine toxicity to insects, TMX was also found to be toxic to mammals. Moreover, oocytes are vulnerable to reactive oxygen species (ROS). Excessive ROS production can override antioxidant defenses and produce oxidative stress and DNA damage that trigger apoptosis and necrosis in organisms. In this study, we exposed bovine oocytes to TMX during maturation. Microscopic examination showed that 1.6 mM TMX significantly inhibited maturation at the germinal vesicle (GV) and metaphase I (MI) stages. Immunofluorescence staining and enzyme activity analysis revealed that TMX induced a reduction in CDC25 and CDC2 activity. Furthermore, time-lapse tracking and immunofluorescence staining indicated the maintenance of cyclin B in the cytoplasm, persistence of Bub3 at kinetochores, and absence of actin caps after TMX-exposed oocytes reached the MI stage. In addition, metaphase II (MII) oocytes exposed to TMX showed disordered chromosomes and spindles. These oocytes accumulated excess ROS and showed significantly decreased mitochondrial membrane potential and increased apoptotic signals. Parthenogenetic embryos from these oocytes showed decreased percentages of morulae and blastocysts. These results indicate that TMX delays bovine oocyte progression to MI stage, blocks them at the MI stage, triggers disordered chromosomes and spindles at MII stage, and ultimately results in MII oocytes with poor cleavage ability and inhibited development to morulae and blastocysts.